From 0c18b8d55bffcc7b33458146d77fd697b4be424d Mon Sep 17 00:00:00 2001
From: Lorenzo Vagliano
Date: Tue, 22 Oct 2024 16:26:39 +0200
Subject: [PATCH] pre-commit formating
---
dags/aps/aps_params.py | 10 +-
dags/aps/aps_pull_api.py | 6 +-
dags/aps/parser.py | 13 +-
dags/aps/repository.py | 1 -
dags/clean/cleanup_logs.py | 3 +-
dags/common/cleanup.py | 2 +-
dags/common/constants.py | 42 ++---
dags/common/enhancer.py | 9 +-
dags/common/parsing/xml_extractors.py | 8 +-
dags/common/scoap3_s3.py | 16 +-
dags/common/utils.py | 18 +-
dags/elsevier/elsevier_pull_sftp.py | 6 +-
dags/executor_config.py | 4 +-
dags/hindawi/hindawi_api_client.py | 2 +-
dags/hindawi/hindawi_params.py | 12 +-
dags/hindawi/parser.py | 4 +-
dags/iop/iop_process_file.py | 14 +-
dags/iop/parser.py | 6 +-
dags/oup/oup_pull_ftp.py | 6 +-
dags/oup/parser.py | 4 +-
dags/springer/parser.py | 8 +-
.../Hindawi/hindawi_fields_mapping.md | 155 ++++++++---------
documentation/IOP/iop_fields_mapping.md | 4 +-
requirements-airflow.txt | 1 -
.../integration/iop/test_iop_dag_pull_sftp.py | 21 ++-
tests/integration/iop/test_repo.py | 29 ++--
.../oup/test_oup_dag_process_file.py | 1 +
tests/units/aps/test_aps_parser.py | 28 +--
.../aps/test_trigger_files_processing.py | 1 +
tests/units/aps/test_utils.py | 2 +-
tests/units/clean/test_clean.py | 2 +-
tests/units/common/data/file_with_mathML.xml | 4 +-
tests/units/common/test_enhancer.py | 5 +-
.../S0370269323000643/main.xml | 2 +-
.../S0370269323000850/main.xml | 2 +-
.../S0550321323000354/main.xml | 2 +-
.../S0550321323000366/main.xml | 2 +-
.../data/address-line-valid/main_rjjlr.xml | 2 +-
.../elsevier/data/j.physletb.2023.138109.xml | 2 +-
tests/units/elsevier/data/main.xml | 2 +-
tests/units/elsevier/data/main2.xml | 2 +-
tests/units/elsevier/data/main_rjjlr.xml | 2 +-
tests/units/hindawi/data/example1.xml | 1 -
tests/units/hindawi/data/example2.xml | 1 -
tests/units/hindawi/data/example3.xml | 1 -
tests/units/hindawi/data/without_doi.xml | 1 -
tests/units/hindawi/data/without_page_nr.xml | 1 -
tests/units/hindawi/test_hindawi_enhance.py | 5 +-
tests/units/iop/data/aca95c.xml | 164 +++++++++---------
.../data/title_and_abstract_with_cdata.xml | 2 +-
tests/units/iop/test_iop_parser.py | 21 +--
tests/units/oup/data/oup_orcid.xml | 2 +-
...b170_no_journal_title_and_pubmed_value.xml | 2 +-
tests/units/oup/test_oup_parser.py | 52 ++----
tests/units/springer/test_parser.py | 162 ++++++++---------
.../test_parser/s10052-024-12692-y.xml | 2 +-
56 files changed, 440 insertions(+), 442 deletions(-)
diff --git a/dags/aps/aps_params.py b/dags/aps/aps_params.py
index 56a37e87..78c304d7 100644
--- a/dags/aps/aps_params.py
+++ b/dags/aps/aps_params.py
@@ -4,11 +4,11 @@
class APSParams:
def __init__(
self,
- from_date= (date.today() - timedelta(days=1)).strftime("%Y-%m-%d"),
- until_date= date.today().strftime("%Y-%m-%d"),
- date= "modified",
- journals= "",
- set= "scoap3",
+ from_date=(date.today() - timedelta(days=1)).strftime("%Y-%m-%d"),
+ until_date=date.today().strftime("%Y-%m-%d"),
+ date="modified",
+ journals="",
+ set="scoap3",
per_page: int = 100,
):
self.from_date = from_date
diff --git a/dags/aps/aps_pull_api.py b/dags/aps/aps_pull_api.py
index 5e85b928..8b6302e0 100644
--- a/dags/aps/aps_pull_api.py
+++ b/dags/aps/aps_pull_api.py
@@ -18,11 +18,11 @@
)
def aps_pull_api():
@task()
- def set_fetching_intervals(repo = APSRepository(), **kwargs):
+ def set_fetching_intervals(repo=APSRepository(), **kwargs):
return set_harvesting_interval(repo=repo, **kwargs)
@task()
- def save_json_in_s3(dates: dict, repo = APSRepository(), **kwargs):
+ def save_json_in_s3(dates: dict, repo=APSRepository(), **kwargs):
parameters = APSParams(
from_date=dates["from_date"],
until_date=dates["until_date"],
@@ -40,7 +40,7 @@ def save_json_in_s3(dates: dict, repo = APSRepository(), **kwargs):
return None
@task()
- def trigger_files_processing(key, repo = APSRepository()):
+ def trigger_files_processing(key, repo=APSRepository()):
if key is None:
logging.warning("No new files were downloaded to s3")
return
diff --git a/dags/aps/parser.py b/dags/aps/parser.py
index c46d4506..14c8d20a 100644
--- a/dags/aps/parser.py
+++ b/dags/aps/parser.py
@@ -97,26 +97,27 @@ def _form_authors(self, article):
if author["type"] == "Person"
]
-
def extract_organization_and_ror(self, text):
pattern = r'(.*?) '
-
+
ror_url = None
-
+
def replace_and_capture(match):
nonlocal ror_url
ror_url = match.group(1)
return match.group(2)
-
+
modified_text = re.sub(pattern, replace_and_capture, text)
-
+
return modified_text, ror_url
def _get_affiliations(self, article, affiliationIds):
parsed_affiliations = [
{
"value": affiliation["name"],
- "organization": self.extract_organization_and_ror(affiliation["name"])[0],
+ "organization": self.extract_organization_and_ror(affiliation["name"])[
+ 0
+ ],
"ror": self.extract_organization_and_ror(affiliation["name"])[1],
}
for affiliation in article["affiliations"]
diff --git a/dags/aps/repository.py b/dags/aps/repository.py
index d07f9f26..cc96e8c9 100644
--- a/dags/aps/repository.py
+++ b/dags/aps/repository.py
@@ -1,6 +1,5 @@
import io
import os
-from typing import IO
from common.repository import IRepository
from common.s3_service import S3Service
diff --git a/dags/clean/cleanup_logs.py b/dags/clean/cleanup_logs.py
index e8b4db93..04e68c11 100644
--- a/dags/clean/cleanup_logs.py
+++ b/dags/clean/cleanup_logs.py
@@ -2,7 +2,6 @@
import pendulum
from airflow.decorators import dag
-from airflow.operators.bash import BashOperator
from airflow.operators.bash_operator import BashOperator
AIRFLOW_HOME = os.getenv("AIRFLOW_HOME")
@@ -14,7 +13,7 @@
def cleanup_logs():
BashOperator(
task_id="cleanup_logs",
- bash_command=f"""
+ bash_command=rf"""
logs_dir="{logs_dir}"
find "$logs_dir" -type d -mtime +30 -exec rm -r {{}} \;
""",
diff --git a/dags/common/cleanup.py b/dags/common/cleanup.py
index a2b4eb8f..1e795054 100644
--- a/dags/common/cleanup.py
+++ b/dags/common/cleanup.py
@@ -35,7 +35,7 @@ def replace_cdata_format(text):
CDATA_PATTERN = re.compile(r"<\?CDATA(.*)\?>")
# pattern = re.compile(r'<\?CDATA\s(.*?)\s\?>', re.DOTALL)
- replaced_text = CDATA_PATTERN.sub(r'', text)
+ replaced_text = CDATA_PATTERN.sub(r"", text)
return replaced_text
diff --git a/dags/common/constants.py b/dags/common/constants.py
index 77e7a2b4..4e9ed959 100644
--- a/dags/common/constants.py
+++ b/dags/common/constants.py
@@ -91,7 +91,7 @@
("Brazil", "Brazil"),
("Brasil", "Brazil"),
("Benin", "Benin"),
- (u"Bénin", "Benin"),
+ ("Bénin", "Benin"),
("Bulgaria", "Bulgaria"),
("Bosnia and Herzegovina", "Bosnia and Herzegovina"),
("Canada", "Canada"),
@@ -141,7 +141,7 @@
("Luxembourg", "Luxembourg"),
("Macedonia", "Macedonia"),
("Mexico", "Mexico"),
- (u"México", "Mexico"),
+ ("México", "Mexico"),
("Monaco", "Monaco"),
("Montenegro", "Montenegro"),
("Morocco", "Morocco"),
@@ -161,7 +161,7 @@
("Portugalo", "Portugal"),
("Portugal", "Portugal"),
("P.R.China", "China"),
- (u"People’s Republic of China", "China"),
+ ("People’s Republic of China", "China"),
("Republic of Belarus", "Belarus"),
("Republic of Benin", "Benin"),
("Republic of Korea", "South Korea"),
@@ -181,7 +181,7 @@
("Slovenia", "Slovenia"),
("South Africa", "South Africa"),
("Africa", "South Africa"),
- (u"España", "Spain"),
+ ("España", "Spain"),
("Spain", "Spain"),
("Sudan", "Sudan"),
("Sweden", "Sweden"),
@@ -233,19 +233,21 @@
]
)
-INSTITUTIONS_AND_COUNTRIES_MAPPING = OrderedDict([
- ("INFN", "Italy"),
- ("European Organization for Nuclear Research", "CERN"),
- ("Conseil Européen pour la Recherche Nucléaire", "CERN"),
- ("CERN", "CERN"),
- ("KEK", "Japan"),
- ("DESY", "Germany"),
- ("FERMILAB", "USA"),
- ("FNAL", "USA"),
- ("SLACK", "USA"),
- ("Stanford Linear Accelerator Center", "USA"),
- ("Joint Institute for Nuclear Research", "JINR"),
- ("JINR", "JINR"),
- ("ROC", "Taiwan"),
- ("R.O.C", "Taiwan"),
-])
+INSTITUTIONS_AND_COUNTRIES_MAPPING = OrderedDict(
+ [
+ ("INFN", "Italy"),
+ ("European Organization for Nuclear Research", "CERN"),
+ ("Conseil Européen pour la Recherche Nucléaire", "CERN"),
+ ("CERN", "CERN"),
+ ("KEK", "Japan"),
+ ("DESY", "Germany"),
+ ("FERMILAB", "USA"),
+ ("FNAL", "USA"),
+ ("SLACK", "USA"),
+ ("Stanford Linear Accelerator Center", "USA"),
+ ("Joint Institute for Nuclear Research", "JINR"),
+ ("JINR", "JINR"),
+ ("ROC", "Taiwan"),
+ ("R.O.C", "Taiwan"),
+ ]
+)
diff --git a/dags/common/enhancer.py b/dags/common/enhancer.py
index b49ebb61..1e9f6b57 100644
--- a/dags/common/enhancer.py
+++ b/dags/common/enhancer.py
@@ -2,7 +2,7 @@
import re
from common.constants import FN_REGEX
-from common.utils import parse_country_from_value, get_country_ISO_name
+from common.utils import get_country_ISO_name, parse_country_from_value
class Enhancer:
@@ -47,7 +47,7 @@ def __construct_titles(self, item, publisher):
def __construct_authors(self, item):
# add_nations(item)
pattern_for_cern_cooperation_agreement = re.compile(
- r'cooperation agreement with cern', re.IGNORECASE
+ r"cooperation agreement with cern", re.IGNORECASE
)
for author in item.get("authors", []):
for affiliation in author.get("affiliations", []):
@@ -65,11 +65,12 @@ def __construct_authors(self, item):
affiliation["country"] = _parsed_country
if affiliation.get("country"):
- affiliation["country"] = get_country_ISO_name(affiliation["country"])
+ affiliation["country"] = get_country_ISO_name(
+ affiliation["country"]
+ )
return item
-
def __call__(self, publisher, item):
creation_date = datetime.datetime.now().isoformat()
item_copy = item.copy()
diff --git a/dags/common/parsing/xml_extractors.py b/dags/common/parsing/xml_extractors.py
index 4f878f0c..8195424d 100644
--- a/dags/common/parsing/xml_extractors.py
+++ b/dags/common/parsing/xml_extractors.py
@@ -17,7 +17,7 @@ def __init__(
extra_function=lambda s: s,
prefixes=None,
all_content_between_tags=False,
- remove_tags=False
+ remove_tags=False,
):
super().__init__(destination)
@@ -92,7 +92,7 @@ def __init__(
default_value=None,
required=False,
extra_function=lambda x: x,
- ) :
+ ):
super().__init__(destination)
self.destination = destination
self.source = source
@@ -132,7 +132,7 @@ def extract(self, article):
class CustomExtractor(IExtractor):
def __init__(
self, destination, extraction_function, required=False, default_value=None
- ) :
+ ):
super().__init__(destination)
self.destination = destination
self.extraction_function = extraction_function
@@ -154,7 +154,7 @@ def __init__(
destination,
value,
required=False,
- ) :
+ ):
super().__init__(destination)
self.destination = destination
self.required = required
diff --git a/dags/common/scoap3_s3.py b/dags/common/scoap3_s3.py
index f5c933ea..49f9c4b6 100644
--- a/dags/common/scoap3_s3.py
+++ b/dags/common/scoap3_s3.py
@@ -8,11 +8,8 @@
logger = get_logger()
-FILE_EXTENSIONS = {
- "pdf": ".pdf",
- "xml": ".xml",
- "pdfa": ".pdf"
-}
+FILE_EXTENSIONS = {"pdf": ".pdf", "xml": ".xml", "pdfa": ".pdf"}
+
def update_filename_extension(filename, type):
extension = FILE_EXTENSIONS.get(type, "")
@@ -20,9 +17,10 @@ def update_filename_extension(filename, type):
return filename
elif extension:
if type == "pdfa":
- extension = f".a-2b.pdf"
+ extension = ".a-2b.pdf"
return f"{filename}{extension}"
+
class Scoap3Repository(IRepository):
def __init__(self):
super().__init__()
@@ -55,7 +53,7 @@ def copy_file(self, source_bucket, source_key, prefix=None, type=None):
"source_key": source_key,
},
"MetadataDirective": "REPLACE",
- "ACL": "public-read"
+ "ACL": "public-read",
},
)
logger.info(
@@ -67,7 +65,9 @@ def copy_files(self, bucket, files, prefix=None):
copied_files = {}
for type, path in files.items():
try:
- copied_files[type] = self.copy_file(bucket, path, prefix=prefix, type=type)
+ copied_files[type] = self.copy_file(
+ bucket, path, prefix=prefix, type=type
+ )
except Exception as e:
logger.error("Failed to copy file.", error=str(e), type=type, path=path)
return copied_files
diff --git a/dags/common/utils.py b/dags/common/utils.py
index 76f36c2c..71847c1f 100644
--- a/dags/common/utils.py
+++ b/dags/common/utils.py
@@ -1,4 +1,3 @@
-from datetime import date, datetime
import io
import json
import os
@@ -6,10 +5,10 @@
import tarfile
import xml.etree.ElementTree as ET
import zipfile
+from datetime import date, datetime
from ftplib import error_perm
from io import StringIO
from stat import S_ISDIR, S_ISREG
-from inspire_utils.record import get_value
import backoff
import country_converter as coco
@@ -20,7 +19,6 @@
BY_PATTERN,
CDATA_PATTERN,
COUNTRIES_DEFAULT_MAPPING,
- COUNTRY_PARSING_PATTERN,
CREATIVE_COMMONS_PATTERN,
INSTITUTIONS_AND_COUNTRIES_MAPPING,
LICENSE_PATTERN,
@@ -30,11 +28,13 @@
UnknownFileExtension,
UnknownLicense,
)
+from inspire_utils.record import get_value
from structlog import get_logger
logger = get_logger()
cc = coco.CountryConverter()
+
def set_harvesting_interval(repo, **kwargs):
if (
"params" in kwargs
@@ -268,7 +268,7 @@ def iterate_element(item):
iterate_element(item)
title_part = [i for i in title_parts if i]
- full_text = ' '.join(title_part).strip()
+ full_text = " ".join(title_part).strip()
return full_text
@@ -311,10 +311,12 @@ def parse_country_from_value(affiliation_value):
country_code = cc.convert(country, to="iso2")
mapped_countries = []
if country_code != "not found":
- mapped_countries = [{
- "code": country_code,
- "name": cc.convert(country, to="name_short"),
- }]
+ mapped_countries = [
+ {
+ "code": country_code,
+ "name": cc.convert(country, to="name_short"),
+ }
+ ]
if len(mapped_countries) > 1 or len(mapped_countries) == 0:
raise FoundMoreThanOneMatchOrNone(affiliation_value)
diff --git a/dags/elsevier/elsevier_pull_sftp.py b/dags/elsevier/elsevier_pull_sftp.py
index e9b539c1..3a1e4504 100644
--- a/dags/elsevier/elsevier_pull_sftp.py
+++ b/dags/elsevier/elsevier_pull_sftp.py
@@ -23,9 +23,7 @@ def elsevier_pull_sftp():
@task(executor_config=kubernetes_executor_config)
def migrate_from_ftp(
- sftp = ElsevierSFTPService(),
- repo = ElsevierRepository(),
- **kwargs
+ sftp=ElsevierSFTPService(), repo=ElsevierRepository(), **kwargs
):
params = kwargs["params"]
specific_files = (
@@ -44,7 +42,7 @@ def migrate_from_ftp(
@task(executor_config=kubernetes_executor_config)
def trigger_file_processing(
- repo = ElsevierRepository(),
+ repo=ElsevierRepository(),
filenames=None,
):
return trigger_file_processing_elsevier(
diff --git a/dags/executor_config.py b/dags/executor_config.py
index db0feb33..971ddecf 100644
--- a/dags/executor_config.py
+++ b/dags/executor_config.py
@@ -9,9 +9,9 @@
resources=k8s.V1ResourceRequirements(
requests={"memory": "1500Mi"},
limits={"memory": "2Gi"},
- )
+ ),
)
],
)
),
-}
\ No newline at end of file
+}
diff --git a/dags/hindawi/hindawi_api_client.py b/dags/hindawi/hindawi_api_client.py
index 0546e11c..a1883da0 100644
--- a/dags/hindawi/hindawi_api_client.py
+++ b/dags/hindawi/hindawi_api_client.py
@@ -29,7 +29,7 @@ def get_articles_metadata(self, parameters, doi=None):
base_url=self.base_url,
headers={
"Accept": "application/xml",
- "User-Agent": "Mozilla/5.0 (Windows NT 10.0; Win64; x64; rv:89.0) Gecko/20100101 Firefox/89.0"
+ "User-Agent": "Mozilla/5.0 (Windows NT 10.0; Win64; x64; rv:89.0) Gecko/20100101 Firefox/89.0",
},
path_segments=path_segments,
parameters=parameters,
diff --git a/dags/hindawi/hindawi_params.py b/dags/hindawi/hindawi_params.py
index 4d7df7c8..39b0b569 100644
--- a/dags/hindawi/hindawi_params.py
+++ b/dags/hindawi/hindawi_params.py
@@ -4,12 +4,12 @@
class HindawiParams:
def __init__(
self,
- from_date= (date.today() - timedelta(days=1)).strftime("%Y-%m-%d"),
- until_date= date.today().strftime("%Y-%m-%d"),
- verb= "listrecords",
- set= "HINDAWI.AHEP",
- metadataprefix= "marc21",
- record= "",
+ from_date=(date.today() - timedelta(days=1)).strftime("%Y-%m-%d"),
+ until_date=date.today().strftime("%Y-%m-%d"),
+ verb="listrecords",
+ set="HINDAWI.AHEP",
+ metadataprefix="marc21",
+ record="",
):
self.from_date = from_date
self.until_date = until_date
diff --git a/dags/hindawi/parser.py b/dags/hindawi/parser.py
index a3b706f7..a72b9123 100644
--- a/dags/hindawi/parser.py
+++ b/dags/hindawi/parser.py
@@ -216,9 +216,7 @@ def _get_publication_info(self, article):
"journal_volume": journal.find(
"./ns0:subfield/[@code='v']", self.prefixes
).text,
- "year": journal.find(
- "./ns0:subfield/[@code='y']", self.prefixes
- ).text,
+ "year": journal.find("./ns0:subfield/[@code='y']", self.prefixes).text,
}
for journal in journals
]
diff --git a/dags/iop/iop_process_file.py b/dags/iop/iop_process_file.py
index 47a80e3f..62f0084a 100644
--- a/dags/iop/iop_process_file.py
+++ b/dags/iop/iop_process_file.py
@@ -3,16 +3,16 @@
import pendulum
from airflow.decorators import dag, task
+from common.cleanup import (
+ convert_html_italics_to_latex,
+ convert_html_subscripts_to_latex,
+ replace_cdata_format,
+)
from common.enhancer import Enhancer
from common.enricher import Enricher
from common.exceptions import EmptyOutputFromPreviousTask
from common.scoap3_s3 import Scoap3Repository
from common.utils import create_or_update_article, upload_json_to_s3
-from common.cleanup import (
- replace_cdata_format,
- convert_html_subscripts_to_latex,
- convert_html_italics_to_latex,
-)
from inspire_utils.record import get_value
from iop.parser import IOPParser
from iop.repository import IOPRepository
@@ -20,12 +20,14 @@
logger = get_logger()
+
def process_xml(input):
input = convert_html_subscripts_to_latex(input)
input = convert_html_italics_to_latex(input)
input = replace_cdata_format(input)
return input
+
def iop_parse_file(**kwargs):
if "params" not in kwargs or "file" not in kwargs["params"]:
raise Exception("There was no 'file' parameter. Exiting run.")
@@ -33,7 +35,7 @@ def iop_parse_file(**kwargs):
file_name = kwargs["params"]["file_name"]
xml_bytes = base64.b64decode(encoded_xml)
if isinstance(xml_bytes, bytes):
- xml_bytes = xml_bytes.decode('utf-8')
+ xml_bytes = xml_bytes.decode("utf-8")
xml_bytes = process_xml(xml_bytes)
xml = ET.fromstring(xml_bytes)
diff --git a/dags/iop/parser.py b/dags/iop/parser.py
index 6161ac2c..57ba642b 100644
--- a/dags/iop/parser.py
+++ b/dags/iop/parser.py
@@ -17,8 +17,8 @@
extract_text,
get_license_type,
get_license_type_and_version_from_url,
+ parse_country_from_value,
parse_to_int,
- parse_country_from_value
)
from idutils import is_arxiv
from inspire_utils.date import PartialDate
@@ -114,7 +114,7 @@ def __init__(self, file_path=None):
required=True,
all_content_between_tags=True,
source="front/article-meta/title-group/article-title",
- remove_tags=True
+ remove_tags=True,
),
TextExtractor(
destination="subtitle",
@@ -131,7 +131,7 @@ def __init__(self, file_path=None):
source="front/article-meta/abstract/p",
all_content_between_tags=True,
extra_function=lambda x: x,
- remove_tags=True
+ remove_tags=True,
),
CustomExtractor(
destination="files",
diff --git a/dags/oup/oup_pull_ftp.py b/dags/oup/oup_pull_ftp.py
index a8fc9444..8b868f2c 100644
--- a/dags/oup/oup_pull_ftp.py
+++ b/dags/oup/oup_pull_ftp.py
@@ -19,9 +19,7 @@ def oup_pull_ftp():
logger = get_logger().bind(class_name="oup_pull_ftp")
@task()
- def migrate_from_ftp(
- ftp = OUPFTPService(), repo = OUPRepository(), **kwargs
- ):
+ def migrate_from_ftp(ftp=OUPFTPService(), repo=OUPRepository(), **kwargs):
params = kwargs["params"]
specific_files = (
"filenames_pull" in params
@@ -37,7 +35,7 @@ def migrate_from_ftp(
@task()
def trigger_file_processing(
- repo = OUPRepository(),
+ repo=OUPRepository(),
filenames=None,
):
return pull_ftp.trigger_file_processing(
diff --git a/dags/oup/parser.py b/dags/oup/parser.py
index b961dcfb..f0c3f27c 100644
--- a/dags/oup/parser.py
+++ b/dags/oup/parser.py
@@ -163,7 +163,9 @@ def _get_authors(self, article):
)
authors = []
for contribution in contributions:
- orcid = get_text_value(contribution.find("contrib-id[@contrib-id-type='orcid']"))
+ orcid = get_text_value(
+ contribution.find("contrib-id[@contrib-id-type='orcid']")
+ )
surname = get_text_value(contribution.find("name/surname"))
given_names = get_text_value(contribution.find("name/given-names"))
email = get_text_value(contribution.find("email"))
diff --git a/dags/springer/parser.py b/dags/springer/parser.py
index 9fd62a64..64180b46 100644
--- a/dags/springer/parser.py
+++ b/dags/springer/parser.py
@@ -9,7 +9,7 @@
CustomExtractor,
TextExtractor,
)
-from common.utils import construct_license, clean_text
+from common.utils import clean_text, construct_license
from structlog import get_logger
@@ -211,7 +211,11 @@ def _get_affiliations(self, author_group, contrib):
affiliations.append(cleaned_aff)
mapped_affiliations = [
- {"value": clean_text(aff), "organization": clean_text(org), **({"country": country} if country else {})}
+ {
+ "value": clean_text(aff),
+ "organization": clean_text(org),
+ **({"country": country} if country else {}),
+ }
for aff, org, country, in affiliations
]
diff --git a/documentation/Hindawi/hindawi_fields_mapping.md b/documentation/Hindawi/hindawi_fields_mapping.md
index 1824eaaf..a8746b09 100644
--- a/documentation/Hindawi/hindawi_fields_mapping.md
+++ b/documentation/Hindawi/hindawi_fields_mapping.md
@@ -5,16 +5,16 @@
| dois | generic_parsing : [33] | value | |
| arxiv_eprints | enricher : [67] | value | |
| | | categories | |
-| page_nr | parsing : [6] | | |
+| page_nr | parsing : [6] | | |
| authors | parsing : [6] generic_parsing : [22] | surname | |
| | | given_names | |
| | | full_name | |
| | | affiliations | country |
| | | | institution |
-| collections | parsing [12] | | |
+| collections | parsing [12] | | |
| license | parsing [11] | url | |
| | | license | |
-| publication_info | generic_parsing : [40]] | journal_title | |
+| publication_info | generic_parsing : [40]] | journal_title | |
| | | journal_volume | |
| | | year | |
| abstracts | enhancer : [46] | value | |
@@ -30,8 +30,8 @@
| | | source | |
| $schema | enricher : [66] | | |
-
# [Enricher](#enricher)
+
| | | |
| ------------------------------ | ------------- | ----------------------------------------------------- |
| Reference | Field | Enricher |
@@ -65,24 +65,24 @@
### [\_\_construct_abstracts](#__construct_abstracts)
-| Reference | Subfield | Value |
-| ------------------------------ | -------- | ------------------------------------------------------------------------------ |
+| Reference | Subfield | Value |
+| ------------------------------ | -------- | ---------------------------------------------------------------------------------------------- |
| [53] | value | Take value from generic parsing abstract [23] |
-| [54] | source | Constant: Hindawi |
+| [54] | source | Constant: Hindawi |
### [\_\_construct_acquisition_source](#__construct_acquisition_source)
| Reference | Subfield | Value |
| ------------------------------ | -------- | ------------------------------------------------ |
-| [55] | source | Constant: Hindawi |
-| [56] | method | Constant: Hindawi |
+| [55] | source | Constant: Hindawi |
+| [56] | method | Constant: Hindawi |
| [57] | date | datetime.datetime.now().isoformat()
|
### [\_\_construct_copyright](#__construct_copyright)
-| Reference | Subfield | Value |
-| ------------------------------ | --------- | ----------------------------------------------------------------------------------------- |
-| [58] | year | Take value from parsing copyright_year [10] |
+| Reference | Subfield | Value |
+| ------------------------------ | --------- | --------------------------------------------------------------------------------------- |
+| [58] | year | Take value from parsing copyright_year [10] |
| [59] | statement | Take value from parsing copyright_statement [9] |
### [\_\_construct_imprints](#__construct_imprints)
@@ -107,99 +107,90 @@
### [\_\_remove_country](#__remove_country)
-| | | | |
-| ------------------------------ | ---------------------------------------------------------------------------------------- | ----- | -------------------------------------------- |
-| Reference | Field | Value | Processing |
-| [65] | from parsed affiliation country [55] | | removes county if the value has: |
-
+| | | | |
+| ------------------------------ | ----------------------------------------------------------------------------- | ----- | -------------------------------- |
+| Reference | Field | Value | Processing |
+| [65] | from parsed affiliation country [55] | | removes county if the value has: |
# [Generic parsing](#generic_parsing)
-| Reference | Field | Subfield | Processing | Default value |
-|-----------|------------------------|----------------------|--------------------------------------------------------------------------------------------------------------------------------------|---------------|
+| Reference | Field | Subfield | Processing | Default value |
+| ------------------------------ | ---------------------- | -------------------- | -------------------------------------------------------------------------------------------------------------------------------------------- | ------------- |
| [22] | authors | surname, given_names | takes authors [2] and splits raw_name: if there is a comma, it means that the surname and given_name are in the second part | |
-| [23] | abstract | | takes abstract [3] and cleans white space characters | |
-| [24] | collaborations | | NO SUCH A FIELD IN HINDAWI | |
-| [25] | title | | takes title [4] and cleans white space characters | |
-| [26] | subtitle | | NO SUCH A FIELD IN HINDAWI | |
-| [27] | journal_year | | | |
-| [28] | preprint_date | | NO SUCH A FIELD IN HINDAWI | |
-| [29] | date_published | | takes date_published [5] and forms it f"{tmp_date.year:04d}-{tmp_date.month:02d}-{tmp_date.day:02d}"
| |
-| [30] | related_article_doi | | NO SUCH A FIELD IN HINDAWI | |
-| [31] | free_keywords | | NO SUCH A FIELD IN HINDAWI | |
-| [32] | classification_numbers | | NO SUCH A FIELD IN HINDAWI | |
-| [33] | dois | | takes dois | |
-| [34] | thesis_supervisor | | NO SUCH A FIELD IN HINDAWI | |
-| [35] | thesis | | NO SUCH A FIELD IN HINDAWI | |
-| [36] | urls | | NO SUCH A FIELD IN HINDAWI | |
-| [37] | local_files | | NO SUCH A FIELD IN HINDAWI | |
-| [38] | record_creation_date | | NO SUCH A FIELD IN HINDAWI | |
-| [39] | control_field | | NO SUCH A FIELD IN HINDAWI | |
-| [40] | publication_info | | | |
-| [41] | | journal_title | takes journal title [16] | |
-| [42] | | journal_volume | takes journal volume [17] | |
-| [43] | | journal_year | takes journal year [18] | |
-| [44] | | journal_issue | NO SUCH A FIELD IN HINDAWI | |
-| [45] | | journal_doctype | NO SUCH A FIELD IN HINDAWI | |
-
+| [23] | abstract | | takes abstract [3] and cleans white space characters | |
+| [24] | collaborations | | NO SUCH A FIELD IN HINDAWI | |
+| [25] | title | | takes title [4] and cleans white space characters | |
+| [26] | subtitle | | NO SUCH A FIELD IN HINDAWI | |
+| [27] | journal_year | | | |
+| [28] | preprint_date | | NO SUCH A FIELD IN HINDAWI | |
+| [29] | date_published | | takes date_published [5] and forms it f"{tmp_date.year:04d}-{tmp_date.month:02d}-{tmp_date.day:02d}"
| |
+| [30] | related_article_doi | | NO SUCH A FIELD IN HINDAWI | |
+| [31] | free_keywords | | NO SUCH A FIELD IN HINDAWI | |
+| [32] | classification_numbers | | NO SUCH A FIELD IN HINDAWI | |
+| [33] | dois | | takes dois | |
+| [34] | thesis_supervisor | | NO SUCH A FIELD IN HINDAWI | |
+| [35] | thesis | | NO SUCH A FIELD IN HINDAWI | |
+| [36] | urls | | NO SUCH A FIELD IN HINDAWI | |
+| [37] | local_files | | NO SUCH A FIELD IN HINDAWI | |
+| [38] | record_creation_date | | NO SUCH A FIELD IN HINDAWI | |
+| [39] | control_field | | NO SUCH A FIELD IN HINDAWI | |
+| [40] | publication_info | | | |
+| [41] | | journal_title | takes journal title [16] | |
+| [42] | | journal_volume | takes journal volume [17] | |
+| [43] | | journal_year | takes journal year [18] | |
+| [44] | | journal_issue | NO SUCH A FIELD IN HINDAWI | |
+| [45] | | journal_doctype | NO SUCH A FIELD IN HINDAWI | |
# [Parsing](#parsing)
-
-| Reference | Field | Source | Parsing |
-|-----------|---------------------|-----------------------------------------------------------------------------|------------------------------------------------------------------|
-| [1] | dois | ns0:metadata/ns1:record/ns0:datafield/[@tag='024']/ns0:subfield/[@code='a'] | lambda x: [x]
|
-| [2] | authors | | authors_parsing |
-| [3] | abstract | ns0:metadata/ns1:record/ns0:datafield/[@tag='520']/ns0:subfield/[@code='a'] | lambda x: " ".join(x.split())
|
-| [4] | title | ns0:metadata/ns1:record/ns0:datafield/[@tag='245']/ns0:subfield/[@code='a'] | lambda x: x
|
-| [5] | date_published | ns0:metadata/ns1:record/ns0:datafield/[@tag='260']/ns0:subfield/[@code='c'] | lambda x: x
|
-| [6] | page_nr | ns0:metadata/ns1:record/ns0:datafield/[@tag='300']/ns0:subfield/[@code='a'] | lambda x: [int(x)]
|
-| [7] | publication_info | | _get_publication_info |
-| [8] | arxiv_eprints | | _get_arxiv |
-| [9] | copyright_statement | ns0:metadata/ns1:record/ns0:datafield/[@tag='542']/ns0:subfield/[@code='f'] | |
-| [10] | copyright_year | ns0:metadata/ns1:record/ns0:datafield/[@tag='542']/ns0:subfield/ | re.search(r"[0-9]{4}", value).group(0)
|
-| [11] | license | | _get_license |
-| [12] | collections | | constant: "Advances in High Energy Physics" |
-
+| Reference | Field | Source | Parsing |
+| ------------------------------ | ------------------- | --------------------------------------------------------------------------- | ----------------------------------------------------------- |
+| [1] | dois | ns0:metadata/ns1:record/ns0:datafield/[@tag='024']/ns0:subfield/[@code='a'] | lambda x: [x]
|
+| [2] | authors | | authors_parsing |
+| [3] | abstract | ns0:metadata/ns1:record/ns0:datafield/[@tag='520']/ns0:subfield/[@code='a'] | lambda x: " ".join(x.split())
|
+| [4] | title | ns0:metadata/ns1:record/ns0:datafield/[@tag='245']/ns0:subfield/[@code='a'] | lambda x: x
|
+| [5] | date_published | ns0:metadata/ns1:record/ns0:datafield/[@tag='260']/ns0:subfield/[@code='c'] | lambda x: x
|
+| [6] | page_nr | ns0:metadata/ns1:record/ns0:datafield/[@tag='300']/ns0:subfield/[@code='a'] | lambda x: [int(x)]
|
+| [7] | publication_info | | \_get_publication_info |
+| [8] | arxiv_eprints | | \_get_arxiv |
+| [9] | copyright_statement | ns0:metadata/ns1:record/ns0:datafield/[@tag='542']/ns0:subfield/[@code='f'] | |
+| [10] | copyright_year | ns0:metadata/ns1:record/ns0:datafield/[@tag='542']/ns0:subfield/ | re.search(r"[0-9]{4}", value).group(0)
|
+| [11] | license | | \_get_license |
+| [12] | collections | | constant: "Advances in High Energy Physics" |
### [authors_parsing](#authors_parsing)
-| Reference | Field | Source | Parsing |
-|-----------|--------------|-------------------------------------------------|-----------------------------------------------------|
+| Reference | Field | Source | Parsing |
+| ------------------------------ | ------------ | ----------------------------------------------- | --------------------------------------------------- |
| [13] | raw_name | ns0:subfield[@code='a'] | lambda x: [x]
|
-| [14] | affiliations | | _get_affiliations |
+| [14] | affiliations | | \_get_affiliations |
| [15] | orcid | ns0:subfield[@code='a']/ns0:subfield[@code='j'] | lambda x: " ".join(x.split())
|
+### [\_get_publication_info](#_get_publication_info)
-
-### [_get_publication_info](#_get_publication_info)
-
-| Reference | Field | Source | Parsing |
-|-----------|----------------|-----------------------------------------------------------------------------|---------|
+| Reference | Field | Source | Parsing |
+| ------------------------------ | -------------- | --------------------------------------------------------------------------- | ------- |
| [16] | journal_title | ns0:metadata/ns1:record/ns0:datafield/[@tag='773']/ns0:subfield/[@code='p'] | |
| [17] | journal_volume | ns0:metadata/ns1:record/ns0:datafield/[@tag='773']/ns0:subfield/[@code='v'] | |
| [18] | journal_year | ns0:metadata/ns1:record/ns0:datafield/[@tag='773']/ns0:subfield/[@code='y'] | |
+### [\_get_arxiv](#_get_arxiv)
+| Reference | Field | Source | Parsing |
+| ------------------------------ | ----- | ---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- | ------------------------------------------------ |
+| [19] | value | "ns0:metadata/ns1:record/ns0:datafield/[@tag='037']/ns0:subfield/[@code='a']" if the field above == 'arxiv' field above: ns0:metadata/ns1:record/ns0:datafield/[@tag='037']/ns0:subfield/[@code='9'] | Removing "arxiv" from value, leaving just digits |
-### [_get_arxiv](#_get_arxiv)
-
-| Reference | Field | Source | Parsing |
-|-----------|-------|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|--------------------------------------------------|
-| [19] | value | "ns0:metadata/ns1:record/ns0:datafield/[@tag='037']/ns0:subfield/[@code='a']" if the field above == 'arxiv' field above: ns0:metadata/ns1:record/ns0:datafield/[@tag='037']/ns0:subfield/[@code='9'] | Removing "arxiv" from value, leaving just digits |
-
-
+### [\_get_license](#_get_arxiv)
-### [_get_license](#_get_arxiv)
-| Reference | Field | Source | Parsing |
-|-----------|---------|---------------------------------------------------------------------------------------------|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
+| Reference | Field | Source | Parsing |
+| ------------------------------ | ------- | ------------------------------------------------------------------------------------------- | -------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- |
| [20] | url | License urls: "ns0:metadata/ns1:record/ns0:datafield/[@tag='540']/ns0:subfield/[@code='u']" | |
| [21] | license | license text = ns0:metadata/ns1:record/ns0:datafield/[@tag='540']/ns0:subfield/[@code='a'] | url_parts = license_url.text.split("/") clean_url_parts = list(filter(bool, url_parts)) version = clean_url_parts.pop() license_type = clean_url_parts.pop() f"CC-{license_type}-{version}"
|
-### [_get_affiliations](#_get_affiliations)
+### [\_get_affiliations](#_get_affiliations)
-| Reference | Field | Source | Parsing |
-|-----------|--------------|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------|---------------------------------------------------------------------|
-| [53] | value | ns0:metadata/ns1:record/ns0:datafield/[@tag='100']/ns0:subfield[@code='u'] ns0:metadata/ns1:record/ns0:datafield/[@tag='700']/ns0:subfield[@code='u'] | |
-| [54] | organization | same as value: ns0:metadata/ns1:record/ns0:datafield/[@tag='100']/ns0:subfield[@code='u'] ns0:metadata/ns1:record/ns0:datafield/[@tag='700']/ns0:subfield[@code='u'] | takes the string before the last comma |
+| Reference | Field | Source | Parsing |
+| ------------------------------ | ------------ | -------------------------------------------------------------------------------------------------------------------------------------------------------------------------- | --------------------------------------------------------------------- |
+| [53] | value | ns0:metadata/ns1:record/ns0:datafield/[@tag='100']/ns0:subfield[@code='u'] ns0:metadata/ns1:record/ns0:datafield/[@tag='700']/ns0:subfield[@code='u'] | |
+| [54] | organization | same as value: ns0:metadata/ns1:record/ns0:datafield/[@tag='100']/ns0:subfield[@code='u'] ns0:metadata/ns1:record/ns0:datafield/[@tag='700']/ns0:subfield[@code='u'] | takes the string before the last comma |
| [55] | country | same as value: ns0:metadata/ns1:record/ns0:datafield/[@tag='100']/ns0:subfield[@code='u'] ns0:metadata/ns1:record/ns0:datafield/[@tag='700']/ns0:subfield[@code='u'] | takes the last string after comma, which starts with a capital letter |
diff --git a/documentation/IOP/iop_fields_mapping.md b/documentation/IOP/iop_fields_mapping.md
index f713d35b..c67366c0 100644
--- a/documentation/IOP/iop_fields_mapping.md
+++ b/documentation/IOP/iop_fields_mapping.md
@@ -80,8 +80,8 @@
| Reference | Subfield | Value |
| ------------------------------ | -------- | ------------------------------------------------ |
-| [57] | source | Constant: IOP |
-| [58] | method | Constant: IOP |
+| [57] | source | Constant: IOP |
+| [58] | method | Constant: IOP |
| [59] | date | datetime.datetime.now().isoformat()
|
### [\_\_construct_copyright](#__construct_copyright)
diff --git a/requirements-airflow.txt b/requirements-airflow.txt
index eceea4bc..88265bbb 100644
--- a/requirements-airflow.txt
+++ b/requirements-airflow.txt
@@ -1,3 +1,2 @@
-c https://raw.githubusercontent.com/apache/airflow/constraints-2.8.3/constraints-3.10.txt
apache-airflow[celery, postgres, redis, cncf.kubernetes, sentry]==2.8.3
-
diff --git a/tests/integration/iop/test_iop_dag_pull_sftp.py b/tests/integration/iop/test_iop_dag_pull_sftp.py
index 30236786..71dfde84 100644
--- a/tests/integration/iop/test_iop_dag_pull_sftp.py
+++ b/tests/integration/iop/test_iop_dag_pull_sftp.py
@@ -1,10 +1,11 @@
+import time
+
import pytest
from airflow.models import DagBag
from common.pull_ftp import migrate_from_ftp, trigger_file_processing
from iop.repository import IOPRepository
from iop.sftp_service import IOPSFTPService
from structlog import get_logger
-import time
DAG_NAME = "iop_pull_sftp"
@@ -94,7 +95,7 @@ def test_dag_run(dag, dag_was_paused: bool, iop_empty_repo):
def test_dag_migrate_from_FTP(iop_empty_repo):
iop_empty_repo.delete_all()
assert len(iop_empty_repo.find_all()) == 0
-
+
with IOPSFTPService() as sftp:
migrate_from_ftp(
sftp,
@@ -165,12 +166,20 @@ def test_dag_migrate_from_FTP(iop_empty_repo):
assert len(iop_empty_repo.find_all()) == len(expected_files)
- iop_pdf_files = sorted(item["pdf"] for item in iop_empty_repo.find_all() if "pdf" in item)
- expected_pdf_files = sorted(item["pdf"] for item in expected_files if "pdf" in item)
+ iop_pdf_files = sorted(
+ item["pdf"] for item in iop_empty_repo.find_all() if "pdf" in item
+ )
+ expected_pdf_files = sorted(
+ item["pdf"] for item in expected_files if "pdf" in item
+ )
assert iop_pdf_files == expected_pdf_files
- iop_xml_files = sorted(item["xml"] for item in iop_empty_repo.find_all() if "xml" in item)
- expected_xml_files = sorted(item["xml"] for item in expected_files if "xml" in item)
+ iop_xml_files = sorted(
+ item["xml"] for item in iop_empty_repo.find_all() if "xml" in item
+ )
+ expected_xml_files = sorted(
+ item["xml"] for item in expected_files if "xml" in item
+ )
assert iop_xml_files == expected_xml_files
diff --git a/tests/integration/iop/test_repo.py b/tests/integration/iop/test_repo.py
index ae757e72..e9245bc6 100644
--- a/tests/integration/iop/test_repo.py
+++ b/tests/integration/iop/test_repo.py
@@ -1,9 +1,10 @@
+import time
+
from common.pull_ftp import migrate_from_ftp
from iop.repository import IOPRepository
from iop.sftp_service import IOPSFTPService
from pytest import fixture
from structlog import get_logger
-import time
@fixture
@@ -16,7 +17,7 @@ def iop_empty_repo():
def test_pull_from_sftp(iop_empty_repo):
iop_empty_repo.delete_all()
assert len(iop_empty_repo.find_all()) == 0
-
+
with IOPSFTPService() as sftp:
migrate_from_ftp(
sftp,
@@ -49,7 +50,8 @@ def test_pull_from_sftp(iop_empty_repo):
{
"pdf": "extracted/2022-07-30T03_02_01_content/1674-1137/1674-1137_46/1674-1137_46_8/1674-1137_46_8_085106/cpc_46_8_085106.pdf",
"xml": "extracted/2022-07-30T03_02_01_content/1674-1137/1674-1137_46/1674-1137_46_8/1674-1137_46_8_085106/cpc_46_8_085106.xml",
- }, {
+ },
+ {
"pdf": "extracted/2022-09-01T03_01_40_content/1674-1137/1674-1137_46/1674-1137_46_9/1674-1137_46_9_093111/cpc_46_9_093111.pdf",
"xml": "extracted/2022-09-01T03_01_40_content/1674-1137/1674-1137_46/1674-1137_46_9/1674-1137_46_9_093111/cpc_46_9_093111.xml",
},
@@ -82,17 +84,24 @@ def test_pull_from_sftp(iop_empty_repo):
"xml": "extracted/2022-09-24T03_01_43_content/1674-1137/1674-1137_46/1674-1137_46_10/1674-1137_46_10_103108/cpc_46_10_103108.xml",
},
{"xml": "extracted/aca95c/aca95c.xml"},
-
]
-
+
assert len(iop_empty_repo.find_all()) == len(expected_files)
- iop_pdf_files = sorted(item["pdf"] for item in iop_empty_repo.find_all() if "pdf" in item)
- expected_pdf_files = sorted(item["pdf"] for item in expected_files if "pdf" in item)
+ iop_pdf_files = sorted(
+ item["pdf"] for item in iop_empty_repo.find_all() if "pdf" in item
+ )
+ expected_pdf_files = sorted(
+ item["pdf"] for item in expected_files if "pdf" in item
+ )
assert iop_pdf_files == expected_pdf_files
- iop_xml_files = sorted(item["xml"] for item in iop_empty_repo.find_all() if "xml" in item)
- expected_xml_files = sorted(item["xml"] for item in expected_files if "xml" in item)
+ iop_xml_files = sorted(
+ item["xml"] for item in iop_empty_repo.find_all() if "xml" in item
+ )
+ expected_xml_files = sorted(
+ item["xml"] for item in expected_files if "xml" in item
+ )
assert iop_xml_files == expected_xml_files
assert sorted(iop_empty_repo.get_all_raw_filenames()) == sorted(
@@ -103,4 +112,4 @@ def test_pull_from_sftp(iop_empty_repo):
"2022-09-24T03_01_43_content.zip",
"aca95c.zip",
]
- )
\ No newline at end of file
+ )
diff --git a/tests/integration/oup/test_oup_dag_process_file.py b/tests/integration/oup/test_oup_dag_process_file.py
index 60c3937c..0e641e93 100644
--- a/tests/integration/oup/test_oup_dag_process_file.py
+++ b/tests/integration/oup/test_oup_dag_process_file.py
@@ -79,6 +79,7 @@ def test_affiliation_countries_in_enriched(parser, articles):
for aff in author.get("affiliations"):
assert aff.get("country") is not None
+
def test_dag_loaded(dag):
assert dag
assert len(dag.tasks) == 6
diff --git a/tests/units/aps/test_aps_parser.py b/tests/units/aps/test_aps_parser.py
index 987ba470..745abdbc 100644
--- a/tests/units/aps/test_aps_parser.py
+++ b/tests/units/aps/test_aps_parser.py
@@ -61,9 +61,9 @@ def parsed_articles(parser, articles):
"surname": "Wu",
"affiliations": [
{
- "value": "Department of Physics and Astronomy, Vanderbilt University , Nashville, Tennessee 37240, USA",
+ "value": 'Department of Physics and Astronomy, Vanderbilt University , Nashville, Tennessee 37240, USA',
"organization": "Department of Physics and Astronomy, Vanderbilt University, Nashville, Tennessee 37240, USA",
- "ror": "https://ror.org/02vm5rt34"
+ "ror": "https://ror.org/02vm5rt34",
}
],
},
@@ -73,9 +73,9 @@ def parsed_articles(parser, articles):
"surname": "Turner",
"affiliations": [
{
- "value": "Department of Physics and Astronomy, Vanderbilt University , Nashville, Tennessee 37240, USA",
+ "value": 'Department of Physics and Astronomy, Vanderbilt University , Nashville, Tennessee 37240, USA',
"organization": "Department of Physics and Astronomy, Vanderbilt University, Nashville, Tennessee 37240, USA",
- "ror": "https://ror.org/02vm5rt34"
+ "ror": "https://ror.org/02vm5rt34",
}
],
},
@@ -85,9 +85,9 @@ def parsed_articles(parser, articles):
"surname": "Wang",
"affiliations": [
{
- "value": "Department of Physics and Astronomy, Vanderbilt University , Nashville, Tennessee 37240, USA",
+ "value": 'Department of Physics and Astronomy, Vanderbilt University , Nashville, Tennessee 37240, USA',
"organization": "Department of Physics and Astronomy, Vanderbilt University, Nashville, Tennessee 37240, USA",
- "ror": "https://ror.org/02vm5rt34"
+ "ror": "https://ror.org/02vm5rt34",
}
],
},
@@ -97,9 +97,9 @@ def parsed_articles(parser, articles):
"surname": "Borel",
"affiliations": [
{
- "value": "Department of Physics and Astronomy, Vanderbilt University , Nashville, Tennessee 37240, USA",
+ "value": 'Department of Physics and Astronomy, Vanderbilt University , Nashville, Tennessee 37240, USA',
"organization": "Department of Physics and Astronomy, Vanderbilt University, Nashville, Tennessee 37240, USA",
- "ror": "https://ror.org/02vm5rt34"
+ "ror": "https://ror.org/02vm5rt34",
}
],
},
@@ -111,9 +111,9 @@ def parsed_articles(parser, articles):
"surname": "Boudjada",
"affiliations": [
{
- "value": "Department of Physics and Astronomy, Vanderbilt University , Nashville, Tennessee 37240, USA",
+ "value": 'Department of Physics and Astronomy, Vanderbilt University , Nashville, Tennessee 37240, USA',
"organization": "Department of Physics and Astronomy, Vanderbilt University, Nashville, Tennessee 37240, USA",
- "ror": "https://ror.org/02vm5rt34"
+ "ror": "https://ror.org/02vm5rt34",
}
],
},
@@ -123,9 +123,9 @@ def parsed_articles(parser, articles):
"surname": "Buessen",
"affiliations": [
{
- "value": "Department of Physics and Astronomy, Vanderbilt University , Nashville, Tennessee 37240, USA",
+ "value": 'Department of Physics and Astronomy, Vanderbilt University , Nashville, Tennessee 37240, USA',
"organization": "Department of Physics and Astronomy, Vanderbilt University, Nashville, Tennessee 37240, USA",
- "ror": "https://ror.org/02vm5rt34"
+ "ror": "https://ror.org/02vm5rt34",
}
],
},
@@ -135,9 +135,9 @@ def parsed_articles(parser, articles):
"surname": "Paramekanti",
"affiliations": [
{
- "value": "Department of Physics and Astronomy, Vanderbilt University , Nashville, Tennessee 37240, USA",
+ "value": 'Department of Physics and Astronomy, Vanderbilt University , Nashville, Tennessee 37240, USA',
"organization": "Department of Physics and Astronomy, Vanderbilt University, Nashville, Tennessee 37240, USA",
- "ror": "https://ror.org/02vm5rt34"
+ "ror": "https://ror.org/02vm5rt34",
}
],
},
diff --git a/tests/units/aps/test_trigger_files_processing.py b/tests/units/aps/test_trigger_files_processing.py
index 07192369..e71d7593 100644
--- a/tests/units/aps/test_trigger_files_processing.py
+++ b/tests/units/aps/test_trigger_files_processing.py
@@ -7,6 +7,7 @@
from airflow.models.dagrun import DagRun
from aps.utils import trigger_file_processing_DAG
+
class S3BucketResultObj:
def __init__(self, key):
self.key = key
diff --git a/tests/units/aps/test_utils.py b/tests/units/aps/test_utils.py
index 5b93f4eb..9745f3ff 100644
--- a/tests/units/aps/test_utils.py
+++ b/tests/units/aps/test_utils.py
@@ -61,6 +61,6 @@ def test_save_file_in_s3():
@freeze_time("2023-12-04 10:00")
def test_split_json():
ids_and_articles = split_json(repo=MockedRepo(), key="key/key")
- expected_id = f"APS__2023-12-04T10:00:00.000000+0000"
+ expected_id = "APS__2023-12-04T10:00:00.000000+0000"
assert ids_and_articles[0]["id"] == expected_id
assert len(ids_and_articles) == 1
diff --git a/tests/units/clean/test_clean.py b/tests/units/clean/test_clean.py
index 449b29bc..a24e78eb 100644
--- a/tests/units/clean/test_clean.py
+++ b/tests/units/clean/test_clean.py
@@ -9,7 +9,7 @@
@fixture
def dag():
dagbag = DagBag(dag_folder="dags/", include_examples=False)
- assert dagbag.import_errors.get(f"dags/cleanup_logs.py") is None
+ assert dagbag.import_errors.get("dags/cleanup_logs.py") is None
clean_dag = dagbag.get_dag(dag_id="cleanup_logs")
return clean_dag
diff --git a/tests/units/common/data/file_with_mathML.xml b/tests/units/common/data/file_with_mathML.xml
index 106dd94f..dddc4760 100644
--- a/tests/units/common/data/file_with_mathML.xml
+++ b/tests/units/common/data/file_with_mathML.xml
@@ -1,6 +1,6 @@
-
+
A
@@ -31,7 +31,7 @@
-
+
diff --git a/tests/units/common/test_enhancer.py b/tests/units/common/test_enhancer.py
index c5206acc..6c54ec39 100644
--- a/tests/units/common/test_enhancer.py
+++ b/tests/units/common/test_enhancer.py
@@ -834,6 +834,7 @@ def test_all_contructors(test_input, expected, publisher):
enhanced = enhancement(item=test_input, publisher=publisher)
assert enhanced == expected
+
@pytest.mark.skip
@pytest.mark.parametrize(
"test_input, expected, publisher",
@@ -846,7 +847,9 @@ def test_all_contructors(test_input, expected, publisher):
],
)
@freeze_time("2022-05-20")
-def test_all_contructors_failing_with_wrong_affiliation_value(test_input, expected, publisher):
+def test_all_contructors_failing_with_wrong_affiliation_value(
+ test_input, expected, publisher
+):
enhancement = Enhancer()
enhanced = enhancement(item=test_input, publisher=publisher)
assert enhanced == expected
diff --git a/tests/units/elsevier/data/CERNQ000000010011/S0370269323000643/main.xml b/tests/units/elsevier/data/CERNQ000000010011/S0370269323000643/main.xml
index d6a8d731..206c477b 100644
--- a/tests/units/elsevier/data/CERNQ000000010011/S0370269323000643/main.xml
+++ b/tests/units/elsevier/data/CERNQ000000010011/S0370269323000643/main.xml
@@ -1 +1 @@
-]>PLB 137730 137730 S0370-2693(23)00064-3 10.1016/j.physletb.2023.137730 European Center of Nuclear Research, ALICE experiment Experiments Fig. 1 Charged-particle pseudorapidity density in Pb Pb [2] and p Pb for the 5% most central collisions, and for pp collisions with INEL>0 trigger class. For symmetric collision systems (Pb Pb and pp) the data has been symmetrised around η = 0 and points for η > 3.5 have been reflected around η = 0. The boxes around the points and to the right reflect the uncorrelated and correlated, with respect to pseudorapidity, systematic uncertainty, respectively. The relative correlated, normalisation, uncertainties are evaluated at dN ch /dη |η =0 . The lines show fits of Eq. (1) (Pb Pb and pp) and Eq. (2) (p Pb) to the data (discussed in Section 4 ). Please note that the ordinate has been cut twice to accommodate for the very different ranges of the charged-particle pseudorapidity densities. Fig. 1 Fig. 2 Charged-particle pseudorapidity density in p Pb collisions at s NN = 5.02 TeV in seven centrality classes based on the V0A and V0C estimators. The lines are obtained using a fit of a scaled, normal distribution in rapidity Eq. (2) to the data (discussed in Section 4 ). Fig. 2 Fig. 3 Ratio r X of the charged-particle pseudorapidity density in Pb Pb (top) and p Pb (bottom) in different centrality classes to the charged-particle pseudorapidity density in pp in the INEL>0 event class. Note, for Pb Pb η lab is the same as the centre-of-mass pseudorapidity. Fig. 3 Fig. 4 The width (top) and effective p T /m (bottom) fit parameters as a function of the mean number of participants in pp, p Pb, and Pb Pb collisions at s NN = 5.02 TeV . Vertical uncertainties are the standard error on the best-fit parameter values, while horizontal uncertainties reflect the uncertainty on 〈N part 〉 from the Glauber calculations. Also shown are similar fit parameters from the same parameterisation of EPOS-LHC calculations as well as direct calculations of the standard deviation of the dN ch /dy distributions and the 〈p T 〉/〈m 〉 ratio from the EPOS-LHC calculations. Fig. 4 Fig. 5 The transverse area S T as calculated in a numerical Glauber model for two extreme cases: a) only the exclusive overlap of nucleons is considered (∩, open markers) and b) the inclusive area of participating nucleons contribute (∪, closed markers) in both p Pb and Pb Pb at s NN = 5.02 TeV . Fig. 5 Fig. 6 Estimate of the lower bound on the Bjorken transverse energy density in pp, p Pb, and Pb Pb collisions at s NN = 5.02 TeV , considering the exclusive (∩, open markers) and inclusive (∪, full markers) overlap area S T of the nucleons. The expression C N part p is fitted to case ∪, and we find C = (0.8 ± 0.3) GeV/(fm2 c ) and p = 0.44 ± 0.08. Also shown is an estimate, via dE T /dy , of ε Bj from Pb Pb collisions at s NN = 2.76 TeV (stars with uncertainty band) [31] . Fig. 6 System-size dependence of the charged-particle pseudorapidity density at s NN = 5.02 TeV for pp, p Pb, and Pb Pb collisions ALICE Collaboration ⋆ S. Acharya 142 D. Adamová 96 A. Adler 74 J. Adolfsson 81 G. Aglieri Rinella 34 M. Agnello 30 N. Agrawal 54 Z. Ahammed 142 S. Ahmad 16 S.U. Ahn 76 I. Ahuja 38 Z. Akbar 51 A. Akindinov 93 M. Al-Turany 108 S.N. Alam 16 D. Aleksandrov 89 B. Alessandro 59 H.M. Alfanda 7 R. Alfaro Molina 71 B. Ali 16 Y. Ali 14 A. Alici 25 N. Alizadehvandchali 125 A. Alkin 34 J. Alme 21 G. Alocco 55 T. Alt 68 I. Altsybeev 113 M.N. Anaam 7 C. Andrei 48 A. Andronic 145 V. Anguelov 105 F. Antinori 57 P. Antonioli 54 C. Anuj 16 N. Apadula 80 L. Aphecetche 115 H. Appelshäuser 68 S. Arcelli 25 R. Arnaldi 59 I.C. Arsene 20 M. Arslandok 147 A. Augustinus 34 R. Averbeck 108 S. Aziz 78 M.D. Azmi 16 A. Badalà 56 Y.W. Baek 41 X. Bai 129 108 R. Bailhache 68 Y. Bailung 50 R. Bala 102 A. Balbino 30 A. Baldisseri 139 B. Balis 2 D. Banerjee 4 Z. Banoo 102 R. Barbera 26 L. Barioglio 106 M. Barlou 85 G.G. Barnaföldi 146 L.S. Barnby 95 V. Barret 136 C. Bartels 128 K. Barth 34 E. Bartsch 68 F. Baruffaldi 27 N. Bastid 136 S. Basu 81 G. Batigne 115 D. Battistini 106 B. Batyunya 75 D. Bauri 49 J.L. Bazo Alba 112 I.G. Bearden 90 C. Beattie 147 P. Becht 108 I. Belikov 138 A.D.C. Bell Hechavarria 145 F. Bellini 25 R. Bellwied 125 S. Belokurova 113 V. Belyaev 94 G. Bencedi 146 69 S. Beole 24 A. Bercuci 48 Y. Berdnikov 99 A. Berdnikova 105 L. Bergmann 105 M.G. Besoiu 67 L. Betev 34 P.P. Bhaduri 142 A. Bhasin 102 I.R. Bhat 102 M.A. Bhat 4 B. Bhattacharjee 42 L. Bianchi 24 N. Bianchi 52 J. Bielčík 37 J. Bielčíková 96 J. Biernat 118 A. Bilandzic 106 G. Biro 146 S. Biswas 4 J.T. Blair 119 D. Blau 89 82 M.B. Blidaru 108 C. Blume 68 G. Boca 28 58 F. Bock 97 A. Bogdanov 94 S. Boi 22 J. Bok 61 L. Boldizsár 146 A. Bolozdynya 94 M. Bombara 38 P.M. Bond 34 G. Bonomi 141 58 H. Borel 139 A. Borissov 82 H. Bossi 147 E. Botta 24 L. Bratrud 68 P. Braun-Munzinger 108 M. Bregant 121 M. Broz 37 G.E. Bruno 107 33 M.D. Buckland 23 128 D. Budnikov 109 H. Buesching 68 S. Bufalino 30 O. Bugnon 115 P. Buhler 114 Z. Buthelezi 72 132 J.B. Butt 14 A. Bylinkin 21 127 S.A. Bysiak 118 M. Cai 27 7 H. Caines 147 A. Caliva 108 E. Calvo Villar 112 J.M.M. Camacho 120 R.S. Camacho 45 P. Camerini 23 F.D.M. Canedo 121 M. Carabas 135 F. Carnesecchi 34 25 R. Caron 137 139 J. Castillo Castellanos 139 F. Catalano 30 C. Ceballos Sanchez 75 I. Chakaberia 80 P. Chakraborty 49 S. Chandra 142 S. Chapeland 34 M. Chartier 128 S. Chattopadhyay 142 S. Chattopadhyay 110 T.G. Chavez 45 T. Cheng 7 C. Cheshkov 137 B. Cheynis 137 V. Chibante Barroso 34 D.D. Chinellato 122 E.S. Chizzali 106 S. Cho 61 P. Chochula 34 P. Christakoglou 91 C.H. Christensen 90 P. Christiansen 81 T. Chujo 134 C. Cicalo 55 L. Cifarelli 25 F. Cindolo 54 M.R. Ciupek 108 G. Clai 54 II J. Cleymans 124 I F. Colamaria 53 J.S. Colburn 111 D. Colella 53 107 33 A. Collu 80 M. Colocci 25 34 M. Concas 59 III G. Conesa Balbastre 79 Z. Conesa del Valle 78 G. Contin 23 J.G. Contreras 37 M.L. Coquet 139 T.M. Cormier 97 P. Cortese 31 M.R. Cosentino 123 F. Costa 34 S. Costanza 28 58 P. Crochet 136 R. Cruz-Torres 80 E. Cuautle 69 P. Cui 7 L. Cunqueiro 97 A. Dainese 57 M.C. Danisch 105 A. Danu 67 P. Das 87 P. Das 4 S. Das 4 S. Dash 49 A. De Caro 29 G. de Cataldo 53 L. De Cilladi 24 J. de Cuveland 39 A. De Falco 22 D. De Gruttola 29 N. De Marco 59 C. De Martin 23 S. De Pasquale 29 S. Deb 50 H.F. Degenhardt 121 K.R. Deja 143 R. Del Grande 106 L. Dello Stritto 29 W. Deng 7 P. Dhankher 19 D. Di Bari 33 A. Di Mauro 34 R.A. Diaz 75 8 T. Dietel 124 Y. Ding 137 7 R. Divià 34 D.U. Dixit 19 Ø. Djuvsland 21 U. Dmitrieva 63 A. Dobrin 67 B. Dönigus 68 A.K. Dubey 142 A. Dubla 108 91 S. Dudi 101 P. Dupieux 136 M. Durkac 117 N. Dzalaiova 13 T.M. Eder 145 R.J. Ehlers 97 V.N. Eikeland 21 F. Eisenhut 68 D. Elia 53 B. Erazmus 115 F. Ercolessi 25 E. Eremenko 96 F. Erhardt 100 A. Erokhin 113 M.R. Ersdal 21 B. Espagnon 78 G. Eulisse 34 D. Evans 111 S. Evdokimov 92 L. Fabbietti 106 M. Faggin 27 J. Faivre 79 F. Fan 7 W. Fan 80 A. Fantoni 52 M. Fasel 97 P. Fecchio 30 A. Feliciello 59 G. Feofilov 113 A. Fernández Téllez 45 A. Ferrero 139 A. Ferretti 24 V.J.G. Feuillard 105 J. Figiel 118 V. Filova 37 D. Finogeev 63 F.M. Fionda 55 G. Fiorenza 34 F. Flor 125 A.N. Flores 119 S. Foertsch 72 S. Fokin 89 E. Fragiacomo 60 E. Frajna 146 A. Francisco 136 U. Fuchs 34 N. Funicello 29 C. Furget 79 A. Furs 63 J.J. Gaardhøje 90 M. Gagliardi 24 A.M. Gago 112 A. Gal 138 C.D. Galvan 120 P. Ganoti 85 C. Garabatos 108 J.R.A. Garcia 45 E. Garcia-Solis 10 K. Garg 115 C. Gargiulo 34 A. Garibli 88 K. Garner 145 P. Gasik 108 E.F. Gauger 119 A. Gautam 127 M.B. Gay Ducati 70 M. Germain 115 S.K. Ghosh 4 M. Giacalone 25 P. Gianotti 52 P. Giubellino 108 59 P. Giubilato 27 A.M.C. Glaenzer 139 P. Glässel 105 E. Glimos 131 D.J.Q. Goh 83 V. Gonzalez 144 L.H. González-Trueba 71 S. Gorbunov 39 M. Gorgon 2 L. Görlich 118 S. Gotovac 35 V. Grabski 71 L.K. Graczykowski 143 L. Greiner 80 A. Grelli 62 C. Grigoras 34 V. Grigoriev 94 S. Grigoryan 75 1 F. Grosa 34 59 J.F. Grosse-Oetringhaus 34 R. Grosso 108 D. Grund 37 G.G. Guardiano 122 R. Guernane 79 M. Guilbaud 115 K. Gulbrandsen 90 T. Gunji 133 W. Guo 7 A. Gupta 102 R. Gupta 102 S.P. Guzman 45 L. Gyulai 146 M.K. Habib 108 C. Hadjidakis 78 H. Hamagaki 83 M. Hamid 7 R. Hannigan 119 M.R. Haque 143 A. Harlenderova 108 J.W. Harris 147 A. Harton 10 J.A. Hasenbichler 34 H. Hassan 97 D. Hatzifotiadou 54 P. Hauer 43 L.B. Havener 147 S.T. Heckel 106 E. Hellbär 108 H. Helstrup 36 T. Herman 37 G. Herrera Corral 9 F. Herrmann 145 K.F. Hetland 36 B. Heybeck 68 H. Hillemanns 34 C. Hills 128 B. Hippolyte 138 B. Hofman 62 B. Hohlweger 91 J. Honermann 145 G.H. Hong 148 D. Horak 37 S. Hornung 108 A. Horzyk 2 R. Hosokawa 15 Y. Hou 7 P. Hristov 34 C. Hughes 131 P. Huhn 68 L.M. Huhta 126 C.V. Hulse 78 T.J. Humanic 98 H. Hushnud 110 L.A. Husova 145 A. Hutson 125 J.P. Iddon 128 R. Ilkaev 109 H. Ilyas 14 M. Inaba 134 G.M. Innocenti 34 M. Ippolitov 89 A. Isakov 96 T. Isidori 127 M.S. Islam 110 M. Ivanov 108 V. Ivanov 99 V. Izucheev 92 M. Jablonski 2 B. Jacak 80 N. Jacazio 34 P.M. Jacobs 80 S. Jadlovska 117 J. Jadlovsky 117 S. Jaelani 62 C. Jahnke 122 M.J. Jakubowska 143 A. Jalotra 102 M.A. Janik 143 T. Janson 74 M. Jercic 100 O. Jevons 111 A.A.P. Jimenez 69 F. Jonas 97 145 P.G. Jones 111 J.M. Jowett 34 108 J. Jung 68 M. Jung 68 A. Junique 34 A. Jusko 111 M.J. Kabus 143 J. Kaewjai 116 P. Kalinak 64 A.S. Kalteyer 108 A. Kalweit 34 V. Kaplin 94 A. Karasu Uysal 77 D. Karatovic 100 O. Karavichev 63 T. Karavicheva 63 P. Karczmarczyk 143 E. Karpechev 63 V. Kashyap 87 A. Kazantsev 89 U. Kebschull 74 R. Keidel 47 D.L.D. Keijdener 62 M. Keil 34 B. Ketzer 43 A.M. Khan 7 S. Khan 16 A. Khanzadeev 99 Y. Kharlov 92 82 A. Khatun 16 A. Khuntia 118 B. Kileng 36 B. Kim 17 C. Kim 17 D.J. Kim 126 E.J. Kim 73 J. Kim 148 J.S. Kim 41 J. Kim 105 J. Kim 73 M. Kim 105 S. Kim 18 T. Kim 148 S. Kirsch 68 I. Kisel 39 S. Kiselev 93 A. Kisiel 143 J.P. Kitowski 2 J.L. Klay 6 J. Klein 34 S. Klein 80 C. Klein-Bösing 145 M. Kleiner 68 T. Klemenz 106 A. Kluge 34 A.G. Knospe 125 C. Kobdaj 116 T. Kollegger 108 A. Kondratyev 75 N. Kondratyeva 94 E. Kondratyuk 92 J. Konig 68 S.A. Konigstorfer 106 P.J. Konopka 34 G. Kornakov 143 S.D. Koryciak 2 A. Kotliarov 96 O. Kovalenko 86 V. Kovalenko 113 M. Kowalski 118 I. Králik 64 A. Kravčáková 38 L. Kreis 108 M. Krivda 111 64 F. Krizek 96 K. Krizkova Gajdosova 37 M. Kroesen 105 M. Krüger 68 D.M. Krupova 37 E. Kryshen 99 M. Krzewicki 39 V. Kučera 34 C. Kuhn 138 P.G. Kuijer 91 T. Kumaoka 134 D. Kumar 142 L. Kumar 101 N. Kumar 101 S. Kundu 34 P. Kurashvili 86 A. Kurepin 63 A.B. Kurepin 63 A. Kuryakin 109 S. Kushpil 96 J. Kvapil 111 M.J. Kweon 61 J.Y. Kwon 61 Y. Kwon 148 S.L. La Pointe 39 P. La Rocca 26 Y.S. Lai 80 A. Lakrathok 116 M. Lamanna 34 R. Langoy 130 P. Larionov 34 52 E. Laudi 34 L. Lautner 34 106 R. Lavicka 114 37 T. Lazareva 113 R. Lea 141 58 J. Lehrbach 39 R.C. Lemmon 95 I. León Monzón 120 M.M. Lesch 106 E.D. Lesser 19 M. Lettrich 34 106 P. Lévai 146 X. Li 11 X.L. Li 7 J. Lien 130 R. Lietava 111 B. Lim 17 S.H. Lim 17 V. Lindenstruth 39 A. Lindner 48 C. Lippmann 108 A. Liu 19 D.H. Liu 7 J. Liu 128 I.M. Lofnes 21 V. Loginov 94 C. Loizides 97 P. Loncar 35 J.A. Lopez 105 X. Lopez 136 E. López Torres 8 J.R. Luhder 145 M. Lunardon 27 G. Luparello 60 Y.G. Ma 40 A. Maevskaya 63 M. Mager 34 T. Mahmoud 43 A. Maire 138 M. Malaev 99 N.M. Malik 102 Q.W. Malik 20 S.K. Malik 102 L. Malinina 75 IV D. Mal'Kevich 93 D. Mallick 87 N. Mallick 50 G. Mandaglio 32 56 V. Manko 89 F. Manso 136 V. Manzari 53 Y. Mao 7 G.V. Margagliotti 23 A. Margotti 54 A. Marín 108 C. Markert 119 M. Marquard 68 N.A. Martin 105 P. Martinengo 34 J.L. Martinez 125 M.I. Martínez 45 G. Martínez García 115 S. Masciocchi 108 M. Masera 24 A. Masoni 55 L. Massacrier 78 A. Mastroserio 140 53 A.M. Mathis 106 O. Matonoha 81 P.F.T. Matuoka 121 A. Matyja 118 C. Mayer 118 A.L. Mazuecos 34 F. Mazzaschi 24 M. Mazzilli 34 J.E. Mdhluli 132 A.F. Mechler 68 Y. Melikyan 63 A. Menchaca-Rocha 71 E. Meninno 114 29 A.S. Menon 125 M. Meres 13 S. Mhlanga 124 72 Y. Miake 134 L. Micheletti 59 L.C. Migliorin 137 D.L. Mihaylov 106 K. Mikhaylov 75 93 A.N. Mishra 146 D. Miśkowiec 108 A. Modak 4 A.P. Mohanty 62 B. Mohanty 87 M. Mohisin Khan 16 V M.A. Molander 44 Z. Moravcova 90 C. Mordasini 106 D.A. Moreira De Godoy 145 I. Morozov 63 A. Morsch 34 T. Mrnjavac 34 V. Muccifora 52 E. Mudnic 35 S. Muhuri 142 J.D. Mulligan 80 A. Mulliri 22 M.G. Munhoz 121 R.H. Munzer 68 H. Murakami 133 S. Murray 124 L. Musa 34 J. Musinsky 64 J.W. Myrcha 143 B. Naik 132 R. Nair 86 B.K. Nandi 49 R. Nania 54 E. Nappi 53 A.F. Nassirpour 81 A. Nath 105 C. Nattrass 131 A. Neagu 20 A. Negru 135 L. Nellen 69 S.V. Nesbo 36 G. Neskovic 39 D. Nesterov 113 B.S. Nielsen 90 E.G. Nielsen 90 S. Nikolaev 89 S. Nikulin 89 V. Nikulin 99 F. Noferini 54 S. Noh 12 P. Nomokonov 75 J. Norman 128 N. Novitzky 134 P. Nowakowski 143 A. Nyanin 89 J. Nystrand 21 M. Ogino 83 A. Ohlson 81 V.A. Okorokov 94 J. Oleniacz 143 A.C. Oliveira Da Silva 131 M.H. Oliver 147 A. Onnerstad 126 C. Oppedisano 59 A. Ortiz Velasquez 69 T. Osako 46 A. Oskarsson 81 J. Otwinowski 118 M. Oya 46 K. Oyama 83 Y. Pachmayer 105 S. Padhan 49 D. Pagano 141 58 G. Paić 69 A. Palasciano 53 S. Panebianco 139 J. Park 61 J.E. Parkkila 126 S.P. Pathak 125 R.N. Patra 102 34 B. Paul 22 H. Pei 7 T. Peitzmann 62 X. Peng 7 L.G. Pereira 70 H. Pereira Da Costa 139 D. Peresunko 89 82 G.M. Perez 8 S. Perrin 139 Y. Pestov 5 V. Petráček 37 V. Petrov 113 M. Petrovici 48 R.P. Pezzi 115 70 S. Piano 60 M. Pikna 13 P. Pillot 115 O. Pinazza 54 34 L. Pinsky 125 C. Pinto 26 S. Pisano 52 M. Płoskoń 80 M. Planinic 100 F. Pliquett 68 M.G. Poghosyan 97 B. Polichtchouk 92 S. Politano 30 N. Poljak 100 A. Pop 48 S. Porteboeuf-Houssais 136 J. Porter 80 V. Pozdniakov 75 S.K. Prasad 4 R. Preghenella 54 F. Prino 59 C.A. Pruneau 144 I. Pshenichnov 63 M. Puccio 34 S. Qiu 91 L. Quaglia 24 R.E. Quishpe 125 S. Ragoni 111 A. Rakotozafindrabe 139 L. Ramello 31 F. Rami 138 S.A.R. Ramirez 45 T.A. Rancien 79 R. Raniwala 103 S. Raniwala 103 S.S. Räsänen 44 R. Rath 50 I. Ravasenga 91 K.F. Read 97 131 A.R. Redelbach 39 K. Redlich 86 VI A. Rehman 21 P. Reichelt 68 F. Reidt 34 H.A. Reme-ness 36 Z. Rescakova 38 K. Reygers 105 A. Riabov 99 V. Riabov 99 T. Richert 81 M. Richter 20 W. Riegler 34 F. Riggi 26 C. Ristea 67 M. Rodríguez Cahuantzi 45 K. Røed 20 R. Rogalev 92 E. Rogochaya 75 T.S. Rogoschinski 68 D. Rohr 34 D. Röhrich 21 P.F. Rojas 45 S. Rojas Torres 37 P.S. Rokita 143 F. Ronchetti 52 A. Rosano 32 56 E.D. Rosas 69 A. Rossi 57 A. Roy 50 P. Roy 110 S. Roy 49 N. Rubini 25 O.V. Rueda 81 D. Ruggiano 143 R. Rui 23 B. Rumyantsev 75 P.G. Russek 2 R. Russo 91 A. Rustamov 88 E. Ryabinkin 89 Y. Ryabov 99 A. Rybicki 118 H. Rytkonen 126 W. Rzesa 143 O.A.M. Saarimaki 44 R. Sadek 115 S. Sadovsky 92 J. Saetre 21 K. Šafařík 37 S.K. Saha 142 S. Saha 87 B. Sahoo 49 P. Sahoo 49 R. Sahoo 50 S. Sahoo 65 D. Sahu 50 P.K. Sahu 65 J. Saini 142 S. Sakai 134 M.P. Salvan 108 S. Sambyal 102 T.B. Saramela 121 D. Sarkar 144 N. Sarkar 142 P. Sarma 42 V.M. Sarti 106 M.H.P. Sas 147 J. Schambach 97 H.S. Scheid 68 C. Schiaua 48 R. Schicker 105 A. Schmah 105 C. Schmidt 108 H.R. Schmidt 104 M.O. Schmidt 34 105 M. Schmidt 104 N.V. Schmidt 97 68 A.R. Schmier 131 R. Schotter 138 J. Schukraft 34 K. Schwarz 108 K. Schweda 108 G. Scioli 25 E. Scomparin 59 J.E. Seger 15 Y. Sekiguchi 133 D. Sekihata 133 I. Selyuzhenkov 108 94 S. Senyukov 138 J.J. Seo 61 D. Serebryakov 63 L. Šerkšnytė 106 A. Sevcenco 67 T.J. Shaba 72 A. Shabanov 63 A. Shabetai 115 R. Shahoyan 34 W. Shaikh 110 A. Shangaraev 92 A. Sharma 101 D. Sharma 49 H. Sharma 118 M. Sharma 102 N. Sharma 101 S. Sharma 102 U. Sharma 102 A. Shatat 78 O. Sheibani 125 K. Shigaki 46 M. Shimomura 84 S. Shirinkin 93 Q. Shou 40 Y. Sibiriak 89 S. Siddhanta 55 T. Siemiarczuk 86 T.F. Silva 121 D. Silvermyr 81 T. Simantathammakul 116 G. Simonetti 34 B. Singh 106 R. Singh 87 R. Singh 102 R. Singh 50 V.K. Singh 142 V. Singhal 142 T. Sinha 110 B. Sitar 13 M. Sitta 31 T.B. Skaali 20 G. Skorodumovs 105 M. Slupecki 44 N. Smirnov 147 R.J.M. Snellings 62 C. Soncco 112 J. Song 125 A. Songmoolnak 116 F. Soramel 27 S. Sorensen 131 I. Sputowska 118 J. Stachel 105 I. Stan 67 P.J. Steffanic 131 S.F. Stiefelmaier 105 D. Stocco 115 I. Storehaug 20 M.M. Storetvedt 36 P. Stratmann 145 S. Strazzi 25 C.P. Stylianidis 91 A.A.P. Suaide 121 C. Suire 78 M. Sukhanov 63 M. Suljic 34 R. Sultanov 93 V. Sumberia 102 S. Sumowidagdo 51 S. Swain 65 A. Szabo 13 I. Szarka 13 U. Tabassam 14 S.F. Taghavi 106 G. Taillepied 108 136 J. Takahashi 122 G.J. Tambave 21 S. Tang 136 7 Z. Tang 129 J.D. Tapia Takaki 127 VII N. Tapus 135 M.G. Tarzila 48 A. Tauro 34 G. Tejeda Muñoz 45 A. Telesca 34 L. Terlizzi 24 C. Terrevoli 125 G. Tersimonov 3 S. Thakur 142 D. Thomas 119 R. Tieulent 137 A. Tikhonov 63 A.R. Timmins 125 M. Tkacik 117 A. Toia 68 N. Topilskaya 63 M. Toppi 52 F. Torales-Acosta 19 T. Tork 78 A.G. Torres Ramos 33 A. Trifiró 32 56 A.S. Triolo 32 S. Tripathy 54 T. Tripathy 49 S. Trogolo 34 V. Trubnikov 3 W.H. Trzaska 126 T.P. Trzcinski 143 A. Tumkin 109 R. Turrisi 57 T.S. Tveter 20 K. Ullaland 21 A. Uras 137 M. Urioni 58 141 G.L. Usai 22 M. Vala 38 N. Valle 28 S. Vallero 59 L.V.R. van Doremalen 62 M. van Leeuwen 91 P. Vande Vyvre 34 D. Varga 146 Z. Varga 146 M. Varga-Kofarago 146 M. Vasileiou 85 A. Vasiliev 89 O. Vázquez Doce 52 106 V. Vechernin 113 A. Velure 21 E. Vercellin 24 S. Vergara Limón 45 L. Vermunt 62 R. Vértesi 146 M. Verweij 62 L. Vickovic 35 Z. Vilakazi 132 O. Villalobos Baillie 111 G. Vino 53 A. Vinogradov 89 T. Virgili 29 V. Vislavicius 90 A. Vodopyanov 75 B. Volkel 34 M.A. Völkl 105 K. Voloshin 93 S.A. Voloshin 144 G. Volpe 33 B. von Haller 34 I. Vorobyev 106 N. Vozniuk 63 J. Vrláková 38 B. Wagner 21 C. Wang 40 D. Wang 40 M. Weber 114 R.J.G.V. Weelden 91 A. Wegrzynek 34 S.C. Wenzel 34 J.P. Wessels 145 S.L. Weyhmiller 147 J. Wiechula 68 J. Wikne 20 G. Wilk 86 J. Wilkinson 108 G.A. Willems 145 B. Windelband 105 M. Winn 139 W.E. Witt 131 J.R. Wright 119 W. Wu 40 Y. Wu 129 R. Xu 7 A.K. Yadav 142 S. Yalcin 77 Y. Yamaguchi 46 K. Yamakawa 46 S. Yang 21 S. Yano 46 Z. Yin 7 I.-K. Yoo 17 J.H. Yoon 61 S. Yuan 21 A. Yuncu 105 V. Zaccolo 23 C. Zampolli 34 H.J.C. Zanoli 62 F. Zanone 105 N. Zardoshti 34 111 A. Zarochentsev 113 P. Závada 66 N. Zaviyalov 109 M. Zhalov 99 B. Zhang 7 S. Zhang 40 X. Zhang 7 Y. Zhang 129 V. Zherebchevskii 113 Y. Zhi 11 N. Zhigareva 93 D. Zhou 7 Y. Zhou 90 J. Zhu 108 7 Y. Zhu 7 G. Zinovjev 3 I N. Zurlo 141 58 1 A.I. Alikhanyan National Science Laboratory (Yerevan Physics Institute) Foundation, Yerevan, Armenia A.I. Alikhanyan National Science Laboratory (Yerevan Physics Institute) Foundation Yerevan Armenia A.I. Alikhanyan National Science Laboratory (Yerevan Physics Institute) Foundation, Yerevan, Armenia 2 AGH University of Science and Technology, Cracow, Poland AGH University of Science and Technology Cracow Poland AGH University of Science and Technology, Cracow, Poland 3 Bogolyubov Institute for Theoretical Physics, National Academy of Sciences of Ukraine, Kiev, Ukraine Bogolyubov Institute for Theoretical Physics National Academy of Sciences of Ukraine Kiev Ukraine Bogolyubov Institute for Theoretical Physics, National Academy of Sciences of Ukraine, Kiev, Ukraine 4 Bose Institute, Department of Physics and Centre for Astroparticle Physics and Space Science (CAPSS), Kolkata, India Bose Institute Department of Physics Centre for Astroparticle Physics and Space Science (CAPSS) Kolkata India Bose Institute, Department of Physics and Centre for Astroparticle Physics and Space Science (CAPSS), Kolkata, India 5 Budker Institute for Nuclear Physics, Novosibirsk, Russia Budker Institute for Nuclear Physics Novosibirsk Russia Budker Institute for Nuclear Physics, Novosibirsk, Russia 6 California Polytechnic State University, San Luis Obispo, CA, United States California Polytechnic State University San Luis Obispo CA United States California Polytechnic State University, San Luis Obispo, California, United States 7 Central China Normal University, Wuhan, China Central China Normal University Wuhan China Central China Normal University, Wuhan, China 8 Centro de Aplicaciones Tecnológicas y Desarrollo Nuclear (CEADEN), Havana, Cuba Centro de Aplicaciones Tecnológicas y Desarrollo Nuclear (CEADEN) Havana Cuba Centro de Aplicaciones Tecnológicas y Desarrollo Nuclear (CEADEN), Havana, Cuba 9 Centro de Investigación y de Estudios Avanzados (CINVESTAV), Mexico City and Mérida, Mexico Centro de Investigación y de Estudios Avanzados (CINVESTAV) Mexico City and Mérida Mexico Centro de Investigación y de Estudios Avanzados (CINVESTAV), Mexico City and Mérida, Mexico 10 Chicago State University, Chicago, IL, United States Chicago State University Chicago IL United States Chicago State University, Chicago, Illinois, United States 11 China Institute of Atomic Energy, Beijing, China China Institute of Atomic Energy Beijing China China Institute of Atomic Energy, Beijing, China 12 Chungbuk National University, Cheongju, Republic of Korea Chungbuk National University Cheongju Republic of Korea Chungbuk National University, Cheongju, Republic of Korea 13 Comenius University Bratislava, Faculty of Mathematics, Physics and Informatics, Bratislava, Slovakia Comenius University Bratislava Faculty of Mathematics, Physics and Informatics Bratislava Slovakia Comenius University Bratislava, Faculty of Mathematics, Physics and Informatics, Bratislava, Slovakia 14 COMSATS University Islamabad, Islamabad, Pakistan COMSATS University Islamabad Islamabad Pakistan COMSATS University Islamabad, Islamabad, Pakistan 15 Creighton University, Omaha, NE, United States Creighton University Omaha NE United States Creighton University, Omaha, Nebraska, United States 16 Department of Physics, Aligarh Muslim University, Aligarh, India Department of Physics Aligarh Muslim University Aligarh India Department of Physics, Aligarh Muslim University, Aligarh, India 17 Department of Physics, Pusan National University, Pusan, Republic of Korea Department of Physics Pusan National University Pusan Republic of Korea Department of Physics, Pusan National University, Pusan, Republic of Korea 18 Department of Physics, Sejong University, Seoul, Republic of Korea Department of Physics Sejong University Seoul Republic of Korea Department of Physics, Sejong University, Seoul, Republic of Korea 19 Department of Physics, University of California, Berkeley, CA, United States Department of Physics University of California Berkeley CA United States Department of Physics, University of California, Berkeley, California, United States 20 Department of Physics, University of Oslo, Oslo, Norway Department of Physics University of Oslo Oslo Norway Department of Physics, University of Oslo, Oslo, Norway 21 Department of Physics and Technology, University of Bergen, Bergen, Norway Department of Physics and Technology University of Bergen Bergen Norway Department of Physics and Technology, University of Bergen, Bergen, Norway 22 Dipartimento di Fisica dell'Università and Sezione INFN, Cagliari, Italy Dipartimento di Fisica dell'Università Sezione INFN Cagliari Italy Dipartimento di Fisica dell'Università and Sezione INFN, Cagliari, Italy 23 Dipartimento di Fisica dell'Università and Sezione INFN, Trieste, Italy Dipartimento di Fisica dell'Università Sezione INFN Trieste Italy Dipartimento di Fisica dell'Università and Sezione INFN, Trieste, Italy 24 Dipartimento di Fisica dell'Università and Sezione INFN, Turin, Italy Dipartimento di Fisica dell'Università Sezione INFN Turin Italy Dipartimento di Fisica dell'Università and Sezione INFN, Turin, Italy 25 Dipartimento di Fisica e Astronomia dell'Università and Sezione INFN, Bologna, Italy Dipartimento di Fisica e Astronomia dell'Università Sezione INFN Bologna Italy Dipartimento di Fisica e Astronomia dell'Università and Sezione INFN, Bologna, Italy 26 Dipartimento di Fisica e Astronomia dell'Università and Sezione INFN, Catania, Italy Dipartimento di Fisica e Astronomia dell'Università Sezione INFN Catania Italy Dipartimento di Fisica e Astronomia dell'Università and Sezione INFN, Catania, Italy 27 Dipartimento di Fisica e Astronomia dell'Università and Sezione INFN, Padova, Italy Dipartimento di Fisica e Astronomia dell'Università Sezione INFN Padova Italy Dipartimento di Fisica e Astronomia dell'Università and Sezione INFN, Padova, Italy 28 Dipartimento di Fisica e Nucleare e Teorica, Università di Pavia, Pavia, Italy Dipartimento di Fisica e Nucleare e Teorica Università di Pavia Pavia Italy Dipartimento di Fisica e Nucleare e Teorica, Università di Pavia, Pavia, Italy 29 Dipartimento di Fisica ‘E.R. Caianiello’ dell'Università and Gruppo Collegato INFN, Salerno, Italy Dipartimento di Fisica ‘E.R. Caianiello’ dell'Università Gruppo Collegato INFN Salerno Italy Dipartimento di Fisica ‘E.R. Caianiello’ dell'Università and Gruppo Collegato INFN, Salerno, Italy 30 Dipartimento DISAT del Politecnico and Sezione INFN, Turin, Italy Dipartimento DISAT del Politecnico Sezione INFN Turin Italy Dipartimento DISAT del Politecnico and Sezione INFN, Turin, Italy 31 Dipartimento di Scienze e Innovazione Tecnologica dell'Università del Piemonte Orientale and INFN Sezione di Torino, Alessandria, Italy Dipartimento di Scienze e Innovazione Tecnologica dell'Università del Piemonte Orientale INFN Sezione di Torino Alessandria Italy Dipartimento di Scienze e Innovazione Tecnologica dell'Università del Piemonte Orientale and INFN Sezione di Torino, Alessandria, Italy 32 Dipartimento di Scienze MIFT, Università di Messina, Messina, Italy Dipartimento di Scienze MIFT Università di Messina Messina Italy Dipartimento di Scienze MIFT, Università di Messina, Messina, Italy 33 Dipartimento Interateneo di Fisica ‘M. Merlin’ and Sezione INFN, Bari, Italy Dipartimento Interateneo di Fisica ‘M. Merlin’ Sezione INFN Bari Italy Dipartimento Interateneo di Fisica ‘M. Merlin’ and Sezione INFN, Bari, Italy 34 European Organization for Nuclear Research (CERN), Geneva, Switzerland European Organization for Nuclear Research (CERN) Geneva Switzerland European Organization for Nuclear Research (CERN), Geneva, Switzerland 35 Faculty of Electrical Engineering, Mechanical Engineering and Naval Architecture, University of Split, Split, Croatia Faculty of Electrical Engineering, Mechanical Engineering and Naval Architecture University of Split Split Croatia Faculty of Electrical Engineering, Mechanical Engineering and Naval Architecture, University of Split, Split, Croatia 36 Faculty of Engineering and Science, Western Norway University of Applied Sciences, Bergen, Norway Faculty of Engineering and Science Western Norway University of Applied Sciences Bergen Norway Faculty of Engineering and Science, Western Norway University of Applied Sciences, Bergen, Norway 37 Faculty of Nuclear Sciences and Physical Engineering, Czech Technical University in Prague, Prague, Czech Republic Faculty of Nuclear Sciences and Physical Engineering Czech Technical University in Prague Prague Czech Republic Faculty of Nuclear Sciences and Physical Engineering, Czech Technical University in Prague, Prague, Czech Republic 38 Faculty of Science, P.J. Šafárik University, Košice, Slovakia Faculty of Science P.J. Šafárik University Košice Slovakia Faculty of Science, P.J. Šafárik University, Košice, Slovakia 39 Frankfurt Institute for Advanced Studies, Johann Wolfgang Goethe-Universität Frankfurt, Frankfurt, Germany Frankfurt Institute for Advanced Studies Johann Wolfgang Goethe-Universität Frankfurt Frankfurt Germany Frankfurt Institute for Advanced Studies, Johann Wolfgang Goethe-Universität Frankfurt, Frankfurt, Germany 40 Fudan University, Shanghai, China Fudan University Shanghai China Fudan University, Shanghai, China 41 Gangneung-Wonju National University, Gangneung, Republic of Korea Gangneung-Wonju National University Gangneung Republic of Korea Gangneung-Wonju National University, Gangneung, Republic of Korea 42 Gauhati University, Department of Physics, Guwahati, India Gauhati University Department of Physics Guwahati India Gauhati University, Department of Physics, Guwahati, India 43 Helmholtz-Institut für Strahlen- und Kernphysik, Rheinische Friedrich-Wilhelms-Universität Bonn, Bonn, Germany Helmholtz-Institut für Strahlen- und Kernphysik Rheinische Friedrich-Wilhelms-Universität Bonn Bonn Germany Helmholtz-Institut für Strahlen- und Kernphysik, Rheinische Friedrich-Wilhelms-Universität Bonn, Bonn, Germany 44 Helsinki Institute of Physics (HIP), Helsinki, Finland Helsinki Institute of Physics (HIP) Helsinki Finland Helsinki Institute of Physics (HIP), Helsinki, Finland 45 High Energy Physics Group, Universidad Autónoma de Puebla, Puebla, Mexico High Energy Physics Group Universidad Autónoma de Puebla Puebla Mexico High Energy Physics Group, Universidad Autónoma de Puebla, Puebla, Mexico 46 Hiroshima University, Hiroshima, Japan Hiroshima University Hiroshima Japan Hiroshima University, Hiroshima, Japan 47 Hochschule Worms, Zentrum für Technologietransfer und Telekommunikation (ZTT), Worms, Germany Hochschule Worms Zentrum für Technologietransfer und Telekommunikation (ZTT) Worms Germany Hochschule Worms, Zentrum für Technologietransfer und Telekommunikation (ZTT), Worms, Germany 48 Horia Hulubei National Institute of Physics and Nuclear Engineering, Bucharest, Romania Horia Hulubei National Institute of Physics and Nuclear Engineering Bucharest Romania Horia Hulubei National Institute of Physics and Nuclear Engineering, Bucharest, Romania 49 Indian Institute of Technology Bombay (IIT), Mumbai, India Indian Institute of Technology Bombay (IIT) Mumbai India Indian Institute of Technology Bombay (IIT), Mumbai, India 50 Indian Institute of Technology Indore, Indore, India Indian Institute of Technology Indore Indore India Indian Institute of Technology Indore, Indore, India 51 Indonesian Institute of Sciences, Jakarta, Indonesia Indonesian Institute of Sciences Jakarta Indonesia Indonesian Institute of Sciences, Jakarta, Indonesia 52 INFN, Laboratori Nazionali di Frascati, Frascati, Italy INFN, Laboratori Nazionali di Frascati Frascati Italy INFN, Laboratori Nazionali di Frascati, Frascati, Italy 53 INFN, Sezione di Bari, Bari, Italy INFN, Sezione di Bari Bari Italy INFN, Sezione di Bari, Bari, Italy 54 INFN, Sezione di Bologna, Bologna, Italy INFN, Sezione di Bologna Bologna Italy INFN, Sezione di Bologna, Bologna, Italy 55 INFN, Sezione di Cagliari, Cagliari, Italy INFN, Sezione di Cagliari Cagliari Italy INFN, Sezione di Cagliari, Cagliari, Italy 56 INFN, Sezione di Catania, Catania, Italy INFN, Sezione di Catania Catania Italy INFN, Sezione di Catania, Catania, Italy 57 INFN, Sezione di Padova, Padova, Italy INFN, Sezione di Padova Padova Italy INFN, Sezione di Padova, Padova, Italy 58 INFN, Sezione di Pavia, Pavia, Italy INFN, Sezione di Pavia Pavia Italy INFN, Sezione di Pavia, Pavia, Italy 59 INFN, Sezione di Torino, Turin, Italy INFN, Sezione di Torino Turin Italy INFN, Sezione di Torino, Turin, Italy 60 INFN, Sezione di Trieste, Trieste, Italy INFN, Sezione di Trieste Trieste Italy INFN, Sezione di Trieste, Trieste, Italy 61 Inha University, Incheon, Republic of Korea Inha University Incheon Republic of Korea Inha University, Incheon, Republic of Korea 62 Institute for Gravitational and Subatomic Physics (GRASP), Utrecht University/Nikhef, Utrecht, Netherlands Institute for Gravitational and Subatomic Physics (GRASP) Utrecht University/Nikhef Utrecht Netherlands Institute for Gravitational and Subatomic Physics (GRASP), Utrecht University/Nikhef, Utrecht, Netherlands 63 Institute for Nuclear Research, Academy of Sciences, Moscow, Russia Institute for Nuclear Research Academy of Sciences Moscow Russia Institute for Nuclear Research, Academy of Sciences, Moscow, Russia 64 Institute of Experimental Physics, Slovak Academy of Sciences, Košice, Slovakia Institute of Experimental Physics Slovak Academy of Sciences Košice Slovakia Institute of Experimental Physics, Slovak Academy of Sciences, Košice, Slovakia 65 Institute of Physics, Homi Bhabha National Institute, Bhubaneswar, India Institute of Physics Homi Bhabha National Institute Bhubaneswar India Institute of Physics, Homi Bhabha National Institute, Bhubaneswar, India 66 Institute of Physics of the Czech Academy of Sciences, Prague, Czech Republic Institute of Physics of the Czech Academy of Sciences Prague Czech Republic Institute of Physics of the Czech Academy of Sciences, Prague, Czech Republic 67 Institute of Space Science (ISS), Bucharest, Romania Institute of Space Science (ISS) Bucharest Romania Institute of Space Science (ISS), Bucharest, Romania 68 Institut für Kernphysik, Johann Wolfgang Goethe-Universität Frankfurt, Frankfurt, Germany Institut für Kernphysik Johann Wolfgang Goethe-Universität Frankfurt Frankfurt Germany Institut für Kernphysik, Johann Wolfgang Goethe-Universität Frankfurt, Frankfurt, Germany 69 Instituto de Ciencias Nucleares, Universidad Nacional Autónoma de México, Mexico City, Mexico Instituto de Ciencias Nucleares Universidad Nacional Autónoma de México Mexico City Mexico Instituto de Ciencias Nucleares, Universidad Nacional Autónoma de México, Mexico City, Mexico 70 Instituto de Física, Universidade Federal do Rio Grande do Sul (UFRGS), Porto Alegre, Brazil Instituto de Física Universidade Federal do Rio Grande do Sul (UFRGS) Porto Alegre Brazil Instituto de Física, Universidade Federal do Rio Grande do Sul (UFRGS), Porto Alegre, Brazil 71 Instituto de Física, Universidad Nacional Autónoma de México, Mexico City, Mexico Instituto de Física Universidad Nacional Autónoma de México Mexico City Mexico Instituto de Física, Universidad Nacional Autónoma de México, Mexico City, Mexico 72 iThemba LABS, National Research Foundation, Somerset West, South Africa iThemba LABS National Research Foundation Somerset West South Africa iThemba LABS, National Research Foundation, Somerset West, South Africa 73 Jeonbuk National University, Jeonju, Republic of Korea Jeonbuk National University Jeonju Republic of Korea Jeonbuk National University, Jeonju, Republic of Korea 74 Johann-Wolfgang-Goethe Universität Frankfurt Institut für Informatik, Fachbereich Informatik und Mathematik, Frankfurt, Germany Johann-Wolfgang-Goethe Universität Frankfurt Institut für Informatik Fachbereich Informatik und Mathematik Frankfurt Germany Johann-Wolfgang-Goethe Universität Frankfurt Institut für Informatik, Fachbereich Informatik und Mathematik, Frankfurt, Germany 75 Joint Institute for Nuclear Research (JINR), Dubna, Russia Joint Institute for Nuclear Research (JINR) Dubna Russia Joint Institute for Nuclear Research (JINR), Dubna, Russia 76 Korea Institute of Science and Technology Information, Daejeon, Republic of Korea Korea Institute of Science and Technology Information Daejeon Republic of Korea Korea Institute of Science and Technology Information, Daejeon, Republic of Korea 77 KTO Karatay University, Konya, Turkey KTO Karatay University Konya Turkey KTO Karatay University, Konya, Turkey 78 Laboratoire de Physique des 2 Infinis, Irène Joliot-Curie, Orsay, France Laboratoire de Physique des 2 Infinis, Irène Joliot-Curie Orsay France Laboratoire de Physique des 2 Infinis, Irène Joliot-Curie, Orsay, France 79 Laboratoire de Physique Subatomique et de Cosmologie, Université Grenoble-Alpes, CNRS-IN2P3, Grenoble, France Laboratoire de Physique Subatomique et de Cosmologie Université Grenoble-Alpes CNRS-IN2P3 Grenoble France Laboratoire de Physique Subatomique et de Cosmologie, Université Grenoble-Alpes, CNRS-IN2P3, Grenoble, France 80 Lawrence Berkeley National Laboratory, Berkeley, CA, United States Lawrence Berkeley National Laboratory Berkeley CA United States Lawrence Berkeley National Laboratory, Berkeley, California, United States 81 Lund University Department of Physics, Division of Particle Physics, Lund, Sweden Lund University Department of Physics Division of Particle Physics Lund Sweden Lund University Department of Physics, Division of Particle Physics, Lund, Sweden 82 Moscow Institute for Physics and Technology, Moscow, Russia Moscow Institute for Physics and Technology Moscow Russia Moscow Institute for Physics and Technology, Moscow, Russia 83 Nagasaki Institute of Applied Science, Nagasaki, Japan Nagasaki Institute of Applied Science Nagasaki Japan Nagasaki Institute of Applied Science, Nagasaki, Japan 84 Nara Women's University (NWU), Nara, Japan Nara Women's University (NWU) Nara Japan Nara Women's University (NWU), Nara, Japan 85 National and Kapodistrian University of Athens, School of Science, Department of Physics , Athens, Greece National and Kapodistrian University of Athens School of Science Department of Physics Athens Greece National and Kapodistrian University of Athens, School of Science, Department of Physics , Athens, Greece 86 National Centre for Nuclear Research, Warsaw, Poland National Centre for Nuclear Research Warsaw Poland National Centre for Nuclear Research, Warsaw, Poland 87 National Institute of Science Education and Research, Homi Bhabha National Institute, Jatni, India National Institute of Science Education and Research Homi Bhabha National Institute Jatni India National Institute of Science Education and Research, Homi Bhabha National Institute, Jatni, India 88 National Nuclear Research Center, Baku, Azerbaijan National Nuclear Research Center Baku Azerbaijan National Nuclear Research Center, Baku, Azerbaijan 89 National Research Centre Kurchatov Institute, Moscow, Russia National Research Centre Kurchatov Institute Moscow Russia National Research Centre Kurchatov Institute, Moscow, Russia 90 Niels Bohr Institute, University of Copenhagen, Copenhagen, Denmark Niels Bohr Institute University of Copenhagen Copenhagen Denmark Niels Bohr Institute, University of Copenhagen, Copenhagen, Denmark 91 Nikhef, National Institute for Subatomic Physics, Amsterdam, Netherlands Nikhef National Institute for Subatomic Physics Amsterdam Netherlands Nikhef, National institute for subatomic physics, Amsterdam, Netherlands 92 NRC Kurchatov Institute IHEP, Protvino, Russia NRC Kurchatov Institute IHEP Protvino Russia NRC Kurchatov Institute IHEP, Protvino, Russia 93 NRC «Kurchatov»Institute – ITEP, Moscow, Russia NRC «Kurchatov»Institute – ITEP Moscow Russia NRC «Kurchatov»Institute – ITEP, Moscow, Russia 94 NRNU Moscow Engineering Physics Institute, Moscow, Russia NRNU Moscow Engineering Physics Institute Moscow Russia NRNU Moscow Engineering Physics Institute, Moscow, Russia 95 Nuclear Physics Group, STFC Daresbury Laboratory, Daresbury, United Kingdom Nuclear Physics Group STFC Daresbury Laboratory Daresbury United Kingdom Nuclear Physics Group, STFC Daresbury Laboratory, Daresbury, United Kingdom 96 Nuclear Physics Institute of the Czech Academy of Sciences, Řež u Prahy, Czech Republic Nuclear Physics Institute of the Czech Academy of Sciences Řež u Prahy Czech Republic Nuclear Physics Institute of the Czech Academy of Sciences, Řež u Prahy, Czech Republic 97 Oak Ridge National Laboratory, Oak Ridge, TN, United States Oak Ridge National Laboratory Oak Ridge TN United States Oak Ridge National Laboratory, Oak Ridge, Tennessee, United States 98 Ohio State University, Columbus, OH, United States Ohio State University Columbus OH United States Ohio State University, Columbus, Ohio, United States 99 Petersburg Nuclear Physics Institute, Gatchina, Russia Petersburg Nuclear Physics Institute Gatchina Russia Petersburg Nuclear Physics Institute, Gatchina, Russia 100 Physics Department, Faculty of Science, University of Zagreb, Zagreb, Croatia Physics Department Faculty of Science University of Zagreb Zagreb Croatia Physics department, Faculty of science, University of Zagreb, Zagreb, Croatia 101 Physics Department, Panjab University, Chandigarh, India Physics Department Panjab University Chandigarh India Physics Department, Panjab University, Chandigarh, India 102 Physics Department, University of Jammu, Jammu, India Physics Department University of Jammu Jammu India Physics Department, University of Jammu, Jammu, India 103 Physics Department, University of Rajasthan, Jaipur, India Physics Department University of Rajasthan Jaipur India Physics Department, University of Rajasthan, Jaipur, India 104 Physikalisches Institut, Eberhard-Karls-Universität Tübingen, Tübingen, Germany Physikalisches Institut Eberhard-Karls-Universität Tübingen Tübingen Germany Physikalisches Institut, Eberhard-Karls-Universität Tübingen, Tübingen, Germany 105 Physikalisches Institut, Ruprecht-Karls-Universität Heidelberg, Heidelberg, Germany Physikalisches Institut Ruprecht-Karls-Universität Heidelberg Heidelberg Germany Physikalisches Institut, Ruprecht-Karls-Universität Heidelberg, Heidelberg, Germany 106 Physik Department, Technische Universität München, Munich, Germany Physik Department Technische Universität München Munich Germany Physik Department, Technische Universität München, Munich, Germany 107 Politecnico di Bari and Sezione INFN, Bari, Italy Politecnico di Bari Sezione INFN Bari Italy Politecnico di Bari and Sezione INFN, Bari, Italy 108 Research Division and ExtreMe Matter Institute EMMI, GSI Helmholtzzentrum für Schwerionenforschung GmbH, Darmstadt, Germany Research Division ExtreMe Matter Institute EMMI GSI Helmholtzzentrum für Schwerionenforschung GmbH Darmstadt Germany Research Division and ExtreMe Matter Institute EMMI, GSI Helmholtzzentrum für Schwerionenforschung GmbH, Darmstadt, Germany 109 Russian Federal Nuclear Center (VNIIEF), Sarov, Russia Russian Federal Nuclear Center (VNIIEF) Sarov Russia Russian Federal Nuclear Center (VNIIEF), Sarov, Russia 110 Saha Institute of Nuclear Physics, Homi Bhabha National Institute, Kolkata, India Saha Institute of Nuclear Physics Homi Bhabha National Institute Kolkata India Saha Institute of Nuclear Physics, Homi Bhabha National Institute, Kolkata, India 111 School of Physics and Astronomy, University of Birmingham, Birmingham, United Kingdom School of Physics and Astronomy University of Birmingham Birmingham United Kingdom School of Physics and Astronomy, University of Birmingham, Birmingham, United Kingdom 112 Sección Física, Departamento de Ciencias, Pontificia Universidad Católica del Perú, Lima, Peru Sección Física Departamento de Ciencias Pontificia Universidad Católica del Perú Lima Peru Sección Física, Departamento de Ciencias, Pontificia Universidad Católica del Perú, Lima, Peru 113 St. Petersburg State University, St. Petersburg, Russia St. Petersburg State University St. Petersburg Russia St. Petersburg State University, St. Petersburg, Russia 114 Stefan Meyer Institut für Subatomare Physik (SMI), Vienna, Austria Stefan Meyer Institut für Subatomare Physik (SMI) Vienna Austria Stefan Meyer Institut für Subatomare Physik (SMI), Vienna, Austria 115 SUBATECH, IMT Atlantique, Université de Nantes, CNRS-IN2P3, Nantes, France SUBATECH IMT Atlantique Université de Nantes CNRS-IN2P3 Nantes France SUBATECH, IMT Atlantique, Université de Nantes, CNRS-IN2P3, Nantes, France 116 Suranaree University of Technology, Nakhon Ratchasima, Thailand Suranaree University of Technology Nakhon Ratchasima Thailand Suranaree University of Technology, Nakhon Ratchasima, Thailand 117 Technical University of Košice, Košice, Slovakia Technical University of Košice Košice Slovakia Technical University of Košice, Košice, Slovakia 118 The Henryk Niewodniczanski Institute of Nuclear Physics, Polish Academy of Sciences, Cracow, Poland The Henryk Niewodniczanski Institute of Nuclear Physics Polish Academy of Sciences Cracow Poland The Henryk Niewodniczanski Institute of Nuclear Physics, Polish Academy of Sciences, Cracow, Poland 119 The University of Texas at Austin, Austin, TX, United States The University of Texas at Austin Austin TX United States The University of Texas at Austin, Austin, Texas, United States 120 Universidad Autónoma de Sinaloa, Culiacán, Mexico Universidad Autónoma de Sinaloa Culiacán Mexico Universidad Autónoma de Sinaloa, Culiacán, Mexico 121 Universidade de São Paulo (USP), São Paulo, Brazil Universidade de São Paulo (USP) São Paulo Brazil Universidade de São Paulo (USP), São Paulo, Brazil 122 Universidade Estadual de Campinas (UNICAMP), Campinas, Brazil Universidade Estadual de Campinas (UNICAMP) Campinas Brazil Universidade Estadual de Campinas (UNICAMP), Campinas, Brazil 123 Universidade Federal do ABC, Santo Andre, Brazil Universidade Federal do ABC Santo Andre Brazil Universidade Federal do ABC, Santo Andre, Brazil 124 University of Cape Town, Cape Town, South Africa University of Cape Town Cape Town South Africa University of Cape Town, Cape Town, South Africa 125 University of Houston, Houston, TX, United States University of Houston Houston TX United States University of Houston, Houston, Texas, United States 126 University of Jyväskylä, Jyväskylä, Finland University of Jyväskylä Jyväskylä Finland University of Jyväskylä, Jyväskylä, Finland 127 University of Kansas, Lawrence, KS, United States University of Kansas Lawrence KS United States University of Kansas, Lawrence, Kansas, United States 128 University of Liverpool, Liverpool, United Kingdom University of Liverpool Liverpool United Kingdom University of Liverpool, Liverpool, United Kingdom 129 University of Science and Technology of China, Hefei, China University of Science and Technology of China Hefei China University of Science and Technology of China, Hefei, China 130 University of South-Eastern Norway, Tonsberg, Norway University of South-Eastern Norway Tonsberg Norway University of South-Eastern Norway, Tonsberg, Norway 131 University of Tennessee, Knoxville, TN, United States University of Tennessee Knoxville TN United States University of Tennessee, Knoxville, Tennessee, United States 132 University of the Witwatersrand, Johannesburg, South Africa University of the Witwatersrand Johannesburg South Africa University of the Witwatersrand, Johannesburg, South Africa 133 University of Tokyo, Tokyo, Japan University of Tokyo Tokyo Japan University of Tokyo, Tokyo, Japan 134 University of Tsukuba, Tsukuba, Japan University of Tsukuba Tsukuba Japan University of Tsukuba, Tsukuba, Japan 135 University Politehnica of Bucharest, Bucharest, Romania University Politehnica of Bucharest Bucharest Romania University Politehnica of Bucharest, Bucharest, Romania 136 Université Clermont Auvergne, CNRS/IN2P3, LPC, Clermont-Ferrand, France Université Clermont Auvergne CNRS/IN2P3 LPC Clermont-Ferrand France Université Clermont Auvergne, CNRS/IN2P3, LPC, Clermont-Ferrand, France 137 Université de Lyon, CNRS/IN2P3, Institut de Physique des 2 Infinis de Lyon, Lyon, France Université de Lyon CNRS/IN2P3 Institut de Physique des 2 Infinis de Lyon Lyon France Université de Lyon, CNRS/IN2P3, Institut de Physique des 2 Infinis de Lyon, Lyon, France 138 Université de Strasbourg, CNRS, IPHC UMR 7178, F-67000 Strasbourg, France Université de Strasbourg CNRS IPHC UMR 7178 Strasbourg F-67000 France Université de Strasbourg, CNRS, IPHC UMR 7178, F-67000 Strasbourg, France, Strasbourg, France 139 Université Paris-Saclay Centre d'Etudes de Saclay (CEA), IRFU, Départment de Physique Nucléaire (DPhN), Saclay, France Université Paris-Saclay Centre d'Etudes de Saclay (CEA) IRFU Départment de Physique Nucléaire (DPhN) Saclay France Université Paris-Saclay Centre d'Etudes de Saclay (CEA), IRFU, Départment de Physique Nucléaire (DPhN), Saclay, France 140 Università degli Studi di Foggia, Foggia, Italy Università degli Studi di Foggia Foggia Italy Università degli Studi di Foggia, Foggia, Italy 141 Università di Brescia, Brescia, Italy Università di Brescia Brescia Italy Università di Brescia, Brescia, Italy 142 Variable Energy Cyclotron Centre, Homi Bhabha National Institute, Kolkata, India Variable Energy Cyclotron Centre Homi Bhabha National Institute Kolkata India Variable Energy Cyclotron Centre, Homi Bhabha National Institute, Kolkata, India 143 Warsaw University of Technology, Warsaw, Poland Warsaw University of Technology Warsaw Poland Warsaw University of Technology, Warsaw, Poland 144 Wayne State University, Detroit, MI, United States Wayne State University Detroit MI United States Wayne State University, Detroit, Michigan, United States 145 Westfälische Wilhelms-Universität Münster, Institut für Kernphysik, Münster, Germany Westfälische Wilhelms-Universität Münster Institut für Kernphysik Münster Germany Westfälische Wilhelms-Universität Münster, Institut für Kernphysik, Münster, Germany 146 Wigner Research Centre for Physics, Budapest, Hungary Wigner Research Centre for Physics Budapest Hungary Wigner Research Centre for Physics, Budapest, Hungary 147 Yale University, New Haven, CT, United States Yale University New Haven CT United States Yale University, New Haven, Connecticut, United States 148 Yonsei University, Seoul, Republic of Korea Yonsei University Seoul Republic of Korea Yonsei University, Seoul, Republic of Korea I Deceased. II Also at: Italian National Agency for New Technologies, Energy and Sustainable Economic Development (ENEA), Bologna, Italy. III Also at: Dipartimento DET del Politecnico di Torino, Turin, Italy. IV Also at: M.V. Lomonosov Moscow State University, D.V. Skobeltsyn Institute of Nuclear, Physics, Moscow, Russia. V Also at: Department of Applied Physics, Aligarh Muslim University, Aligarh, India. VI Also at: Institute of Theoretical Physics, University of Wroclaw, Poland. VII Also at: University of Kansas, Lawrence, Kansas, United States. ⋆ E-mail address: alice-publications@cern.ch .Editor: M. Doser Abstract We present the first systematic comparison of the charged-particle pseudorapidity densities for three widely different collision systems, pp, p Pb, and Pb Pb, at the top energy of the Large Hadron Collider (s NN = 5.02 TeV ) measured over a wide pseudorapidity range (− 3.5 < η < 5 ), the widest possible among the four experiments at that facility. The systematic uncertainties are minimised since the measurements are recorded by the same experimental apparatus (ALICE). The distributions for p Pb and Pb Pb collisions are determined as a function of the centrality of the collisions, while results from pp collisions are reported for inelastic events with at least one charged particle at midrapidity. The charged-particle pseudorapidity densities are, under simple and robust assumptions, transformed to charged-particle rapidity densities. This allows for the calculation and the presentation of the evolution of the width of the rapidity distributions and of a lower bound on the Bjorken energy density, as a function of the number of participants in all three collision systems. We find a decreasing width of the particle production, and roughly a smooth ten fold increase in the energy density, as the system size grows, which is consistent with a gradually higher dense phase of matter. 1 Introduction The number of charged particles produced in energetic nuclear collisions is an important indicator for the strong interaction processes that determine the particle production at the sub-nucleonic level. In particular, the production of charged particles is expected to reflect the number of quark and gluon collisions occurring during the initial stages of the reaction. The total number of particles produced also provides information on the energy transfer available from the initial colliding beams to particle production, as a consequence of nuclear stopping [1] . In order to help unravel this complex scenario it is important to compare the particle production amongst collision systems of different sizes over a wide kinematic range. We present the measured charged-particle pseudorapidity density, d N ch / d η , for pp, p Pb, and Pb Pb (previously published [2] ) collisions at the same collision energy of s NN = 5.02 TeV in the nucleon–nucleon centre-of-mass reference frame. This is, at present, the maximum available energy at CERN's Large Hadron Collider (LHC) for Pb Pb collisions. The measurements were carried out using ALICE at LHC (for earlier d N ch / d η results see for example Refs. [3–5] ). The three studied reactions have different characteristics probing widely different particle production yields and mechanisms. In Pb Pb collisions, the total particle yield for central collisions is of the order 104 [2] , and a strongly coupled plasma of quarks and gluons (sQGP) is formed [6–9] , whose collective and transport properties are currently under intense study. On the other hand, pp collisions represent the simplest possible nuclear collision system, where the average total particle production is much smaller (≈80, by integrating the measured distributions), and is to first approximation much less subject to collective effects [10] . The p Pb system is intermediate to the other reactions, corresponding to the situation where a single nucleon probes the nucleons in a narrow cylinder of the target nucleus. The extent to which p Pb is governed by the initial state cold nuclear matter of the lead ion or whether collective phenomena in the hot and dense medium play an important role is, at present, a matter under scrutiny by the community [10,11] . In this letter, we compare the three reactions and present the ratios of the charged-particle pseudorapidity density distributions (d N ch / d η ) of the more complex reactions to the pp distribution. Owing to ALICE's unique large acceptance in pseudorapidity, and using simple and robust assumptions, we transform the measured charged-particle pseudorapidity density distributions into charged-particle rapidity density distributions (d N ch / d y ). This allows us to calculate the width of the rapidity distributions as a function of the number of participating nucleons. The parameters of the transformation also allow us to estimate a lower bound on the energy density using the well-known formula from Bjorken [12] . An energy density exceeding the critical energy density of roughly 1 GeV / fm 3 [13] is a necessary condition for the formation of deconfined matter of quarks and gluons, and thus it is of the utmost interest to understand the development of these energy densities across different collision systems. 2 Experimental set-up, data sample, analysis method, systematic uncertainties A detailed description of the ALICE detector and its performance can be found elsewhere [14,15] . The present analysis uses the Silicon Pixel Detector (SPD) to determine the pseudorapidity densities in the range − 2 < η < 2 and the Forward Multiplicity Detector (FMD) in the ranges − 3.5 < η < − 1.8 and 1.8 < η < 5 . The V0, comprised of two plastic scintillator discs covering − 3.7 < η < − 1.7 (V0C) and 2.8 < η < 5.1 (V0A), and the ZDC, two zero-degree calorimeters located 112.5 m from the interaction point, measurements determine the collision centrality and are used for offline event selection [2] . The results presented are based on data from collisions at a centre-of-mass energy per nucleon pair of s NN = 5.02 TeV as collected by ALICE during LHC Run 1 (2013) for p Pb, and during Run 2 (2015) for pp and Pb Pb. The FMD suffered high levels of background noise during the 2016 p Pb campaign, due to the high collision rate, and this data is therefore not used for the present analysis. About 105 events with a minimum bias trigger requirement [2] were analysed in the centrality range from 0% to 90% and 0% to 100% of the visible cross section for Pb Pb and p Pb collisions, respectively. The minimum bias trigger for p Pb and Pb Pb collisions in ALICE was defined as a coincidence between the V0A and V0C sides of the V0 detector. The data from the p Pb collisions were taken in two beam configurations: one where the lead ion travelled toward positive pseudorapidity and one where it travelled toward negative pseudorapidity. The results from the latter collisions are mirrored around η = 0 . The centre-of-mass frame in p Pb collisions does not coincide with the laboratory frame, due to the single magnetic field in the LHC, and thus the rapidity of the centre-of-mass is y CM = ± 0.465 for the two directions, respectively, in the laboratory frame. For this reason, pseudorapidity, calculated with respect to the laboratory frame, is denoted η lab whenever p Pb results are presented. Likewise, for the pp collisions, about 105 events with coincidence between V0A and V0C and at least one charged particle in | η | < 1 were analysed. By requiring at least one charged particle at midrapidity, the so-called INEL>0 event class, the systematic uncertainty, related to the absolute normalisation to the full inelastic cross section, is reduced, while still sampling a large fraction (> 75 % ) of the hadronic cross section [16,17] . The standard ALICE event selection [18] and centrality estimator based on the V0 amplitude [19,20] are used in this analysis. The event selection consists of: a) exclusion of background events using the timing information from the ZDC (for Pb Pb and p Pb, e.g., beam–gas interactions) and V0 detectors, b) verification of the trigger conditions, and c) a reconstructed position of the collision (primary vertex). In Pb Pb collisions, centrality is obtained from the sum amplitude in both V0 detector arrays (V0M). For p Pb only the amplitude in the array on the lead-going side (V0A or V0C) is used. In Pb Pb collisions, the 10% most peripheral collisions have substantial contributions from electromagnetic processes and are therefore not included in the results presented here [19] . A primary charged particle is defined as a charged particle with a mean proper lifetime τ larger than 1 cm / c , which is either a) produced directly in the interaction, or b) from decays of particles with τ smaller than 1 cm / c [21] . All quantities reported here are for primary, charged particles, though “primary” is omitted in the following for brevity. The analysis method is identical to that of previous publications [2] : the measurement of the charged-particle pseudorapidity density at midrapidity is obtained from counting particle trajectories determined using the two layers of the SPD. The SPD has a lower transverse momentum acceptance of 50 MeV / c , and the yield is extrapolated down to p T = 0 MeV / c via simulations. In the forward regions, the measurement is provided by the analysis of the deposited energy signal in the FMD and a statistical method is employed to calculate the inclusive number of charged particles. A data-driven correction [22] , based on separate measurements exploiting displaced collision vertices, is applied to remove the background from secondary particles. Systematic uncertainty estimations for the midrapidity measurements are detailed elsewhere [2,16,20] , and are from background suppression, transverse momentum extrapolation, weak decays, and simulations. The estimates are obtained through variation of thresholds and simulation studies. For pp (p Pb), the total systematic uncertainty amounts to 1.5% (2.7%) over the whole pseudorapidity range; while for Pb Pb the total systematic uncertainty is 2.6% at η = 0 and 2.9% at | η | = 2 . The systematic uncertainty is mostly correlated over pseudorapidity for | η | < 2 , and largely independent of centrality. The uncertainty in the forward region, estimated via variations of thresholds and simulation studies, is the same for all collision systems and is uncorrelated across η , amounting to 6.9% for η > 3.5 and 6.4% elsewhere within the forward regions [22] . In the figures of this letter, uncorrelated, local in pseudorapidity, systematic uncertainties are indicated by open boxes on the data points, while correlated systematic uncertainties, those that affect the overall scale and typically from event classification and selection, are indicated by filled boxes to the right of the data. The systematic uncertainty on d N ch / d η , due to the centrality class definition in Pb Pb, is estimated to vary from 0.6% for the most central to 9.5% for the most peripheral class [23] . The 80% to 90% centrality class has residual contamination from electromagnetic processes as detailed elsewhere [19] , which gives rise to an additional 4% systematic uncertainty in the measurements. No overall systematic uncertainty has been estimated for p Pb collisions, as the centrality selection in that collision system is inherently difficult to map to the underlying dynamics of the collisions [20] . 3 Results Fig. 1 shows the measured pseudorapidity densities in pp, and in central p Pb, and the previously published results for Pb Pb [2] collisions at s NN = 5.02 TeV for primary particles.For the 5% most central Pb Pb collisions d N ch / d η ≈ 2000 at midrapidity (η = 0 ) [2] , while for p Pb collisions the distribution peaks at d N ch / d η lab ≈ 60 around η lab = 3 in the lead-going direction (η > 0 ). For pp collisions with the INEL>0 trigger condition discussed above, d N ch / d η = 5.7 ± 0.2 at midrapidity, consistent with previous results derived from p T spectra [24] . Fig. 2 shows, as a function of centrality, the measured charged-particle pseudorapidity densities for p Pb collisions at s NN = 5.02 TeV . The strategy of centrality selection for proton on nucleus reactions is explained elsewhere [20] . The ALICE Collaboration has previously presented d N ch / d η for Pb Pb collisions at this energy [2] .In Fig. 3 , the charged-particle pseudorapidity densities in p Pb and Pb Pb reactions are divided by the pp distributions corresponding to the INEL>0 trigger class. The ratio is r X = ( d N ch / d η | X ) / ( d N ch / d η | pp ) , where X labels p Pb and Pb Pb collisions, in centrality classes, as a function of pseudorapidity. In the ratios, systematic uncertainties, of common origin, are partially cancelled, and, as an estimate, the magnitude of the resulting systematic uncertainties are given only by the uncertainties in the d N ch / d η | X measurements, since the uncertainties are independent of the collision system. In p Pb collisions the rapidity of the centre-of-mass is non-zero, which is not taken into account in the ratios. Such a correction would require prior determination of the full Jacobian of the transformation from pseudorapidity to rapidity, which is not possible to perform reliably with the ALICE apparatus. The ratio of the p Pb relative to the pp distributions increases with pseudorapidity from the p-going to the Pb-going direction for central collisions, which Brodsky et al. and Adil et al. [25,26] suggest is a sign of scaling of the pp distribution with the increasing number of participants as the lead nucleus is probed by the incident proton, and thus independent proton–nucleon scatterings on the lead-ion side. A similar scaling, however, does not hold for the Pb Pb reaction. The ratios cannot be obtained by simple scaling of the elementary pp distributions. Instead, the ratio of the Pb Pb relative to the pp distributions exhibits an enhancement of particle production around midrapidity for the more central collisions which is indicative of the formation of the sQGP [7] . Likewise, r pPb increases for all but the two most peripheral centrality classes as η lab → 3 . In Pb Pb collisions it is seen that the various mechanisms behind the pseudorapidity distributions are more transversely directed than in pp collisions by the increase of r PbPb as | η | → 0 4 Rapidity and energy-density dependence on system size and discussion It has been shown that the charged-particle rapidity density (d N ch / d y ) in Pb Pb collisions, to a good accuracy, follows a normal distribution over the considered rapidity interval (| y | ≲ 5 ) [2,27] . Those results relied on calculating the average Jacobian d N ch / d y = 〈 J 〉 = 〈 β 〉 using the full p T spectra, at midrapidity, of charged pions and kaons as well as protons and antiprotons. Here, we use the approximationy ≈ η − 1 2 m 2 p T 2 cos ϑ , where ϑ is the polar angle of emission, and identify a = p T / m with an effective ratio of transverse momentum over mass. With this, the effective Jacobian can be written asJ ′ ( η , a ) = ( 1 + 1 a 2 1 cosh 2 η ) − 1 / 2 . We further make the ansatz that d N ch / d y is normal distributed for symmetric collision systems (pp and Pb Pb), so that d N ch / d η can be parameterised as(1) f ( η ; A , a , σ ) = J ′ ( η , a ) A 1 2 π σ exp ( − y 2 { η , a } 2 σ 2 ) , where A and σ are the total integral and width of the distribution, respectively, and y the rapidity in the centre-of-mass frame. Motivated by the observed approximate linearity of r pPb (see lower panel of Fig. 3 ), we replace A with ( α y + A ) for the asymmetric system (p Pb) and parameterise d N ch / d η lab as(2) g ( η ; A , a , α , σ ) = J ′ ( η , a ) ( α y { η , a } + A ) × 1 2 π σ exp ( − [ y { η , a } − y CM ] 2 2 σ 2 ) . The functions f and g defined in Eq. (1) and Eq. (2) , respectively, describe the measurements within the measured region with χ 2 per degrees of freedom (ν ) in the range of 0.1 to 0.5. The small χ 2 / ν values are a consequence of the relatively large uncorrelated systematic uncertainties on the measurements. That is, the charged-particle distributions for pp, p Pb, and Pb Pb collisions at s NN = 5.02 TeV follow a normal distribution in rapidity, with free parameters A , a , σ , and α in the asymmetric case. The top panel of Fig. 4 shows the best-fit parameter values of the normal width (σ d N ch / d y ) for all three collision systems as a function of the average number of participating nucleons (〈 N part 〉 ) calculated using a Glauber model [28] . The best-fit parameters are found taking statistical and uncorrelated systematic uncertainties into account. The result using the above procedure, for the most central Pb Pb collisions, is found to be compatible with previous results extracted by unfolding with the mean Jacobian estimated from transverse momentum spectra [2] . The open points (crosses) and dashed lines on the figure are from evaluations of Eq. (1) and Eq. (2) , and direct calculations of σ d N ch / d y , respectively, using model calculations with EPOS-LHC [29] . EPOS-LHC was chosen as it provides predictions for all three collision systems. The parameterisation, in terms of the two functions, of this model calculation generally reproduces the widths of the charged-particle rapidity densities, except in the asymmetric case where a direct evaluation of the standard deviation is less motivated. The general trend is that the widths decrease as 〈 N part 〉 increases, consistent with the behaviour of the r PbPb ratios. Notably, the width of the d N ch / d y distributions in p Pb and Pb Pb, for low number of participant nucleons in the collisions, approaches the width of the pp distribution, which, presumably, is dominated by kinematic and phase space constraints. The lower panel of Fig. 4 shows the dependence of a on the average number of participants. The right-hand ordinate is the same, but multiplied by the average mass 〈 m 〉 = ( 0.215 ± 0.001 ) GeV / c 2 estimated from measurements of identified particles in Pb Pb collisions at s NN = 2.76 TeV [30] . To better understand the parameter a , this parameter extracted from the EPOS-LHC calculations, using the above procedure, is also shown in the figure. The dotted lines show the average p T / m predicted by EPOS-LHC [29] . The EPOS-LHC calculations indicate that the extracted effective transverse momentum to mass ratio a is consistently smaller than the ratio of the average transverse momentum to the average mass. Thus a gives a lower bound on 〈 p T 〉 / 〈 m 〉 . We can estimate the energy density that is reached in the collisions as a function of the number of participants for the three systems. A conventional approach is to use the model originally proposed by Bjorken [12] in which the energy density (ε Bj ) depends on the rapidity density of particles and the volume of a longitudinal cylinder with cross sectional area determined by the overlap between the colliding partners and length determined by a characteristic particle formation timeε Bj = 1 c τ 1 S T 〈 d E T d y 〉 . Here, S T ≈ π R 2 ≈ π N part 2 / 3 is the transverse area spanned by the participating nucleons, d E T / d y is the transverse-energy rapidity density, and τ is the formation time. While a formation time of τ = 1 fm / c is often assumed, it is left as a free parameter here. With 〈 m T 〉 = 〈 m 〉 1 + ( 〈 p T 〉 / 〈 m 〉 ) 2 , the transverse-energy rapidity density can be approximated by〈 d E T d y 〉 ≈ 〈 m T 〉 1 f total d N ch d y = 〈 m 〉 1 + ( 〈 p T 〉 〈 m 〉 ) 2 1 f total d N ch d y , where f total = 0.55 ± 0.01 , the ratio of charged particles to all particles [31] , accounts for neutral particles not measured in the experiment, and is assumed the same for all collision systems. Substituting the derived d N ch / d y and the effective a = p T / m ≲ 〈 p T 〉 / 〈 m 〉 results in a lower bound estimate for the Bjorken energy density (ε LB )(3) ε Bj τ ≥ ε LB τ = 1 c 1 S T 〈 m 〉 1 + a 2 1 f total 1 + 1 a 2 1 cosh 2 η d N ch d η , where a and 〈 m 〉 are as in the top panel of Fig. 4 . The transverse area S T is estimated in a numerical Glauber model [32,33] as shown in Fig. 5 . We consider two extremes for the transverse area spanned by the participating nucleons: a) the exclusive (or direct) overlap between participating nucleons, ∩ and open markers in Fig. 5 , and b) the inclusive (or full) area of all participating nucleons, ∪ and full markers in Fig. 5 . Fig. 6 shows the lower-bound energy density estimate, ε LB τ ≤ ε Bj τ , as a function of the number of participants, which reaches values between 10 and 20 GeV / ( fm 2 c ) in the most central Pb Pb collisions. The uncertainties are from standard error propagation of Eq. (3) of uncertainties on the best-fit parameter values, the number of participants, mean mass, and f total . A rise from roughly 1 GeV / ( fm 2 c ) to over 10 GeV / ( fm 2 c ) is observed if the transverse area is assumed to be the inclusive area of participating nucleons. This trend is illustrated by a power-law (C N part p ) fit to the data in the figure, with the parameter values C = ( 0.8 ± 0.3 ) GeV / ( fm 2 c ) and p = 0.44 ± 0.08 . On the other hand, if the transverse area is assumed to be the smaller exclusive overlap area, we observe a substantially larger lower bound on the energy density, but a less dramatic increase with increasing number of participating nucleons. Also shown in the figure are estimates of the Bjorken energy density ε Bj τ for Pb Pb reactions at s NN = 2.76 TeV [31] . These results where obtained from measurements of the transverse energy in the collisions and using the inclusive estimate of the transverse area S T . The trend of the s NN = 5.02 TeV results is similar to these earlier results. Bearing in mind that for the largest LHC collision energy we show a lower bound estimate of the energy density in Fig. 6 , we find a likely overall increase in the energy density from s NN = 2.76 TeV to 5.02 TeV .5 Summary and conclusions We have measured the charged particle pseudorapidity density in pp, p Pb, and Pb Pb collisions at s NN = 5.02 TeV over the widest possible pseudorapidity range available at the LHC. The distributions where determined using the same experimental apparatus and methods, and systematic uncertainties have been minimised to within the capabilities of the set-up. While the particle production in central Pb Pb collisions clearly exhibits an enhancement as compared to pp collisions, particle production in p Pb collisions is consistent with dominantly incoherent nucleon–nucleon collisions. By transforming the measured pseudorapidity distributions to rapidity distributions we have obtained systematic trends for the width of the rapidity distributions and a lower bound on the energy density, which shows a clear scaling behaviour as a function of the average number of participant nucleons. The decreasing width of the deduced rapidity distributions with increasing participant number suggests that the kinematic spread of particles, including longitudinal degrees of freedom, is reduced due to interactions in the early stages of the collisions. This is also reflected in the accompanying growth of the energy density. Both observations are consistent with the gradual establishment of a high-density phase of matter with increasing size of the collision domain. Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgements The ALICE Collaboration would like to thank all its engineers and technicians for their invaluable contributions to the construction of the experiment and the CERN accelerator teams for the outstanding performance of the LHC complex. The ALICE Collaboration gratefully acknowledges the resources and support provided by all Grid centres and the Worldwide LHC Computing Grid (WLCG) collaboration. The ALICE Collaboration acknowledges the following funding agencies for their support in building and running the ALICE detector: A. I. Alikhanyan National Science Laboratory (Yerevan Physics Institute) Foundation (ANSL) , State Committee of Science and World Federation of Scientists (WFS), Armenia; Austrian Academy of Sciences , Austrian Science Fund (FWF): [M 2467-N36 ] and Nationalstiftung für Forschung, Technologie und Entwicklung , Austria; Ministry of Communications and High Technologies, National Nuclear Research Center , Azerbaijan; Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq ), Financiadora de Estudos e Projetos (Finep), Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP) and Universidade Federal do Rio Grande do Sul (UFRGS), Brazil; Ministry of Education of China (MOEC), Ministry of Science & Technology of China (MSTC) and National Natural Science Foundation of China (NSFC), China; Ministry of Science and Education and Croatian Science Foundation , Croatia; Centro de Aplicaciones Tecnológicas y Desarrollo Nuclear (CEADEN), Cubaenergía , Cuba; Ministry of Education, Youth and Sports of the Czech Republic , Czech Republic; The Danish Council for Independent Research | Natural Sciences , the Villum Fonden and Danish National Research Foundation (DNRF), Denmark; Helsinki Institute of Physics (HIP), Finland; Commissariat à l'Energie Atomique (CEA ) and Institut National de Physique Nucléaire et de Physique des Particules (IN2P3) and Centre National de la Recherche Scientifique (CNRS), France; Bundesministerium für Bildung und Forschung (BMBF) and GSI Helmholtzzentrum für Schwerionenforschung GmbH , Germany; General Secretariat for Research and Technology , Ministry of Education, Research and Religions, Greece; National Research, Development and Innovation Office , Hungary; Department of Atomic Energy Government of India (DAE ), Department of Science and Technology , Government of India (DST), University Grants Commission , Government of India (UGC) and Council of Scientific and Industrial Research (CSIR), India; Indonesian Institute of Sciences , Indonesia; Istituto Nazionale di Fisica Nucleare (INFN ), Italy; Japanese Ministry of Education, Culture, Sports, Science and Technology (MEXT) and Japan Society for the Promotion of Science (JSPS) KAKENHI, Japan; Consejo Nacional de Ciencia (CONACYT) y Tecnología, through Fondo de Cooperación Internacional en Ciencia y Tecnología (FONCICYT) and Dirección General de Asuntos del Personal Académico (DGAPA), Mexico; Nederlandse Organisatie voor Wetenschappelijk Onderzoek (NWO), Netherlands; The Research Council of Norway , Norway; Commission on Science and Technology for Sustainable Development in the South (COMSATS), Pakistan; Pontificia Universidad Católica del Perú , Peru; Ministry of Education and Science , National Science Centre and WUT ID-UB , Poland; Korea Institute of Science and Technology Information and National Research Foundation of Korea (NRF), Republic of Korea; Ministry of Education and Scientific Research , Institute of Atomic Physics , Ministry of Research and Innovation and Institute of Atomic Physics and University Politehnica of Bucharest , Romania; Joint Institute for Nuclear Research (JINR), Ministry of Education and Science of the Russian Federation , National Research Centre Kurchatov Institute , Russian Science Foundation and Russian Foundation for Basic Research , Russia; Ministry of Education, Science, Research and Sport of the Slovak Republic , Slovakia; National Research Foundation of South Africa, South Africa; Swedish Research Council (VR) and Knut and Alice Wallenberg Foundation (KAW), Sweden; European Organization for Nuclear Research , Switzerland; Suranaree University of Technology (SUT), National Science and Technology Development Agency (NSTDA), Thailand Science Research and Innovation (TSRI) and National Science, Research and Innovation Fund (NSRF), Thailand; Turkish Energy, Nuclear and Mineral Research Agency (TENMAK), Turkey; National Academy of Sciences of Ukraine , Ukraine; Science and Technology Facilities Council (STFC), United Kingdom; National Science Foundation of the United States of America (NSF ) and United States Department of Energy, Office of Nuclear Physics (DOE NP ), United States of America. 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+]>PLB 137730 137730 S0370-2693(23)00064-3 10.1016/j.physletb.2023.137730 European Center of Nuclear Research, ALICE experiment Experiments Fig. 1 Charged-particle pseudorapidity density in Pb Pb [2] and p Pb for the 5% most central collisions, and for pp collisions with INEL>0 trigger class. For symmetric collision systems (Pb Pb and pp) the data has been symmetrised around η = 0 and points for η > 3.5 have been reflected around η = 0. The boxes around the points and to the right reflect the uncorrelated and correlated, with respect to pseudorapidity, systematic uncertainty, respectively. The relative correlated, normalisation, uncertainties are evaluated at dN ch /dη |η =0 . The lines show fits of Eq. (1) (Pb Pb and pp) and Eq. (2) (p Pb) to the data (discussed in Section 4 ). Please note that the ordinate has been cut twice to accommodate for the very different ranges of the charged-particle pseudorapidity densities. Fig. 1 Fig. 2 Charged-particle pseudorapidity density in p Pb collisions at s NN = 5.02 TeV in seven centrality classes based on the V0A and V0C estimators. The lines are obtained using a fit of a scaled, normal distribution in rapidity Eq. (2) to the data (discussed in Section 4 ). Fig. 2 Fig. 3 Ratio r X of the charged-particle pseudorapidity density in Pb Pb (top) and p Pb (bottom) in different centrality classes to the charged-particle pseudorapidity density in pp in the INEL>0 event class. Note, for Pb Pb η lab is the same as the centre-of-mass pseudorapidity. Fig. 3 Fig. 4 The width (top) and effective p T /m (bottom) fit parameters as a function of the mean number of participants in pp, p Pb, and Pb Pb collisions at s NN = 5.02 TeV . Vertical uncertainties are the standard error on the best-fit parameter values, while horizontal uncertainties reflect the uncertainty on 〈N part 〉 from the Glauber calculations. Also shown are similar fit parameters from the same parameterisation of EPOS-LHC calculations as well as direct calculations of the standard deviation of the dN ch /dy distributions and the 〈p T 〉/〈m 〉 ratio from the EPOS-LHC calculations. Fig. 4 Fig. 5 The transverse area S T as calculated in a numerical Glauber model for two extreme cases: a) only the exclusive overlap of nucleons is considered (∩, open markers) and b) the inclusive area of participating nucleons contribute (∪, closed markers) in both p Pb and Pb Pb at s NN = 5.02 TeV . Fig. 5 Fig. 6 Estimate of the lower bound on the Bjorken transverse energy density in pp, p Pb, and Pb Pb collisions at s NN = 5.02 TeV , considering the exclusive (∩, open markers) and inclusive (∪, full markers) overlap area S T of the nucleons. The expression C N part p is fitted to case ∪, and we find C = (0.8 ± 0.3) GeV/(fm2 c ) and p = 0.44 ± 0.08. Also shown is an estimate, via dE T /dy , of ε Bj from Pb Pb collisions at s NN = 2.76 TeV (stars with uncertainty band) [31] . Fig. 6 System-size dependence of the charged-particle pseudorapidity density at s NN = 5.02 TeV for pp, p Pb, and Pb Pb collisions ALICE Collaboration ⋆ S. Acharya 142 D. Adamová 96 A. Adler 74 J. Adolfsson 81 G. Aglieri Rinella 34 M. Agnello 30 N. Agrawal 54 Z. Ahammed 142 S. Ahmad 16 S.U. Ahn 76 I. Ahuja 38 Z. Akbar 51 A. Akindinov 93 M. Al-Turany 108 S.N. Alam 16 D. Aleksandrov 89 B. Alessandro 59 H.M. Alfanda 7 R. Alfaro Molina 71 B. Ali 16 Y. Ali 14 A. Alici 25 N. Alizadehvandchali 125 A. Alkin 34 J. Alme 21 G. Alocco 55 T. Alt 68 I. Altsybeev 113 M.N. Anaam 7 C. Andrei 48 A. Andronic 145 V. Anguelov 105 F. Antinori 57 P. Antonioli 54 C. Anuj 16 N. Apadula 80 L. Aphecetche 115 H. Appelshäuser 68 S. Arcelli 25 R. Arnaldi 59 I.C. Arsene 20 M. Arslandok 147 A. Augustinus 34 R. Averbeck 108 S. Aziz 78 M.D. Azmi 16 A. Badalà 56 Y.W. Baek 41 X. Bai 129 108 R. Bailhache 68 Y. Bailung 50 R. Bala 102 A. Balbino 30 A. Baldisseri 139 B. Balis 2 D. Banerjee 4 Z. Banoo 102 R. Barbera 26 L. Barioglio 106 M. Barlou 85 G.G. Barnaföldi 146 L.S. Barnby 95 V. Barret 136 C. Bartels 128 K. Barth 34 E. Bartsch 68 F. Baruffaldi 27 N. Bastid 136 S. Basu 81 G. Batigne 115 D. Battistini 106 B. Batyunya 75 D. Bauri 49 J.L. Bazo Alba 112 I.G. Bearden 90 C. Beattie 147 P. Becht 108 I. Belikov 138 A.D.C. Bell Hechavarria 145 F. Bellini 25 R. Bellwied 125 S. Belokurova 113 V. Belyaev 94 G. Bencedi 146 69 S. Beole 24 A. Bercuci 48 Y. Berdnikov 99 A. Berdnikova 105 L. Bergmann 105 M.G. Besoiu 67 L. Betev 34 P.P. Bhaduri 142 A. Bhasin 102 I.R. Bhat 102 M.A. Bhat 4 B. Bhattacharjee 42 L. Bianchi 24 N. Bianchi 52 J. Bielčík 37 J. Bielčíková 96 J. Biernat 118 A. Bilandzic 106 G. Biro 146 S. Biswas 4 J.T. Blair 119 D. Blau 89 82 M.B. Blidaru 108 C. Blume 68 G. Boca 28 58 F. Bock 97 A. Bogdanov 94 S. Boi 22 J. Bok 61 L. Boldizsár 146 A. Bolozdynya 94 M. Bombara 38 P.M. 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Eisenhut 68 D. Elia 53 B. Erazmus 115 F. Ercolessi 25 E. Eremenko 96 F. Erhardt 100 A. Erokhin 113 M.R. Ersdal 21 B. Espagnon 78 G. Eulisse 34 D. Evans 111 S. Evdokimov 92 L. Fabbietti 106 M. Faggin 27 J. Faivre 79 F. Fan 7 W. Fan 80 A. Fantoni 52 M. Fasel 97 P. Fecchio 30 A. Feliciello 59 G. Feofilov 113 A. Fernández Téllez 45 A. Ferrero 139 A. Ferretti 24 V.J.G. Feuillard 105 J. Figiel 118 V. Filova 37 D. Finogeev 63 F.M. Fionda 55 G. Fiorenza 34 F. Flor 125 A.N. Flores 119 S. Foertsch 72 S. Fokin 89 E. Fragiacomo 60 E. Frajna 146 A. Francisco 136 U. Fuchs 34 N. Funicello 29 C. Furget 79 A. Furs 63 J.J. Gaardhøje 90 M. Gagliardi 24 A.M. Gago 112 A. Gal 138 C.D. Galvan 120 P. Ganoti 85 C. Garabatos 108 J.R.A. Garcia 45 E. Garcia-Solis 10 K. Garg 115 C. Gargiulo 34 A. Garibli 88 K. Garner 145 P. Gasik 108 E.F. Gauger 119 A. Gautam 127 M.B. Gay Ducati 70 M. Germain 115 S.K. Ghosh 4 M. Giacalone 25 P. Gianotti 52 P. Giubellino 108 59 P. Giubilato 27 A.M.C. Glaenzer 139 P. Glässel 105 E. Glimos 131 D.J.Q. Goh 83 V. Gonzalez 144 L.H. González-Trueba 71 S. Gorbunov 39 M. Gorgon 2 L. Görlich 118 S. Gotovac 35 V. Grabski 71 L.K. Graczykowski 143 L. Greiner 80 A. Grelli 62 C. Grigoras 34 V. Grigoriev 94 S. Grigoryan 75 1 F. Grosa 34 59 J.F. Grosse-Oetringhaus 34 R. Grosso 108 D. Grund 37 G.G. Guardiano 122 R. Guernane 79 M. Guilbaud 115 K. Gulbrandsen 90 T. Gunji 133 W. Guo 7 A. Gupta 102 R. Gupta 102 S.P. Guzman 45 L. Gyulai 146 M.K. Habib 108 C. Hadjidakis 78 H. Hamagaki 83 M. Hamid 7 R. Hannigan 119 M.R. Haque 143 A. Harlenderova 108 J.W. Harris 147 A. Harton 10 J.A. Hasenbichler 34 H. Hassan 97 D. Hatzifotiadou 54 P. Hauer 43 L.B. Havener 147 S.T. Heckel 106 E. Hellbär 108 H. Helstrup 36 T. Herman 37 G. Herrera Corral 9 F. Herrmann 145 K.F. Hetland 36 B. Heybeck 68 H. Hillemanns 34 C. Hills 128 B. Hippolyte 138 B. Hofman 62 B. Hohlweger 91 J. Honermann 145 G.H. Hong 148 D. Horak 37 S. Hornung 108 A. Horzyk 2 R. Hosokawa 15 Y. Hou 7 P. Hristov 34 C. Hughes 131 P. Huhn 68 L.M. Huhta 126 C.V. Hulse 78 T.J. Humanic 98 H. Hushnud 110 L.A. Husova 145 A. Hutson 125 J.P. Iddon 128 R. Ilkaev 109 H. Ilyas 14 M. Inaba 134 G.M. Innocenti 34 M. Ippolitov 89 A. Isakov 96 T. Isidori 127 M.S. Islam 110 M. Ivanov 108 V. Ivanov 99 V. Izucheev 92 M. Jablonski 2 B. Jacak 80 N. Jacazio 34 P.M. Jacobs 80 S. Jadlovska 117 J. Jadlovsky 117 S. Jaelani 62 C. Jahnke 122 M.J. Jakubowska 143 A. Jalotra 102 M.A. Janik 143 T. Janson 74 M. Jercic 100 O. Jevons 111 A.A.P. Jimenez 69 F. Jonas 97 145 P.G. Jones 111 J.M. Jowett 34 108 J. Jung 68 M. Jung 68 A. Junique 34 A. Jusko 111 M.J. Kabus 143 J. Kaewjai 116 P. Kalinak 64 A.S. Kalteyer 108 A. Kalweit 34 V. Kaplin 94 A. Karasu Uysal 77 D. Karatovic 100 O. Karavichev 63 T. Karavicheva 63 P. Karczmarczyk 143 E. Karpechev 63 V. Kashyap 87 A. Kazantsev 89 U. Kebschull 74 R. Keidel 47 D.L.D. Keijdener 62 M. Keil 34 B. Ketzer 43 A.M. Khan 7 S. Khan 16 A. Khanzadeev 99 Y. Kharlov 92 82 A. Khatun 16 A. Khuntia 118 B. Kileng 36 B. Kim 17 C. Kim 17 D.J. Kim 126 E.J. Kim 73 J. Kim 148 J.S. Kim 41 J. Kim 105 J. Kim 73 M. Kim 105 S. Kim 18 T. Kim 148 S. Kirsch 68 I. Kisel 39 S. Kiselev 93 A. Kisiel 143 J.P. Kitowski 2 J.L. Klay 6 J. Klein 34 S. Klein 80 C. Klein-Bösing 145 M. Kleiner 68 T. Klemenz 106 A. Kluge 34 A.G. Knospe 125 C. Kobdaj 116 T. Kollegger 108 A. Kondratyev 75 N. Kondratyeva 94 E. Kondratyuk 92 J. Konig 68 S.A. Konigstorfer 106 P.J. Konopka 34 G. Kornakov 143 S.D. Koryciak 2 A. Kotliarov 96 O. Kovalenko 86 V. Kovalenko 113 M. Kowalski 118 I. Králik 64 A. Kravčáková 38 L. Kreis 108 M. Krivda 111 64 F. Krizek 96 K. Krizkova Gajdosova 37 M. Kroesen 105 M. Krüger 68 D.M. Krupova 37 E. Kryshen 99 M. Krzewicki 39 V. Kučera 34 C. Kuhn 138 P.G. Kuijer 91 T. Kumaoka 134 D. Kumar 142 L. Kumar 101 N. Kumar 101 S. Kundu 34 P. Kurashvili 86 A. Kurepin 63 A.B. Kurepin 63 A. Kuryakin 109 S. Kushpil 96 J. Kvapil 111 M.J. Kweon 61 J.Y. Kwon 61 Y. Kwon 148 S.L. La Pointe 39 P. La Rocca 26 Y.S. Lai 80 A. Lakrathok 116 M. Lamanna 34 R. Langoy 130 P. Larionov 34 52 E. Laudi 34 L. Lautner 34 106 R. Lavicka 114 37 T. Lazareva 113 R. Lea 141 58 J. Lehrbach 39 R.C. Lemmon 95 I. León Monzón 120 M.M. Lesch 106 E.D. Lesser 19 M. Lettrich 34 106 P. Lévai 146 X. Li 11 X.L. Li 7 J. Lien 130 R. Lietava 111 B. Lim 17 S.H. Lim 17 V. Lindenstruth 39 A. Lindner 48 C. Lippmann 108 A. Liu 19 D.H. Liu 7 J. Liu 128 I.M. Lofnes 21 V. Loginov 94 C. Loizides 97 P. Loncar 35 J.A. Lopez 105 X. Lopez 136 E. López Torres 8 J.R. Luhder 145 M. Lunardon 27 G. Luparello 60 Y.G. Ma 40 A. Maevskaya 63 M. Mager 34 T. Mahmoud 43 A. Maire 138 M. Malaev 99 N.M. Malik 102 Q.W. Malik 20 S.K. Malik 102 L. Malinina 75 IV D. Mal'Kevich 93 D. Mallick 87 N. Mallick 50 G. Mandaglio 32 56 V. Manko 89 F. Manso 136 V. Manzari 53 Y. Mao 7 G.V. Margagliotti 23 A. Margotti 54 A. Marín 108 C. Markert 119 M. Marquard 68 N.A. Martin 105 P. Martinengo 34 J.L. Martinez 125 M.I. Martínez 45 G. Martínez García 115 S. Masciocchi 108 M. Masera 24 A. Masoni 55 L. Massacrier 78 A. Mastroserio 140 53 A.M. Mathis 106 O. Matonoha 81 P.F.T. Matuoka 121 A. Matyja 118 C. Mayer 118 A.L. Mazuecos 34 F. Mazzaschi 24 M. Mazzilli 34 J.E. Mdhluli 132 A.F. Mechler 68 Y. Melikyan 63 A. Menchaca-Rocha 71 E. Meninno 114 29 A.S. Menon 125 M. Meres 13 S. Mhlanga 124 72 Y. Miake 134 L. Micheletti 59 L.C. Migliorin 137 D.L. Mihaylov 106 K. Mikhaylov 75 93 A.N. Mishra 146 D. Miśkowiec 108 A. Modak 4 A.P. Mohanty 62 B. Mohanty 87 M. Mohisin Khan 16 V M.A. Molander 44 Z. Moravcova 90 C. Mordasini 106 D.A. Moreira De Godoy 145 I. Morozov 63 A. Morsch 34 T. Mrnjavac 34 V. Muccifora 52 E. Mudnic 35 S. Muhuri 142 J.D. Mulligan 80 A. Mulliri 22 M.G. Munhoz 121 R.H. Munzer 68 H. Murakami 133 S. Murray 124 L. Musa 34 J. Musinsky 64 J.W. Myrcha 143 B. Naik 132 R. Nair 86 B.K. Nandi 49 R. Nania 54 E. Nappi 53 A.F. Nassirpour 81 A. Nath 105 C. Nattrass 131 A. Neagu 20 A. Negru 135 L. Nellen 69 S.V. Nesbo 36 G. Neskovic 39 D. Nesterov 113 B.S. Nielsen 90 E.G. Nielsen 90 S. Nikolaev 89 S. Nikulin 89 V. Nikulin 99 F. Noferini 54 S. Noh 12 P. Nomokonov 75 J. Norman 128 N. Novitzky 134 P. Nowakowski 143 A. Nyanin 89 J. Nystrand 21 M. Ogino 83 A. Ohlson 81 V.A. Okorokov 94 J. Oleniacz 143 A.C. Oliveira Da Silva 131 M.H. Oliver 147 A. Onnerstad 126 C. Oppedisano 59 A. Ortiz Velasquez 69 T. Osako 46 A. Oskarsson 81 J. Otwinowski 118 M. Oya 46 K. Oyama 83 Y. Pachmayer 105 S. Padhan 49 D. Pagano 141 58 G. Paić 69 A. Palasciano 53 S. Panebianco 139 J. Park 61 J.E. Parkkila 126 S.P. Pathak 125 R.N. Patra 102 34 B. Paul 22 H. Pei 7 T. Peitzmann 62 X. Peng 7 L.G. Pereira 70 H. Pereira Da Costa 139 D. Peresunko 89 82 G.M. Perez 8 S. Perrin 139 Y. Pestov 5 V. Petráček 37 V. Petrov 113 M. Petrovici 48 R.P. Pezzi 115 70 S. Piano 60 M. Pikna 13 P. Pillot 115 O. Pinazza 54 34 L. Pinsky 125 C. Pinto 26 S. Pisano 52 M. Płoskoń 80 M. Planinic 100 F. Pliquett 68 M.G. Poghosyan 97 B. Polichtchouk 92 S. Politano 30 N. Poljak 100 A. Pop 48 S. Porteboeuf-Houssais 136 J. Porter 80 V. Pozdniakov 75 S.K. Prasad 4 R. Preghenella 54 F. Prino 59 C.A. Pruneau 144 I. Pshenichnov 63 M. Puccio 34 S. Qiu 91 L. Quaglia 24 R.E. Quishpe 125 S. Ragoni 111 A. Rakotozafindrabe 139 L. Ramello 31 F. Rami 138 S.A.R. Ramirez 45 T.A. Rancien 79 R. Raniwala 103 S. Raniwala 103 S.S. Räsänen 44 R. Rath 50 I. Ravasenga 91 K.F. Read 97 131 A.R. Redelbach 39 K. Redlich 86 VI A. Rehman 21 P. Reichelt 68 F. Reidt 34 H.A. Reme-ness 36 Z. Rescakova 38 K. Reygers 105 A. Riabov 99 V. Riabov 99 T. Richert 81 M. Richter 20 W. Riegler 34 F. Riggi 26 C. Ristea 67 M. Rodríguez Cahuantzi 45 K. Røed 20 R. Rogalev 92 E. Rogochaya 75 T.S. Rogoschinski 68 D. Rohr 34 D. Röhrich 21 P.F. Rojas 45 S. Rojas Torres 37 P.S. Rokita 143 F. Ronchetti 52 A. Rosano 32 56 E.D. Rosas 69 A. Rossi 57 A. Roy 50 P. Roy 110 S. Roy 49 N. Rubini 25 O.V. Rueda 81 D. Ruggiano 143 R. Rui 23 B. Rumyantsev 75 P.G. Russek 2 R. Russo 91 A. Rustamov 88 E. Ryabinkin 89 Y. Ryabov 99 A. Rybicki 118 H. Rytkonen 126 W. Rzesa 143 O.A.M. Saarimaki 44 R. Sadek 115 S. Sadovsky 92 J. Saetre 21 K. Šafařík 37 S.K. Saha 142 S. Saha 87 B. Sahoo 49 P. Sahoo 49 R. Sahoo 50 S. Sahoo 65 D. Sahu 50 P.K. Sahu 65 J. Saini 142 S. Sakai 134 M.P. Salvan 108 S. Sambyal 102 T.B. Saramela 121 D. Sarkar 144 N. Sarkar 142 P. Sarma 42 V.M. Sarti 106 M.H.P. Sas 147 J. Schambach 97 H.S. Scheid 68 C. Schiaua 48 R. Schicker 105 A. Schmah 105 C. Schmidt 108 H.R. Schmidt 104 M.O. Schmidt 34 105 M. Schmidt 104 N.V. Schmidt 97 68 A.R. Schmier 131 R. Schotter 138 J. Schukraft 34 K. Schwarz 108 K. Schweda 108 G. Scioli 25 E. Scomparin 59 J.E. Seger 15 Y. Sekiguchi 133 D. Sekihata 133 I. Selyuzhenkov 108 94 S. Senyukov 138 J.J. Seo 61 D. Serebryakov 63 L. Šerkšnytė 106 A. Sevcenco 67 T.J. Shaba 72 A. Shabanov 63 A. Shabetai 115 R. Shahoyan 34 W. Shaikh 110 A. Shangaraev 92 A. Sharma 101 D. Sharma 49 H. Sharma 118 M. Sharma 102 N. Sharma 101 S. Sharma 102 U. Sharma 102 A. Shatat 78 O. Sheibani 125 K. Shigaki 46 M. Shimomura 84 S. Shirinkin 93 Q. Shou 40 Y. Sibiriak 89 S. Siddhanta 55 T. Siemiarczuk 86 T.F. Silva 121 D. Silvermyr 81 T. Simantathammakul 116 G. Simonetti 34 B. Singh 106 R. Singh 87 R. Singh 102 R. Singh 50 V.K. Singh 142 V. Singhal 142 T. Sinha 110 B. Sitar 13 M. Sitta 31 T.B. Skaali 20 G. Skorodumovs 105 M. Slupecki 44 N. Smirnov 147 R.J.M. Snellings 62 C. Soncco 112 J. Song 125 A. Songmoolnak 116 F. Soramel 27 S. Sorensen 131 I. Sputowska 118 J. Stachel 105 I. Stan 67 P.J. Steffanic 131 S.F. Stiefelmaier 105 D. Stocco 115 I. Storehaug 20 M.M. Storetvedt 36 P. Stratmann 145 S. Strazzi 25 C.P. Stylianidis 91 A.A.P. Suaide 121 C. Suire 78 M. Sukhanov 63 M. Suljic 34 R. Sultanov 93 V. Sumberia 102 S. Sumowidagdo 51 S. Swain 65 A. Szabo 13 I. Szarka 13 U. Tabassam 14 S.F. Taghavi 106 G. Taillepied 108 136 J. Takahashi 122 G.J. Tambave 21 S. Tang 136 7 Z. Tang 129 J.D. Tapia Takaki 127 VII N. Tapus 135 M.G. Tarzila 48 A. Tauro 34 G. Tejeda Muñoz 45 A. Telesca 34 L. Terlizzi 24 C. Terrevoli 125 G. Tersimonov 3 S. Thakur 142 D. Thomas 119 R. Tieulent 137 A. Tikhonov 63 A.R. Timmins 125 M. Tkacik 117 A. Toia 68 N. Topilskaya 63 M. Toppi 52 F. Torales-Acosta 19 T. Tork 78 A.G. Torres Ramos 33 A. Trifiró 32 56 A.S. Triolo 32 S. Tripathy 54 T. Tripathy 49 S. Trogolo 34 V. Trubnikov 3 W.H. Trzaska 126 T.P. Trzcinski 143 A. Tumkin 109 R. Turrisi 57 T.S. Tveter 20 K. Ullaland 21 A. Uras 137 M. Urioni 58 141 G.L. Usai 22 M. Vala 38 N. Valle 28 S. Vallero 59 L.V.R. van Doremalen 62 M. van Leeuwen 91 P. Vande Vyvre 34 D. Varga 146 Z. Varga 146 M. Varga-Kofarago 146 M. Vasileiou 85 A. Vasiliev 89 O. Vázquez Doce 52 106 V. Vechernin 113 A. Velure 21 E. Vercellin 24 S. Vergara Limón 45 L. Vermunt 62 R. Vértesi 146 M. Verweij 62 L. Vickovic 35 Z. Vilakazi 132 O. Villalobos Baillie 111 G. Vino 53 A. Vinogradov 89 T. Virgili 29 V. Vislavicius 90 A. Vodopyanov 75 B. Volkel 34 M.A. Völkl 105 K. Voloshin 93 S.A. Voloshin 144 G. Volpe 33 B. von Haller 34 I. Vorobyev 106 N. Vozniuk 63 J. Vrláková 38 B. Wagner 21 C. Wang 40 D. Wang 40 M. Weber 114 R.J.G.V. Weelden 91 A. Wegrzynek 34 S.C. Wenzel 34 J.P. Wessels 145 S.L. Weyhmiller 147 J. Wiechula 68 J. Wikne 20 G. Wilk 86 J. Wilkinson 108 G.A. Willems 145 B. Windelband 105 M. Winn 139 W.E. Witt 131 J.R. Wright 119 W. Wu 40 Y. Wu 129 R. Xu 7 A.K. Yadav 142 S. Yalcin 77 Y. Yamaguchi 46 K. Yamakawa 46 S. Yang 21 S. Yano 46 Z. Yin 7 I.-K. Yoo 17 J.H. Yoon 61 S. Yuan 21 A. Yuncu 105 V. Zaccolo 23 C. Zampolli 34 H.J.C. Zanoli 62 F. Zanone 105 N. Zardoshti 34 111 A. Zarochentsev 113 P. Závada 66 N. Zaviyalov 109 M. Zhalov 99 B. Zhang 7 S. Zhang 40 X. Zhang 7 Y. Zhang 129 V. Zherebchevskii 113 Y. Zhi 11 N. Zhigareva 93 D. Zhou 7 Y. Zhou 90 J. Zhu 108 7 Y. Zhu 7 G. Zinovjev 3 I N. Zurlo 141 58 1 A.I. Alikhanyan National Science Laboratory (Yerevan Physics Institute) Foundation, Yerevan, Armenia A.I. Alikhanyan National Science Laboratory (Yerevan Physics Institute) Foundation Yerevan Armenia A.I. Alikhanyan National Science Laboratory (Yerevan Physics Institute) Foundation, Yerevan, Armenia 2 AGH University of Science and Technology, Cracow, Poland AGH University of Science and Technology Cracow Poland AGH University of Science and Technology, Cracow, Poland 3 Bogolyubov Institute for Theoretical Physics, National Academy of Sciences of Ukraine, Kiev, Ukraine Bogolyubov Institute for Theoretical Physics National Academy of Sciences of Ukraine Kiev Ukraine Bogolyubov Institute for Theoretical Physics, National Academy of Sciences of Ukraine, Kiev, Ukraine 4 Bose Institute, Department of Physics and Centre for Astroparticle Physics and Space Science (CAPSS), Kolkata, India Bose Institute Department of Physics Centre for Astroparticle Physics and Space Science (CAPSS) Kolkata India Bose Institute, Department of Physics and Centre for Astroparticle Physics and Space Science (CAPSS), Kolkata, India 5 Budker Institute for Nuclear Physics, Novosibirsk, Russia Budker Institute for Nuclear Physics Novosibirsk Russia Budker Institute for Nuclear Physics, Novosibirsk, Russia 6 California Polytechnic State University, San Luis Obispo, CA, United States California Polytechnic State University San Luis Obispo CA United States California Polytechnic State University, San Luis Obispo, California, United States 7 Central China Normal University, Wuhan, China Central China Normal University Wuhan China Central China Normal University, Wuhan, China 8 Centro de Aplicaciones Tecnológicas y Desarrollo Nuclear (CEADEN), Havana, Cuba Centro de Aplicaciones Tecnológicas y Desarrollo Nuclear (CEADEN) Havana Cuba Centro de Aplicaciones Tecnológicas y Desarrollo Nuclear (CEADEN), Havana, Cuba 9 Centro de Investigación y de Estudios Avanzados (CINVESTAV), Mexico City and Mérida, Mexico Centro de Investigación y de Estudios Avanzados (CINVESTAV) Mexico City and Mérida Mexico Centro de Investigación y de Estudios Avanzados (CINVESTAV), Mexico City and Mérida, Mexico 10 Chicago State University, Chicago, IL, United States Chicago State University Chicago IL United States Chicago State University, Chicago, Illinois, United States 11 China Institute of Atomic Energy, Beijing, China China Institute of Atomic Energy Beijing China China Institute of Atomic Energy, Beijing, China 12 Chungbuk National University, Cheongju, Republic of Korea Chungbuk National University Cheongju Republic of Korea Chungbuk National University, Cheongju, Republic of Korea 13 Comenius University Bratislava, Faculty of Mathematics, Physics and Informatics, Bratislava, Slovakia Comenius University Bratislava Faculty of Mathematics, Physics and Informatics Bratislava Slovakia Comenius University Bratislava, Faculty of Mathematics, Physics and Informatics, Bratislava, Slovakia 14 COMSATS University Islamabad, Islamabad, Pakistan COMSATS University Islamabad Islamabad Pakistan COMSATS University Islamabad, Islamabad, Pakistan 15 Creighton University, Omaha, NE, United States Creighton University Omaha NE United States Creighton University, Omaha, Nebraska, United States 16 Department of Physics, Aligarh Muslim University, Aligarh, India Department of Physics Aligarh Muslim University Aligarh India Department of Physics, Aligarh Muslim University, Aligarh, India 17 Department of Physics, Pusan National University, Pusan, Republic of Korea Department of Physics Pusan National University Pusan Republic of Korea Department of Physics, Pusan National University, Pusan, Republic of Korea 18 Department of Physics, Sejong University, Seoul, Republic of Korea Department of Physics Sejong University Seoul Republic of Korea Department of Physics, Sejong University, Seoul, Republic of Korea 19 Department of Physics, University of California, Berkeley, CA, United States Department of Physics University of California Berkeley CA United States Department of Physics, University of California, Berkeley, California, United States 20 Department of Physics, University of Oslo, Oslo, Norway Department of Physics University of Oslo Oslo Norway Department of Physics, University of Oslo, Oslo, Norway 21 Department of Physics and Technology, University of Bergen, Bergen, Norway Department of Physics and Technology University of Bergen Bergen Norway Department of Physics and Technology, University of Bergen, Bergen, Norway 22 Dipartimento di Fisica dell'Università and Sezione INFN, Cagliari, Italy Dipartimento di Fisica dell'Università Sezione INFN Cagliari Italy Dipartimento di Fisica dell'Università and Sezione INFN, Cagliari, Italy 23 Dipartimento di Fisica dell'Università and Sezione INFN, Trieste, Italy Dipartimento di Fisica dell'Università Sezione INFN Trieste Italy Dipartimento di Fisica dell'Università and Sezione INFN, Trieste, Italy 24 Dipartimento di Fisica dell'Università and Sezione INFN, Turin, Italy Dipartimento di Fisica dell'Università Sezione INFN Turin Italy Dipartimento di Fisica dell'Università and Sezione INFN, Turin, Italy 25 Dipartimento di Fisica e Astronomia dell'Università and Sezione INFN, Bologna, Italy Dipartimento di Fisica e Astronomia dell'Università Sezione INFN Bologna Italy Dipartimento di Fisica e Astronomia dell'Università and Sezione INFN, Bologna, Italy 26 Dipartimento di Fisica e Astronomia dell'Università and Sezione INFN, Catania, Italy Dipartimento di Fisica e Astronomia dell'Università Sezione INFN Catania Italy Dipartimento di Fisica e Astronomia dell'Università and Sezione INFN, Catania, Italy 27 Dipartimento di Fisica e Astronomia dell'Università and Sezione INFN, Padova, Italy Dipartimento di Fisica e Astronomia dell'Università Sezione INFN Padova Italy Dipartimento di Fisica e Astronomia dell'Università and Sezione INFN, Padova, Italy 28 Dipartimento di Fisica e Nucleare e Teorica, Università di Pavia, Pavia, Italy Dipartimento di Fisica e Nucleare e Teorica Università di Pavia Pavia Italy Dipartimento di Fisica e Nucleare e Teorica, Università di Pavia, Pavia, Italy 29 Dipartimento di Fisica ‘E.R. Caianiello’ dell'Università and Gruppo Collegato INFN, Salerno, Italy Dipartimento di Fisica ‘E.R. Caianiello’ dell'Università Gruppo Collegato INFN Salerno Italy Dipartimento di Fisica ‘E.R. Caianiello’ dell'Università and Gruppo Collegato INFN, Salerno, Italy 30 Dipartimento DISAT del Politecnico and Sezione INFN, Turin, Italy Dipartimento DISAT del Politecnico Sezione INFN Turin Italy Dipartimento DISAT del Politecnico and Sezione INFN, Turin, Italy 31 Dipartimento di Scienze e Innovazione Tecnologica dell'Università del Piemonte Orientale and INFN Sezione di Torino, Alessandria, Italy Dipartimento di Scienze e Innovazione Tecnologica dell'Università del Piemonte Orientale INFN Sezione di Torino Alessandria Italy Dipartimento di Scienze e Innovazione Tecnologica dell'Università del Piemonte Orientale and INFN Sezione di Torino, Alessandria, Italy 32 Dipartimento di Scienze MIFT, Università di Messina, Messina, Italy Dipartimento di Scienze MIFT Università di Messina Messina Italy Dipartimento di Scienze MIFT, Università di Messina, Messina, Italy 33 Dipartimento Interateneo di Fisica ‘M. Merlin’ and Sezione INFN, Bari, Italy Dipartimento Interateneo di Fisica ‘M. Merlin’ Sezione INFN Bari Italy Dipartimento Interateneo di Fisica ‘M. Merlin’ and Sezione INFN, Bari, Italy 34 European Organization for Nuclear Research (CERN), Geneva, Switzerland European Organization for Nuclear Research (CERN) Geneva Switzerland European Organization for Nuclear Research (CERN), Geneva, Switzerland 35 Faculty of Electrical Engineering, Mechanical Engineering and Naval Architecture, University of Split, Split, Croatia Faculty of Electrical Engineering, Mechanical Engineering and Naval Architecture University of Split Split Croatia Faculty of Electrical Engineering, Mechanical Engineering and Naval Architecture, University of Split, Split, Croatia 36 Faculty of Engineering and Science, Western Norway University of Applied Sciences, Bergen, Norway Faculty of Engineering and Science Western Norway University of Applied Sciences Bergen Norway Faculty of Engineering and Science, Western Norway University of Applied Sciences, Bergen, Norway 37 Faculty of Nuclear Sciences and Physical Engineering, Czech Technical University in Prague, Prague, Czech Republic Faculty of Nuclear Sciences and Physical Engineering Czech Technical University in Prague Prague Czech Republic Faculty of Nuclear Sciences and Physical Engineering, Czech Technical University in Prague, Prague, Czech Republic 38 Faculty of Science, P.J. Šafárik University, Košice, Slovakia Faculty of Science P.J. Šafárik University Košice Slovakia Faculty of Science, P.J. Šafárik University, Košice, Slovakia 39 Frankfurt Institute for Advanced Studies, Johann Wolfgang Goethe-Universität Frankfurt, Frankfurt, Germany Frankfurt Institute for Advanced Studies Johann Wolfgang Goethe-Universität Frankfurt Frankfurt Germany Frankfurt Institute for Advanced Studies, Johann Wolfgang Goethe-Universität Frankfurt, Frankfurt, Germany 40 Fudan University, Shanghai, China Fudan University Shanghai China Fudan University, Shanghai, China 41 Gangneung-Wonju National University, Gangneung, Republic of Korea Gangneung-Wonju National University Gangneung Republic of Korea Gangneung-Wonju National University, Gangneung, Republic of Korea 42 Gauhati University, Department of Physics, Guwahati, India Gauhati University Department of Physics Guwahati India Gauhati University, Department of Physics, Guwahati, India 43 Helmholtz-Institut für Strahlen- und Kernphysik, Rheinische Friedrich-Wilhelms-Universität Bonn, Bonn, Germany Helmholtz-Institut für Strahlen- und Kernphysik Rheinische Friedrich-Wilhelms-Universität Bonn Bonn Germany Helmholtz-Institut für Strahlen- und Kernphysik, Rheinische Friedrich-Wilhelms-Universität Bonn, Bonn, Germany 44 Helsinki Institute of Physics (HIP), Helsinki, Finland Helsinki Institute of Physics (HIP) Helsinki Finland Helsinki Institute of Physics (HIP), Helsinki, Finland 45 High Energy Physics Group, Universidad Autónoma de Puebla, Puebla, Mexico High Energy Physics Group Universidad Autónoma de Puebla Puebla Mexico High Energy Physics Group, Universidad Autónoma de Puebla, Puebla, Mexico 46 Hiroshima University, Hiroshima, Japan Hiroshima University Hiroshima Japan Hiroshima University, Hiroshima, Japan 47 Hochschule Worms, Zentrum für Technologietransfer und Telekommunikation (ZTT), Worms, Germany Hochschule Worms Zentrum für Technologietransfer und Telekommunikation (ZTT) Worms Germany Hochschule Worms, Zentrum für Technologietransfer und Telekommunikation (ZTT), Worms, Germany 48 Horia Hulubei National Institute of Physics and Nuclear Engineering, Bucharest, Romania Horia Hulubei National Institute of Physics and Nuclear Engineering Bucharest Romania Horia Hulubei National Institute of Physics and Nuclear Engineering, Bucharest, Romania 49 Indian Institute of Technology Bombay (IIT), Mumbai, India Indian Institute of Technology Bombay (IIT) Mumbai India Indian Institute of Technology Bombay (IIT), Mumbai, India 50 Indian Institute of Technology Indore, Indore, India Indian Institute of Technology Indore Indore India Indian Institute of Technology Indore, Indore, India 51 Indonesian Institute of Sciences, Jakarta, Indonesia Indonesian Institute of Sciences Jakarta Indonesia Indonesian Institute of Sciences, Jakarta, Indonesia 52 INFN, Laboratori Nazionali di Frascati, Frascati, Italy INFN, Laboratori Nazionali di Frascati Frascati Italy INFN, Laboratori Nazionali di Frascati, Frascati, Italy 53 INFN, Sezione di Bari, Bari, Italy INFN, Sezione di Bari Bari Italy INFN, Sezione di Bari, Bari, Italy 54 INFN, Sezione di Bologna, Bologna, Italy INFN, Sezione di Bologna Bologna Italy INFN, Sezione di Bologna, Bologna, Italy 55 INFN, Sezione di Cagliari, Cagliari, Italy INFN, Sezione di Cagliari Cagliari Italy INFN, Sezione di Cagliari, Cagliari, Italy 56 INFN, Sezione di Catania, Catania, Italy INFN, Sezione di Catania Catania Italy INFN, Sezione di Catania, Catania, Italy 57 INFN, Sezione di Padova, Padova, Italy INFN, Sezione di Padova Padova Italy INFN, Sezione di Padova, Padova, Italy 58 INFN, Sezione di Pavia, Pavia, Italy INFN, Sezione di Pavia Pavia Italy INFN, Sezione di Pavia, Pavia, Italy 59 INFN, Sezione di Torino, Turin, Italy INFN, Sezione di Torino Turin Italy INFN, Sezione di Torino, Turin, Italy 60 INFN, Sezione di Trieste, Trieste, Italy INFN, Sezione di Trieste Trieste Italy INFN, Sezione di Trieste, Trieste, Italy 61 Inha University, Incheon, Republic of Korea Inha University Incheon Republic of Korea Inha University, Incheon, Republic of Korea 62 Institute for Gravitational and Subatomic Physics (GRASP), Utrecht University/Nikhef, Utrecht, Netherlands Institute for Gravitational and Subatomic Physics (GRASP) Utrecht University/Nikhef Utrecht Netherlands Institute for Gravitational and Subatomic Physics (GRASP), Utrecht University/Nikhef, Utrecht, Netherlands 63 Institute for Nuclear Research, Academy of Sciences, Moscow, Russia Institute for Nuclear Research Academy of Sciences Moscow Russia Institute for Nuclear Research, Academy of Sciences, Moscow, Russia 64 Institute of Experimental Physics, Slovak Academy of Sciences, Košice, Slovakia Institute of Experimental Physics Slovak Academy of Sciences Košice Slovakia Institute of Experimental Physics, Slovak Academy of Sciences, Košice, Slovakia 65 Institute of Physics, Homi Bhabha National Institute, Bhubaneswar, India Institute of Physics Homi Bhabha National Institute Bhubaneswar India Institute of Physics, Homi Bhabha National Institute, Bhubaneswar, India 66 Institute of Physics of the Czech Academy of Sciences, Prague, Czech Republic Institute of Physics of the Czech Academy of Sciences Prague Czech Republic Institute of Physics of the Czech Academy of Sciences, Prague, Czech Republic 67 Institute of Space Science (ISS), Bucharest, Romania Institute of Space Science (ISS) Bucharest Romania Institute of Space Science (ISS), Bucharest, Romania 68 Institut für Kernphysik, Johann Wolfgang Goethe-Universität Frankfurt, Frankfurt, Germany Institut für Kernphysik Johann Wolfgang Goethe-Universität Frankfurt Frankfurt Germany Institut für Kernphysik, Johann Wolfgang Goethe-Universität Frankfurt, Frankfurt, Germany 69 Instituto de Ciencias Nucleares, Universidad Nacional Autónoma de México, Mexico City, Mexico Instituto de Ciencias Nucleares Universidad Nacional Autónoma de México Mexico City Mexico Instituto de Ciencias Nucleares, Universidad Nacional Autónoma de México, Mexico City, Mexico 70 Instituto de Física, Universidade Federal do Rio Grande do Sul (UFRGS), Porto Alegre, Brazil Instituto de Física Universidade Federal do Rio Grande do Sul (UFRGS) Porto Alegre Brazil Instituto de Física, Universidade Federal do Rio Grande do Sul (UFRGS), Porto Alegre, Brazil 71 Instituto de Física, Universidad Nacional Autónoma de México, Mexico City, Mexico Instituto de Física Universidad Nacional Autónoma de México Mexico City Mexico Instituto de Física, Universidad Nacional Autónoma de México, Mexico City, Mexico 72 iThemba LABS, National Research Foundation, Somerset West, South Africa iThemba LABS National Research Foundation Somerset West South Africa iThemba LABS, National Research Foundation, Somerset West, South Africa 73 Jeonbuk National University, Jeonju, Republic of Korea Jeonbuk National University Jeonju Republic of Korea Jeonbuk National University, Jeonju, Republic of Korea 74 Johann-Wolfgang-Goethe Universität Frankfurt Institut für Informatik, Fachbereich Informatik und Mathematik, Frankfurt, Germany Johann-Wolfgang-Goethe Universität Frankfurt Institut für Informatik Fachbereich Informatik und Mathematik Frankfurt Germany Johann-Wolfgang-Goethe Universität Frankfurt Institut für Informatik, Fachbereich Informatik und Mathematik, Frankfurt, Germany 75 Joint Institute for Nuclear Research (JINR), Dubna, Russia Joint Institute for Nuclear Research (JINR) Dubna Russia Joint Institute for Nuclear Research (JINR), Dubna, Russia 76 Korea Institute of Science and Technology Information, Daejeon, Republic of Korea Korea Institute of Science and Technology Information Daejeon Republic of Korea Korea Institute of Science and Technology Information, Daejeon, Republic of Korea 77 KTO Karatay University, Konya, Turkey KTO Karatay University Konya Turkey KTO Karatay University, Konya, Turkey 78 Laboratoire de Physique des 2 Infinis, Irène Joliot-Curie, Orsay, France Laboratoire de Physique des 2 Infinis, Irène Joliot-Curie Orsay France Laboratoire de Physique des 2 Infinis, Irène Joliot-Curie, Orsay, France 79 Laboratoire de Physique Subatomique et de Cosmologie, Université Grenoble-Alpes, CNRS-IN2P3, Grenoble, France Laboratoire de Physique Subatomique et de Cosmologie Université Grenoble-Alpes CNRS-IN2P3 Grenoble France Laboratoire de Physique Subatomique et de Cosmologie, Université Grenoble-Alpes, CNRS-IN2P3, Grenoble, France 80 Lawrence Berkeley National Laboratory, Berkeley, CA, United States Lawrence Berkeley National Laboratory Berkeley CA United States Lawrence Berkeley National Laboratory, Berkeley, California, United States 81 Lund University Department of Physics, Division of Particle Physics, Lund, Sweden Lund University Department of Physics Division of Particle Physics Lund Sweden Lund University Department of Physics, Division of Particle Physics, Lund, Sweden 82 Moscow Institute for Physics and Technology, Moscow, Russia Moscow Institute for Physics and Technology Moscow Russia Moscow Institute for Physics and Technology, Moscow, Russia 83 Nagasaki Institute of Applied Science, Nagasaki, Japan Nagasaki Institute of Applied Science Nagasaki Japan Nagasaki Institute of Applied Science, Nagasaki, Japan 84 Nara Women's University (NWU), Nara, Japan Nara Women's University (NWU) Nara Japan Nara Women's University (NWU), Nara, Japan 85 National and Kapodistrian University of Athens, School of Science, Department of Physics , Athens, Greece National and Kapodistrian University of Athens School of Science Department of Physics Athens Greece National and Kapodistrian University of Athens, School of Science, Department of Physics , Athens, Greece 86 National Centre for Nuclear Research, Warsaw, Poland National Centre for Nuclear Research Warsaw Poland National Centre for Nuclear Research, Warsaw, Poland 87 National Institute of Science Education and Research, Homi Bhabha National Institute, Jatni, India National Institute of Science Education and Research Homi Bhabha National Institute Jatni India National Institute of Science Education and Research, Homi Bhabha National Institute, Jatni, India 88 National Nuclear Research Center, Baku, Azerbaijan National Nuclear Research Center Baku Azerbaijan National Nuclear Research Center, Baku, Azerbaijan 89 National Research Centre Kurchatov Institute, Moscow, Russia National Research Centre Kurchatov Institute Moscow Russia National Research Centre Kurchatov Institute, Moscow, Russia 90 Niels Bohr Institute, University of Copenhagen, Copenhagen, Denmark Niels Bohr Institute University of Copenhagen Copenhagen Denmark Niels Bohr Institute, University of Copenhagen, Copenhagen, Denmark 91 Nikhef, National Institute for Subatomic Physics, Amsterdam, Netherlands Nikhef National Institute for Subatomic Physics Amsterdam Netherlands Nikhef, National institute for subatomic physics, Amsterdam, Netherlands 92 NRC Kurchatov Institute IHEP, Protvino, Russia NRC Kurchatov Institute IHEP Protvino Russia NRC Kurchatov Institute IHEP, Protvino, Russia 93 NRC «Kurchatov»Institute – ITEP, Moscow, Russia NRC «Kurchatov»Institute – ITEP Moscow Russia NRC «Kurchatov»Institute – ITEP, Moscow, Russia 94 NRNU Moscow Engineering Physics Institute, Moscow, Russia NRNU Moscow Engineering Physics Institute Moscow Russia NRNU Moscow Engineering Physics Institute, Moscow, Russia 95 Nuclear Physics Group, STFC Daresbury Laboratory, Daresbury, United Kingdom Nuclear Physics Group STFC Daresbury Laboratory Daresbury United Kingdom Nuclear Physics Group, STFC Daresbury Laboratory, Daresbury, United Kingdom 96 Nuclear Physics Institute of the Czech Academy of Sciences, Řež u Prahy, Czech Republic Nuclear Physics Institute of the Czech Academy of Sciences Řež u Prahy Czech Republic Nuclear Physics Institute of the Czech Academy of Sciences, Řež u Prahy, Czech Republic 97 Oak Ridge National Laboratory, Oak Ridge, TN, United States Oak Ridge National Laboratory Oak Ridge TN United States Oak Ridge National Laboratory, Oak Ridge, Tennessee, United States 98 Ohio State University, Columbus, OH, United States Ohio State University Columbus OH United States Ohio State University, Columbus, Ohio, United States 99 Petersburg Nuclear Physics Institute, Gatchina, Russia Petersburg Nuclear Physics Institute Gatchina Russia Petersburg Nuclear Physics Institute, Gatchina, Russia 100 Physics Department, Faculty of Science, University of Zagreb, Zagreb, Croatia Physics Department Faculty of Science University of Zagreb Zagreb Croatia Physics department, Faculty of science, University of Zagreb, Zagreb, Croatia 101 Physics Department, Panjab University, Chandigarh, India Physics Department Panjab University Chandigarh India Physics Department, Panjab University, Chandigarh, India 102 Physics Department, University of Jammu, Jammu, India Physics Department University of Jammu Jammu India Physics Department, University of Jammu, Jammu, India 103 Physics Department, University of Rajasthan, Jaipur, India Physics Department University of Rajasthan Jaipur India Physics Department, University of Rajasthan, Jaipur, India 104 Physikalisches Institut, Eberhard-Karls-Universität Tübingen, Tübingen, Germany Physikalisches Institut Eberhard-Karls-Universität Tübingen Tübingen Germany Physikalisches Institut, Eberhard-Karls-Universität Tübingen, Tübingen, Germany 105 Physikalisches Institut, Ruprecht-Karls-Universität Heidelberg, Heidelberg, Germany Physikalisches Institut Ruprecht-Karls-Universität Heidelberg Heidelberg Germany Physikalisches Institut, Ruprecht-Karls-Universität Heidelberg, Heidelberg, Germany 106 Physik Department, Technische Universität München, Munich, Germany Physik Department Technische Universität München Munich Germany Physik Department, Technische Universität München, Munich, Germany 107 Politecnico di Bari and Sezione INFN, Bari, Italy Politecnico di Bari Sezione INFN Bari Italy Politecnico di Bari and Sezione INFN, Bari, Italy 108 Research Division and ExtreMe Matter Institute EMMI, GSI Helmholtzzentrum für Schwerionenforschung GmbH, Darmstadt, Germany Research Division ExtreMe Matter Institute EMMI GSI Helmholtzzentrum für Schwerionenforschung GmbH Darmstadt Germany Research Division and ExtreMe Matter Institute EMMI, GSI Helmholtzzentrum für Schwerionenforschung GmbH, Darmstadt, Germany 109 Russian Federal Nuclear Center (VNIIEF), Sarov, Russia Russian Federal Nuclear Center (VNIIEF) Sarov Russia Russian Federal Nuclear Center (VNIIEF), Sarov, Russia 110 Saha Institute of Nuclear Physics, Homi Bhabha National Institute, Kolkata, India Saha Institute of Nuclear Physics Homi Bhabha National Institute Kolkata India Saha Institute of Nuclear Physics, Homi Bhabha National Institute, Kolkata, India 111 School of Physics and Astronomy, University of Birmingham, Birmingham, United Kingdom School of Physics and Astronomy University of Birmingham Birmingham United Kingdom School of Physics and Astronomy, University of Birmingham, Birmingham, United Kingdom 112 Sección Física, Departamento de Ciencias, Pontificia Universidad Católica del Perú, Lima, Peru Sección Física Departamento de Ciencias Pontificia Universidad Católica del Perú Lima Peru Sección Física, Departamento de Ciencias, Pontificia Universidad Católica del Perú, Lima, Peru 113 St. Petersburg State University, St. Petersburg, Russia St. Petersburg State University St. Petersburg Russia St. Petersburg State University, St. Petersburg, Russia 114 Stefan Meyer Institut für Subatomare Physik (SMI), Vienna, Austria Stefan Meyer Institut für Subatomare Physik (SMI) Vienna Austria Stefan Meyer Institut für Subatomare Physik (SMI), Vienna, Austria 115 SUBATECH, IMT Atlantique, Université de Nantes, CNRS-IN2P3, Nantes, France SUBATECH IMT Atlantique Université de Nantes CNRS-IN2P3 Nantes France SUBATECH, IMT Atlantique, Université de Nantes, CNRS-IN2P3, Nantes, France 116 Suranaree University of Technology, Nakhon Ratchasima, Thailand Suranaree University of Technology Nakhon Ratchasima Thailand Suranaree University of Technology, Nakhon Ratchasima, Thailand 117 Technical University of Košice, Košice, Slovakia Technical University of Košice Košice Slovakia Technical University of Košice, Košice, Slovakia 118 The Henryk Niewodniczanski Institute of Nuclear Physics, Polish Academy of Sciences, Cracow, Poland The Henryk Niewodniczanski Institute of Nuclear Physics Polish Academy of Sciences Cracow Poland The Henryk Niewodniczanski Institute of Nuclear Physics, Polish Academy of Sciences, Cracow, Poland 119 The University of Texas at Austin, Austin, TX, United States The University of Texas at Austin Austin TX United States The University of Texas at Austin, Austin, Texas, United States 120 Universidad Autónoma de Sinaloa, Culiacán, Mexico Universidad Autónoma de Sinaloa Culiacán Mexico Universidad Autónoma de Sinaloa, Culiacán, Mexico 121 Universidade de São Paulo (USP), São Paulo, Brazil Universidade de São Paulo (USP) São Paulo Brazil Universidade de São Paulo (USP), São Paulo, Brazil 122 Universidade Estadual de Campinas (UNICAMP), Campinas, Brazil Universidade Estadual de Campinas (UNICAMP) Campinas Brazil Universidade Estadual de Campinas (UNICAMP), Campinas, Brazil 123 Universidade Federal do ABC, Santo Andre, Brazil Universidade Federal do ABC Santo Andre Brazil Universidade Federal do ABC, Santo Andre, Brazil 124 University of Cape Town, Cape Town, South Africa University of Cape Town Cape Town South Africa University of Cape Town, Cape Town, South Africa 125 University of Houston, Houston, TX, United States University of Houston Houston TX United States University of Houston, Houston, Texas, United States 126 University of Jyväskylä, Jyväskylä, Finland University of Jyväskylä Jyväskylä Finland University of Jyväskylä, Jyväskylä, Finland 127 University of Kansas, Lawrence, KS, United States University of Kansas Lawrence KS United States University of Kansas, Lawrence, Kansas, United States 128 University of Liverpool, Liverpool, United Kingdom University of Liverpool Liverpool United Kingdom University of Liverpool, Liverpool, United Kingdom 129 University of Science and Technology of China, Hefei, China University of Science and Technology of China Hefei China University of Science and Technology of China, Hefei, China 130 University of South-Eastern Norway, Tonsberg, Norway University of South-Eastern Norway Tonsberg Norway University of South-Eastern Norway, Tonsberg, Norway 131 University of Tennessee, Knoxville, TN, United States University of Tennessee Knoxville TN United States University of Tennessee, Knoxville, Tennessee, United States 132 University of the Witwatersrand, Johannesburg, South Africa University of the Witwatersrand Johannesburg South Africa University of the Witwatersrand, Johannesburg, South Africa 133 University of Tokyo, Tokyo, Japan University of Tokyo Tokyo Japan University of Tokyo, Tokyo, Japan 134 University of Tsukuba, Tsukuba, Japan University of Tsukuba Tsukuba Japan University of Tsukuba, Tsukuba, Japan 135 University Politehnica of Bucharest, Bucharest, Romania University Politehnica of Bucharest Bucharest Romania University Politehnica of Bucharest, Bucharest, Romania 136 Université Clermont Auvergne, CNRS/IN2P3, LPC, Clermont-Ferrand, France Université Clermont Auvergne CNRS/IN2P3 LPC Clermont-Ferrand France Université Clermont Auvergne, CNRS/IN2P3, LPC, Clermont-Ferrand, France 137 Université de Lyon, CNRS/IN2P3, Institut de Physique des 2 Infinis de Lyon, Lyon, France Université de Lyon CNRS/IN2P3 Institut de Physique des 2 Infinis de Lyon Lyon France Université de Lyon, CNRS/IN2P3, Institut de Physique des 2 Infinis de Lyon, Lyon, France 138 Université de Strasbourg, CNRS, IPHC UMR 7178, F-67000 Strasbourg, France Université de Strasbourg CNRS IPHC UMR 7178 Strasbourg F-67000 France Université de Strasbourg, CNRS, IPHC UMR 7178, F-67000 Strasbourg, France, Strasbourg, France 139 Université Paris-Saclay Centre d'Etudes de Saclay (CEA), IRFU, Départment de Physique Nucléaire (DPhN), Saclay, France Université Paris-Saclay Centre d'Etudes de Saclay (CEA) IRFU Départment de Physique Nucléaire (DPhN) Saclay France Université Paris-Saclay Centre d'Etudes de Saclay (CEA), IRFU, Départment de Physique Nucléaire (DPhN), Saclay, France 140 Università degli Studi di Foggia, Foggia, Italy Università degli Studi di Foggia Foggia Italy Università degli Studi di Foggia, Foggia, Italy 141 Università di Brescia, Brescia, Italy Università di Brescia Brescia Italy Università di Brescia, Brescia, Italy 142 Variable Energy Cyclotron Centre, Homi Bhabha National Institute, Kolkata, India Variable Energy Cyclotron Centre Homi Bhabha National Institute Kolkata India Variable Energy Cyclotron Centre, Homi Bhabha National Institute, Kolkata, India 143 Warsaw University of Technology, Warsaw, Poland Warsaw University of Technology Warsaw Poland Warsaw University of Technology, Warsaw, Poland 144 Wayne State University, Detroit, MI, United States Wayne State University Detroit MI United States Wayne State University, Detroit, Michigan, United States 145 Westfälische Wilhelms-Universität Münster, Institut für Kernphysik, Münster, Germany Westfälische Wilhelms-Universität Münster Institut für Kernphysik Münster Germany Westfälische Wilhelms-Universität Münster, Institut für Kernphysik, Münster, Germany 146 Wigner Research Centre for Physics, Budapest, Hungary Wigner Research Centre for Physics Budapest Hungary Wigner Research Centre for Physics, Budapest, Hungary 147 Yale University, New Haven, CT, United States Yale University New Haven CT United States Yale University, New Haven, Connecticut, United States 148 Yonsei University, Seoul, Republic of Korea Yonsei University Seoul Republic of Korea Yonsei University, Seoul, Republic of Korea I Deceased. II Also at: Italian National Agency for New Technologies, Energy and Sustainable Economic Development (ENEA), Bologna, Italy. III Also at: Dipartimento DET del Politecnico di Torino, Turin, Italy. IV Also at: M.V. Lomonosov Moscow State University, D.V. Skobeltsyn Institute of Nuclear, Physics, Moscow, Russia. V Also at: Department of Applied Physics, Aligarh Muslim University, Aligarh, India. VI Also at: Institute of Theoretical Physics, University of Wroclaw, Poland. VII Also at: University of Kansas, Lawrence, Kansas, United States. ⋆ E-mail address: alice-publications@cern.ch .Editor: M. Doser Abstract We present the first systematic comparison of the charged-particle pseudorapidity densities for three widely different collision systems, pp, p Pb, and Pb Pb, at the top energy of the Large Hadron Collider (s NN = 5.02 TeV ) measured over a wide pseudorapidity range (− 3.5 < η < 5 ), the widest possible among the four experiments at that facility. The systematic uncertainties are minimised since the measurements are recorded by the same experimental apparatus (ALICE). The distributions for p Pb and Pb Pb collisions are determined as a function of the centrality of the collisions, while results from pp collisions are reported for inelastic events with at least one charged particle at midrapidity. The charged-particle pseudorapidity densities are, under simple and robust assumptions, transformed to charged-particle rapidity densities. This allows for the calculation and the presentation of the evolution of the width of the rapidity distributions and of a lower bound on the Bjorken energy density, as a function of the number of participants in all three collision systems. We find a decreasing width of the particle production, and roughly a smooth ten fold increase in the energy density, as the system size grows, which is consistent with a gradually higher dense phase of matter. 1 Introduction The number of charged particles produced in energetic nuclear collisions is an important indicator for the strong interaction processes that determine the particle production at the sub-nucleonic level. In particular, the production of charged particles is expected to reflect the number of quark and gluon collisions occurring during the initial stages of the reaction. The total number of particles produced also provides information on the energy transfer available from the initial colliding beams to particle production, as a consequence of nuclear stopping [1] . In order to help unravel this complex scenario it is important to compare the particle production amongst collision systems of different sizes over a wide kinematic range. We present the measured charged-particle pseudorapidity density, d N ch / d η , for pp, p Pb, and Pb Pb (previously published [2] ) collisions at the same collision energy of s NN = 5.02 TeV in the nucleon–nucleon centre-of-mass reference frame. This is, at present, the maximum available energy at CERN's Large Hadron Collider (LHC) for Pb Pb collisions. The measurements were carried out using ALICE at LHC (for earlier d N ch / d η results see for example Refs. [3–5] ). The three studied reactions have different characteristics probing widely different particle production yields and mechanisms. In Pb Pb collisions, the total particle yield for central collisions is of the order 104 [2] , and a strongly coupled plasma of quarks and gluons (sQGP) is formed [6–9] , whose collective and transport properties are currently under intense study. On the other hand, pp collisions represent the simplest possible nuclear collision system, where the average total particle production is much smaller (≈80, by integrating the measured distributions), and is to first approximation much less subject to collective effects [10] . The p Pb system is intermediate to the other reactions, corresponding to the situation where a single nucleon probes the nucleons in a narrow cylinder of the target nucleus. The extent to which p Pb is governed by the initial state cold nuclear matter of the lead ion or whether collective phenomena in the hot and dense medium play an important role is, at present, a matter under scrutiny by the community [10,11] . In this letter, we compare the three reactions and present the ratios of the charged-particle pseudorapidity density distributions (d N ch / d η ) of the more complex reactions to the pp distribution. Owing to ALICE's unique large acceptance in pseudorapidity, and using simple and robust assumptions, we transform the measured charged-particle pseudorapidity density distributions into charged-particle rapidity density distributions (d N ch / d y ). This allows us to calculate the width of the rapidity distributions as a function of the number of participating nucleons. The parameters of the transformation also allow us to estimate a lower bound on the energy density using the well-known formula from Bjorken [12] . An energy density exceeding the critical energy density of roughly 1 GeV / fm 3 [13] is a necessary condition for the formation of deconfined matter of quarks and gluons, and thus it is of the utmost interest to understand the development of these energy densities across different collision systems. 2 Experimental set-up, data sample, analysis method, systematic uncertainties A detailed description of the ALICE detector and its performance can be found elsewhere [14,15] . The present analysis uses the Silicon Pixel Detector (SPD) to determine the pseudorapidity densities in the range − 2 < η < 2 and the Forward Multiplicity Detector (FMD) in the ranges − 3.5 < η < − 1.8 and 1.8 < η < 5 . The V0, comprised of two plastic scintillator discs covering − 3.7 < η < − 1.7 (V0C) and 2.8 < η < 5.1 (V0A), and the ZDC, two zero-degree calorimeters located 112.5 m from the interaction point, measurements determine the collision centrality and are used for offline event selection [2] . The results presented are based on data from collisions at a centre-of-mass energy per nucleon pair of s NN = 5.02 TeV as collected by ALICE during LHC Run 1 (2013) for p Pb, and during Run 2 (2015) for pp and Pb Pb. The FMD suffered high levels of background noise during the 2016 p Pb campaign, due to the high collision rate, and this data is therefore not used for the present analysis. About 105 events with a minimum bias trigger requirement [2] were analysed in the centrality range from 0% to 90% and 0% to 100% of the visible cross section for Pb Pb and p Pb collisions, respectively. The minimum bias trigger for p Pb and Pb Pb collisions in ALICE was defined as a coincidence between the V0A and V0C sides of the V0 detector. The data from the p Pb collisions were taken in two beam configurations: one where the lead ion travelled toward positive pseudorapidity and one where it travelled toward negative pseudorapidity. The results from the latter collisions are mirrored around η = 0 . The centre-of-mass frame in p Pb collisions does not coincide with the laboratory frame, due to the single magnetic field in the LHC, and thus the rapidity of the centre-of-mass is y CM = ± 0.465 for the two directions, respectively, in the laboratory frame. For this reason, pseudorapidity, calculated with respect to the laboratory frame, is denoted η lab whenever p Pb results are presented. Likewise, for the pp collisions, about 105 events with coincidence between V0A and V0C and at least one charged particle in | η | < 1 were analysed. By requiring at least one charged particle at midrapidity, the so-called INEL>0 event class, the systematic uncertainty, related to the absolute normalisation to the full inelastic cross section, is reduced, while still sampling a large fraction (> 75 % ) of the hadronic cross section [16,17] . The standard ALICE event selection [18] and centrality estimator based on the V0 amplitude [19,20] are used in this analysis. The event selection consists of: a) exclusion of background events using the timing information from the ZDC (for Pb Pb and p Pb, e.g., beam–gas interactions) and V0 detectors, b) verification of the trigger conditions, and c) a reconstructed position of the collision (primary vertex). In Pb Pb collisions, centrality is obtained from the sum amplitude in both V0 detector arrays (V0M). For p Pb only the amplitude in the array on the lead-going side (V0A or V0C) is used. In Pb Pb collisions, the 10% most peripheral collisions have substantial contributions from electromagnetic processes and are therefore not included in the results presented here [19] . A primary charged particle is defined as a charged particle with a mean proper lifetime τ larger than 1 cm / c , which is either a) produced directly in the interaction, or b) from decays of particles with τ smaller than 1 cm / c [21] . All quantities reported here are for primary, charged particles, though “primary” is omitted in the following for brevity. The analysis method is identical to that of previous publications [2] : the measurement of the charged-particle pseudorapidity density at midrapidity is obtained from counting particle trajectories determined using the two layers of the SPD. The SPD has a lower transverse momentum acceptance of 50 MeV / c , and the yield is extrapolated down to p T = 0 MeV / c via simulations. In the forward regions, the measurement is provided by the analysis of the deposited energy signal in the FMD and a statistical method is employed to calculate the inclusive number of charged particles. A data-driven correction [22] , based on separate measurements exploiting displaced collision vertices, is applied to remove the background from secondary particles. Systematic uncertainty estimations for the midrapidity measurements are detailed elsewhere [2,16,20] , and are from background suppression, transverse momentum extrapolation, weak decays, and simulations. The estimates are obtained through variation of thresholds and simulation studies. For pp (p Pb), the total systematic uncertainty amounts to 1.5% (2.7%) over the whole pseudorapidity range; while for Pb Pb the total systematic uncertainty is 2.6% at η = 0 and 2.9% at | η | = 2 . The systematic uncertainty is mostly correlated over pseudorapidity for | η | < 2 , and largely independent of centrality. The uncertainty in the forward region, estimated via variations of thresholds and simulation studies, is the same for all collision systems and is uncorrelated across η , amounting to 6.9% for η > 3.5 and 6.4% elsewhere within the forward regions [22] . In the figures of this letter, uncorrelated, local in pseudorapidity, systematic uncertainties are indicated by open boxes on the data points, while correlated systematic uncertainties, those that affect the overall scale and typically from event classification and selection, are indicated by filled boxes to the right of the data. The systematic uncertainty on d N ch / d η , due to the centrality class definition in Pb Pb, is estimated to vary from 0.6% for the most central to 9.5% for the most peripheral class [23] . The 80% to 90% centrality class has residual contamination from electromagnetic processes as detailed elsewhere [19] , which gives rise to an additional 4% systematic uncertainty in the measurements. No overall systematic uncertainty has been estimated for p Pb collisions, as the centrality selection in that collision system is inherently difficult to map to the underlying dynamics of the collisions [20] . 3 Results Fig. 1 shows the measured pseudorapidity densities in pp, and in central p Pb, and the previously published results for Pb Pb [2] collisions at s NN = 5.02 TeV for primary particles.For the 5% most central Pb Pb collisions d N ch / d η ≈ 2000 at midrapidity (η = 0 ) [2] , while for p Pb collisions the distribution peaks at d N ch / d η lab ≈ 60 around η lab = 3 in the lead-going direction (η > 0 ). For pp collisions with the INEL>0 trigger condition discussed above, d N ch / d η = 5.7 ± 0.2 at midrapidity, consistent with previous results derived from p T spectra [24] . Fig. 2 shows, as a function of centrality, the measured charged-particle pseudorapidity densities for p Pb collisions at s NN = 5.02 TeV . The strategy of centrality selection for proton on nucleus reactions is explained elsewhere [20] . The ALICE Collaboration has previously presented d N ch / d η for Pb Pb collisions at this energy [2] .In Fig. 3 , the charged-particle pseudorapidity densities in p Pb and Pb Pb reactions are divided by the pp distributions corresponding to the INEL>0 trigger class. The ratio is r X = ( d N ch / d η | X ) / ( d N ch / d η | pp ) , where X labels p Pb and Pb Pb collisions, in centrality classes, as a function of pseudorapidity. In the ratios, systematic uncertainties, of common origin, are partially cancelled, and, as an estimate, the magnitude of the resulting systematic uncertainties are given only by the uncertainties in the d N ch / d η | X measurements, since the uncertainties are independent of the collision system. In p Pb collisions the rapidity of the centre-of-mass is non-zero, which is not taken into account in the ratios. Such a correction would require prior determination of the full Jacobian of the transformation from pseudorapidity to rapidity, which is not possible to perform reliably with the ALICE apparatus. The ratio of the p Pb relative to the pp distributions increases with pseudorapidity from the p-going to the Pb-going direction for central collisions, which Brodsky et al. and Adil et al. [25,26] suggest is a sign of scaling of the pp distribution with the increasing number of participants as the lead nucleus is probed by the incident proton, and thus independent proton–nucleon scatterings on the lead-ion side. A similar scaling, however, does not hold for the Pb Pb reaction. The ratios cannot be obtained by simple scaling of the elementary pp distributions. Instead, the ratio of the Pb Pb relative to the pp distributions exhibits an enhancement of particle production around midrapidity for the more central collisions which is indicative of the formation of the sQGP [7] . Likewise, r pPb increases for all but the two most peripheral centrality classes as η lab → 3 . In Pb Pb collisions it is seen that the various mechanisms behind the pseudorapidity distributions are more transversely directed than in pp collisions by the increase of r PbPb as | η | → 0 4 Rapidity and energy-density dependence on system size and discussion It has been shown that the charged-particle rapidity density (d N ch / d y ) in Pb Pb collisions, to a good accuracy, follows a normal distribution over the considered rapidity interval (| y | ≲ 5 ) [2,27] . Those results relied on calculating the average Jacobian d N ch / d y = 〈 J 〉 = 〈 β 〉 using the full p T spectra, at midrapidity, of charged pions and kaons as well as protons and antiprotons. Here, we use the approximationy ≈ η − 1 2 m 2 p T 2 cos ϑ , where ϑ is the polar angle of emission, and identify a = p T / m with an effective ratio of transverse momentum over mass. With this, the effective Jacobian can be written asJ ′ ( η , a ) = ( 1 + 1 a 2 1 cosh 2 η ) − 1 / 2 . We further make the ansatz that d N ch / d y is normal distributed for symmetric collision systems (pp and Pb Pb), so that d N ch / d η can be parameterised as(1) f ( η ; A , a , σ ) = J ′ ( η , a ) A 1 2 π σ exp ( − y 2 { η , a } 2 σ 2 ) , where A and σ are the total integral and width of the distribution, respectively, and y the rapidity in the centre-of-mass frame. Motivated by the observed approximate linearity of r pPb (see lower panel of Fig. 3 ), we replace A with ( α y + A ) for the asymmetric system (p Pb) and parameterise d N ch / d η lab as(2) g ( η ; A , a , α , σ ) = J ′ ( η , a ) ( α y { η , a } + A ) × 1 2 π σ exp ( − [ y { η , a } − y CM ] 2 2 σ 2 ) . The functions f and g defined in Eq. (1) and Eq. (2) , respectively, describe the measurements within the measured region with χ 2 per degrees of freedom (ν ) in the range of 0.1 to 0.5. The small χ 2 / ν values are a consequence of the relatively large uncorrelated systematic uncertainties on the measurements. That is, the charged-particle distributions for pp, p Pb, and Pb Pb collisions at s NN = 5.02 TeV follow a normal distribution in rapidity, with free parameters A , a , σ , and α in the asymmetric case. The top panel of Fig. 4 shows the best-fit parameter values of the normal width (σ d N ch / d y ) for all three collision systems as a function of the average number of participating nucleons (〈 N part 〉 ) calculated using a Glauber model [28] . The best-fit parameters are found taking statistical and uncorrelated systematic uncertainties into account. The result using the above procedure, for the most central Pb Pb collisions, is found to be compatible with previous results extracted by unfolding with the mean Jacobian estimated from transverse momentum spectra [2] . The open points (crosses) and dashed lines on the figure are from evaluations of Eq. (1) and Eq. (2) , and direct calculations of σ d N ch / d y , respectively, using model calculations with EPOS-LHC [29] . EPOS-LHC was chosen as it provides predictions for all three collision systems. The parameterisation, in terms of the two functions, of this model calculation generally reproduces the widths of the charged-particle rapidity densities, except in the asymmetric case where a direct evaluation of the standard deviation is less motivated. The general trend is that the widths decrease as 〈 N part 〉 increases, consistent with the behaviour of the r PbPb ratios. Notably, the width of the d N ch / d y distributions in p Pb and Pb Pb, for low number of participant nucleons in the collisions, approaches the width of the pp distribution, which, presumably, is dominated by kinematic and phase space constraints. The lower panel of Fig. 4 shows the dependence of a on the average number of participants. The right-hand ordinate is the same, but multiplied by the average mass 〈 m 〉 = ( 0.215 ± 0.001 ) GeV / c 2 estimated from measurements of identified particles in Pb Pb collisions at s NN = 2.76 TeV [30] . To better understand the parameter a , this parameter extracted from the EPOS-LHC calculations, using the above procedure, is also shown in the figure. The dotted lines show the average p T / m predicted by EPOS-LHC [29] . The EPOS-LHC calculations indicate that the extracted effective transverse momentum to mass ratio a is consistently smaller than the ratio of the average transverse momentum to the average mass. Thus a gives a lower bound on 〈 p T 〉 / 〈 m 〉 . We can estimate the energy density that is reached in the collisions as a function of the number of participants for the three systems. A conventional approach is to use the model originally proposed by Bjorken [12] in which the energy density (ε Bj ) depends on the rapidity density of particles and the volume of a longitudinal cylinder with cross sectional area determined by the overlap between the colliding partners and length determined by a characteristic particle formation timeε Bj = 1 c τ 1 S T 〈 d E T d y 〉 . Here, S T ≈ π R 2 ≈ π N part 2 / 3 is the transverse area spanned by the participating nucleons, d E T / d y is the transverse-energy rapidity density, and τ is the formation time. While a formation time of τ = 1 fm / c is often assumed, it is left as a free parameter here. With 〈 m T 〉 = 〈 m 〉 1 + ( 〈 p T 〉 / 〈 m 〉 ) 2 , the transverse-energy rapidity density can be approximated by〈 d E T d y 〉 ≈ 〈 m T 〉 1 f total d N ch d y = 〈 m 〉 1 + ( 〈 p T 〉 〈 m 〉 ) 2 1 f total d N ch d y , where f total = 0.55 ± 0.01 , the ratio of charged particles to all particles [31] , accounts for neutral particles not measured in the experiment, and is assumed the same for all collision systems. Substituting the derived d N ch / d y and the effective a = p T / m ≲ 〈 p T 〉 / 〈 m 〉 results in a lower bound estimate for the Bjorken energy density (ε LB )(3) ε Bj τ ≥ ε LB τ = 1 c 1 S T 〈 m 〉 1 + a 2 1 f total 1 + 1 a 2 1 cosh 2 η d N ch d η , where a and 〈 m 〉 are as in the top panel of Fig. 4 . The transverse area S T is estimated in a numerical Glauber model [32,33] as shown in Fig. 5 . We consider two extremes for the transverse area spanned by the participating nucleons: a) the exclusive (or direct) overlap between participating nucleons, ∩ and open markers in Fig. 5 , and b) the inclusive (or full) area of all participating nucleons, ∪ and full markers in Fig. 5 . Fig. 6 shows the lower-bound energy density estimate, ε LB τ ≤ ε Bj τ , as a function of the number of participants, which reaches values between 10 and 20 GeV / ( fm 2 c ) in the most central Pb Pb collisions. The uncertainties are from standard error propagation of Eq. (3) of uncertainties on the best-fit parameter values, the number of participants, mean mass, and f total . A rise from roughly 1 GeV / ( fm 2 c ) to over 10 GeV / ( fm 2 c ) is observed if the transverse area is assumed to be the inclusive area of participating nucleons. This trend is illustrated by a power-law (C N part p ) fit to the data in the figure, with the parameter values C = ( 0.8 ± 0.3 ) GeV / ( fm 2 c ) and p = 0.44 ± 0.08 . On the other hand, if the transverse area is assumed to be the smaller exclusive overlap area, we observe a substantially larger lower bound on the energy density, but a less dramatic increase with increasing number of participating nucleons. Also shown in the figure are estimates of the Bjorken energy density ε Bj τ for Pb Pb reactions at s NN = 2.76 TeV [31] . These results where obtained from measurements of the transverse energy in the collisions and using the inclusive estimate of the transverse area S T . The trend of the s NN = 5.02 TeV results is similar to these earlier results. Bearing in mind that for the largest LHC collision energy we show a lower bound estimate of the energy density in Fig. 6 , we find a likely overall increase in the energy density from s NN = 2.76 TeV to 5.02 TeV .5 Summary and conclusions We have measured the charged particle pseudorapidity density in pp, p Pb, and Pb Pb collisions at s NN = 5.02 TeV over the widest possible pseudorapidity range available at the LHC. The distributions where determined using the same experimental apparatus and methods, and systematic uncertainties have been minimised to within the capabilities of the set-up. While the particle production in central Pb Pb collisions clearly exhibits an enhancement as compared to pp collisions, particle production in p Pb collisions is consistent with dominantly incoherent nucleon–nucleon collisions. By transforming the measured pseudorapidity distributions to rapidity distributions we have obtained systematic trends for the width of the rapidity distributions and a lower bound on the energy density, which shows a clear scaling behaviour as a function of the average number of participant nucleons. The decreasing width of the deduced rapidity distributions with increasing participant number suggests that the kinematic spread of particles, including longitudinal degrees of freedom, is reduced due to interactions in the early stages of the collisions. This is also reflected in the accompanying growth of the energy density. Both observations are consistent with the gradual establishment of a high-density phase of matter with increasing size of the collision domain. Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgements The ALICE Collaboration would like to thank all its engineers and technicians for their invaluable contributions to the construction of the experiment and the CERN accelerator teams for the outstanding performance of the LHC complex. The ALICE Collaboration gratefully acknowledges the resources and support provided by all Grid centres and the Worldwide LHC Computing Grid (WLCG) collaboration. The ALICE Collaboration acknowledges the following funding agencies for their support in building and running the ALICE detector: A. I. Alikhanyan National Science Laboratory (Yerevan Physics Institute) Foundation (ANSL) , State Committee of Science and World Federation of Scientists (WFS), Armenia; Austrian Academy of Sciences , Austrian Science Fund (FWF): [M 2467-N36 ] and Nationalstiftung für Forschung, Technologie und Entwicklung , Austria; Ministry of Communications and High Technologies, National Nuclear Research Center , Azerbaijan; Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq ), Financiadora de Estudos e Projetos (Finep), Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP) and Universidade Federal do Rio Grande do Sul (UFRGS), Brazil; Ministry of Education of China (MOEC), Ministry of Science & Technology of China (MSTC) and National Natural Science Foundation of China (NSFC), China; Ministry of Science and Education and Croatian Science Foundation , Croatia; Centro de Aplicaciones Tecnológicas y Desarrollo Nuclear (CEADEN), Cubaenergía , Cuba; Ministry of Education, Youth and Sports of the Czech Republic , Czech Republic; The Danish Council for Independent Research | Natural Sciences , the Villum Fonden and Danish National Research Foundation (DNRF), Denmark; Helsinki Institute of Physics (HIP), Finland; Commissariat à l'Energie Atomique (CEA ) and Institut National de Physique Nucléaire et de Physique des Particules (IN2P3) and Centre National de la Recherche Scientifique (CNRS), France; Bundesministerium für Bildung und Forschung (BMBF) and GSI Helmholtzzentrum für Schwerionenforschung GmbH , Germany; General Secretariat for Research and Technology , Ministry of Education, Research and Religions, Greece; National Research, Development and Innovation Office , Hungary; Department of Atomic Energy Government of India (DAE ), Department of Science and Technology , Government of India (DST), University Grants Commission , Government of India (UGC) and Council of Scientific and Industrial Research (CSIR), India; Indonesian Institute of Sciences , Indonesia; Istituto Nazionale di Fisica Nucleare (INFN ), Italy; Japanese Ministry of Education, Culture, Sports, Science and Technology (MEXT) and Japan Society for the Promotion of Science (JSPS) KAKENHI, Japan; Consejo Nacional de Ciencia (CONACYT) y Tecnología, through Fondo de Cooperación Internacional en Ciencia y Tecnología (FONCICYT) and Dirección General de Asuntos del Personal Académico (DGAPA), Mexico; Nederlandse Organisatie voor Wetenschappelijk Onderzoek (NWO), Netherlands; The Research Council of Norway , Norway; Commission on Science and Technology for Sustainable Development in the South (COMSATS), Pakistan; Pontificia Universidad Católica del Perú , Peru; Ministry of Education and Science , National Science Centre and WUT ID-UB , Poland; Korea Institute of Science and Technology Information and National Research Foundation of Korea (NRF), Republic of Korea; Ministry of Education and Scientific Research , Institute of Atomic Physics , Ministry of Research and Innovation and Institute of Atomic Physics and University Politehnica of Bucharest , Romania; Joint Institute for Nuclear Research (JINR), Ministry of Education and Science of the Russian Federation , National Research Centre Kurchatov Institute , Russian Science Foundation and Russian Foundation for Basic Research , Russia; Ministry of Education, Science, Research and Sport of the Slovak Republic , Slovakia; National Research Foundation of South Africa, South Africa; Swedish Research Council (VR) and Knut and Alice Wallenberg Foundation (KAW), Sweden; European Organization for Nuclear Research , Switzerland; Suranaree University of Technology (SUT), National Science and Technology Development Agency (NSTDA), Thailand Science Research and Innovation (TSRI) and National Science, Research and Innovation Fund (NSRF), Thailand; Turkish Energy, Nuclear and Mineral Research Agency (TENMAK), Turkey; National Academy of Sciences of Ukraine , Ukraine; Science and Technology Facilities Council (STFC), United Kingdom; National Science Foundation of the United States of America (NSF ) and United States Department of Energy, Office of Nuclear Physics (DOE NP ), United States of America. 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-PLB 137751 137751 S0370-2693(23)00085-0 10.1016/j.physletb.2023.137751 The Author(s) Astrophysics and Cosmology Cosmological stability in f ( ϕ , G ) gravity Shinji Tsujikawa tsujikawa@waseda.jp Department of Physics, Waseda University, 3-4-1 Okubo, Shinjuku, Tokyo 169-8555, Japan Department of Physics Waseda University 3-4-1 Okubo Tokyo Shinjuku 169-8555 Japan Department of Physics, Waseda University, 3-4-1 Okubo, Shinjuku, Tokyo 169-8555, Japan Editor: M. Trodden Abstract In gravitational theories where a canonical scalar field ϕ with a potential V ( ϕ ) is coupled to a Gauss-Bonnet (GB) term G with the Lagrangian f ( ϕ , G ) , we study the cosmological stability of tensor and scalar perturbations in the presence of a perfect fluid. We show that, in decelerating cosmological epochs with a positive tensor propagation speed squared, the existence of nonlinear functions of G in f always induces Laplacian instability of a dynamical scalar perturbation associated with the GB term. This is also the case for f ( G ) gravity, where the presence of nonlinear GB functions f ( G ) is not allowed during the radiation- and matter-dominated epochs. A linearly coupled GB term with ϕ of the form ξ ( ϕ ) G can be consistent with all the stability conditions, provided that the scalar-GB coupling is subdominant to the background cosmological dynamics. Data availability No data was used for the research described in the article. 1 Introduction General Relativity (GR) is a fundamental theory of gravity whose validity has been probed in Solar System experiments [1] and submillimeter laboratory tests [2,3] . Despite the success of GR describing gravitational interactions in the Solar System, there have been long-standing cosmological problems such as the origins of inflation, dark energy, and dark matter. To address these problems, one typically introduces additional degrees of freedom (DOFs) beyond those appearing in GR [4–10] . One of such new DOFs is a canonical scalar field ϕ with a potential V ( ϕ ) [11–22] . If the scalar field evolves slowly along the potential, it is possible to realize cosmic acceleration responsible for inflation or dark energy. An oscillating scalar field around the potential minimum can be also the source for dark matter. The other way of introducing a new dynamical DOF is to modify the gravitational sector from GR. The Lagrangian in GR is given by an Einstein-Hilbert term M Pl 2 R / 2 , where M Pl is the reduced Planck mass and R is the Ricci scalar. If we consider theories containing nonlinear functions of R of the form f ( R ) , there is one scalar DOF arising from the modification of gravity [23,24] . One well known example is the Starobinsky's model, in which the presence of a quadratic curvature term R 2 drives cosmic acceleration [25] . It is also possible to construct f ( R ) models of late-time cosmic acceleration [26–32] , while being consistent with local gravity constraints. The Einstein tensor G μ ν obtained by varying the Einstein-Hilbert action satisfies the conserved relation ∇ μ G μ ν = 0 (∇ μ is a covariant derivative operator), with the property of second-order field equations of motion in metrics. If we demand such conserved and second-order properties for 2-rank symmetric tensors, GR is the unique theory of gravity in 4 dimensions [33] . In spacetime dimensions higher than 4, there is a particular combination known as a Gauss-Bonnet (GB) term G consistent with those demands [34] . In 4 dimensions, the GB term is a topological surface term and hence it does not contribute to the field equations of motion. In the presence of a coupling between a scalar field ϕ and G of the form ξ ( ϕ ) G , the spacetime dynamics is modified by the time or spatial variation of ϕ . Indeed, this type of scalar-GB coupling appears in the context of low energy effective string theory [35–37] . The cosmological application of the coupling ξ ( ϕ ) G has been extensively performed in the literature [38–63] . Moreover, it is known that the same coupling gives rise to spherically symmetric solutions of hairy black holes and neutron stars [64–80] . The Lagrangian f ( G ) containing nonlinear functions of G also generates nontrivial contributions to the spacetime dynamics [81–90] . In Ref. [91] , De Felice and Suyama studied the stability of scalar perturbations in f ( R , G ) gravity on a spatially flat Friedmann-Lemaître-Robertson-Walker (FLRW) background. In theories with f , R G 2 − f , R R f , G G ≠ 0 , where f , R G = ∂ 2 f / ∂ G ∂ R , f , R R = ∂ 2 f / ∂ R 2 , and f , G G = ∂ 2 f / ∂ G 2 , there is an unusual scale-dependent sound speed which propagates superluminally in the short-wavelength limit, unless the vacuum is in a de Sitter state (see also Ref. [92] for the analysis in an anisotropic cosmological background). We note that this problem does not arise for f ( R ) gravity or M Pl 2 R / 2 + f ( G ) gravity. In Ref. [93] , the same authors extended the analysis to a more general Lagrangian f ( ϕ , R , G ) with a canonical scalar field ϕ and showed that the property of the scale-dependent sound speed is not modified by the presence of ϕ . Taking a perfect fluid (radiation or nonrelativistic matter) into account in f ( R , G ) gravity, the cosmological stability and evolution of matter perturbations were studied in Refs. [94–96] . In Einstein-scalar-GB gravity given by the Lagrangian M Pl 2 R / 2 + f ( ϕ , G ) , where ϕ is a canonical scalar field, the problem of scale-dependent sound speeds mentioned above is not present. In this theory, the propagation of scalar perturbations on the flat FLRW background was studied in Ref. [93] without taking into account matter. While the sound speed associated with the field ϕ is luminal for theories with f , G G ≠ 0 , the propagation speed squared c s 2 arising from a nonlinear GB term deviates from that of light and it can be even negative. In Ref. [93] , the authors discussed the possibility for satisfying the Laplacian stability condition c s 2 > 0 . In the presence of matter, however, the stability conditions are subject to modifications from those in the vacuum. To understand what happens for the dynamics of cosmological perturbations during radiation- and matter-dominated epochs, we need to study their stabilities by incorporating radiation or nonrelativistic matter. In this letter, we will derive general conditions for the absence of ghosts and Laplacian instabilities in M Pl 2 R / 2 + f ( ϕ , G ) gravity, where ϕ is a canonical scalar field with a potential V ( ϕ ) . In theories where the scalar field ϕ is coupled to the linear GB term, i.e., f ( ϕ , G ) = ξ ( ϕ ) G , there is only one dynamical scalar DOF ϕ . In theories with f , G G ≠ 0 , the Lagrangian f ( ϕ , G ) can be expressed in terms of two scalar fields ϕ and χ coupled to the linear GB term, where χ arises from the nonlinearity in G . Hence the latter theory has two dynamical scalar DOFs. To study the cosmological stability of f ( ϕ , G ) theories with f , G G ≠ 0 , we take a perfect fluid into account as a form of the Schutz-Sorkin action [97–99] . We will show that the squared sound speed arising from nonlinear functions of G is negative during decelerating cosmological epochs including radiation and matter eras. To reach this conclusion, we exploit the fact that the propagation speed squared c t 2 of tensor perturbations must be positive to avoid Laplacian instability of gravitational waves. The same Laplacian instability of scalar perturbations is also present in M Pl 2 R / 2 + f ( G ) gravity with any nonlinear function of G in f . We note that, in f ( G ) models of late-time cosmic acceleration, violent instabilities of matter density perturbations during the radiation and matter eras were reported in Ref. [100] . This can be regarded as the consequence of a negative sound speed squared of the scalar perturbation δ G arising from the nonlinearity of G in f . Since δ G is coupled to the matter perturbation δρ , the background cosmological evolution during the radiation and matter eras is spoiled by the rapid growth of δρ . Our analysis in this letter shows that similar catastrophic instabilities persist for more general scalar-GB couplings f ( ϕ , G ) with f , G G ≠ 0 . This letter is organized as follows. In Sec. 2 , we revisit cosmological stability conditions in M Pl 2 R / 2 + ξ ( ϕ ) G gravity with a canonical scalar field ϕ , which can be accommodated in a subclass of Horndeski theories with a single scalar DOF [101–104] . This is an exceptional case satisfying the condition f , G G = 0 , under which the Laplacian instability of scalar perturbations can be avoided. In Sec. 3 , we derive the background equations and stability conditions of tensor perturbations in M Pl 2 R / 2 + f ( ϕ , G ) gravity with f , G G ≠ 0 by incorporating a perfect fluid. In Sec. 4 , we proceed to the derivation of a second-order action of scalar perturbations and obtain conditions for the absence of ghosts and Laplacian instabilities in the scalar sector. In particular, we show that an effective cosmological equation of state w eff needs to be in the range w eff < − ( 2 + c t 2 ) / 6 to ensure Laplacian stabilities of the perturbation δ G . Sec. 5 is devoted to conclusions. 2 ξ ( ϕ ) G gravityWe first briefly revisit the cosmological stability in ξ ( ϕ ) G gravity given by the action(2.1) S = ∫ d 4 x − g [ M Pl 2 2 R + η X − V ( ϕ ) + ξ ( ϕ ) G ] + S m ( g μ ν , Ψ m ) , where g is a determinant of the metric tensor g μ ν , η is a constant, X = − ( 1 / 2 ) g μ ν ∇ μ ϕ ∇ ν ϕ is a kinetic term of the scalar field ϕ , V ( ϕ ) and ξ ( ϕ ) are functions of ϕ , and G is a GB term defined by(2.2) G ≡ R 2 − 4 R μ ν R μ ν + R μ ν ρ σ R μ ν ρ σ , with R μ ν and R μ ν ρ σ being the Ricci and Riemann tensors, respectively. For the matter action S m , we consider a perfect fluid minimally coupled to gravity. The action (2.1) contains one scalar DOF ϕ besides the matter field Ψ m . If we consider Horndeski theories [101] given by the action(2.3) S = ∫ d 4 x − g [ G 2 ( ϕ , X ) − G 3 ( ϕ , X ) □ ϕ + G 4 ( ϕ , X ) R + G 4 , X ( ϕ , X ) { ( □ ϕ ) 2 − ( ∇ μ ∇ ν ϕ ) ( ∇ μ ∇ ν ϕ ) } + G 5 ( ϕ , X ) G μ ν ∇ μ ∇ ν ϕ − 1 6 G 5 , X ( ϕ , X ) { ( □ ϕ ) 3 − 3 ( □ ϕ ) ( ∇ μ ∇ ν ϕ ) ( ∇ μ ∇ ν ϕ ) + 2 ( ∇ μ ∇ α ϕ ) ( ∇ α ∇ β ϕ ) ( ∇ β ∇ μ ϕ ) } ] + S m ( g μ ν , Ψ m ) , then the theory (2.1) can be accommodated by choosing the coupling functions [103] (2.4) G 2 ( ϕ , X ) = η X − V ( ϕ ) + 8 ξ , ϕ ϕ ϕ ϕ ( ϕ ) X 2 ( 3 − ln | X | ) , G 3 ( ϕ , X ) = 4 ξ , ϕ ϕ ϕ ( ϕ ) X ( 7 − 3 ln | X | ) , G 4 ( ϕ , X ) = M Pl 2 2 + 4 ξ , ϕ ϕ ( ϕ ) X ( 2 − ln | X | ) , G 5 ( ϕ , X ) = − 4 ξ , ϕ ( ϕ ) ln | X | , where we use the notations F , X = ∂ F / ∂ X and F , ϕ = ∂ F / ∂ ϕ for any arbitrary function F . Let us consider a spatially flat FLRW background given by the line element d s 2 = − d t 2 + a 2 ( t ) δ i j d x i d x j , where a ( t ) is a time-dependent scale factor. The perfect fluid has a density ρ and pressure P . The background equations as well as the perturbation equations in full Horndeski theories were derived in Refs. [103,105–107] . On using the correspondence (2.4) , the background equations of motion in theories given by the action (2.1) are(2.5) 3 q ˜ t H 2 = 1 2 η ϕ ˙ 2 + V ( ϕ ) + ρ , (2.6) 2 q ˜ t H ˙ = − η ϕ ˙ 2 − H 2 q ˜ t ( c ˜ t 2 − 1 ) − ρ − P , (2.7) η ( ϕ ¨ + 3 H ϕ ˙ ) + V , ϕ − ξ , ϕ G = 0 , (2.8) ρ ˙ + 3 H ( ρ + P ) = 0 , where H = a ˙ / a is the Hubble expansion rate, a dot represents the derivative with respect to t , and(2.9) q ˜ t = M Pl 2 + 8 ξ , ϕ H ϕ ˙ , (2.10) c ˜ t 2 = M Pl 2 + 8 ( ξ , ϕ ϕ ¨ + ξ , ϕ ϕ ϕ ˙ 2 ) M Pl 2 + 8 ξ , ϕ H ϕ ˙ , (2.11) G = 24 H 2 ( H 2 + H ˙ ) . In the presence of tensor perturbations h i j with the perturbed line element d s 2 = − d t 2 + a 2 ( t ) ( δ i j + h i j ) d x i d x j , the second-order action of traceless and divergence-free modes of h i j was already derived in full Horndeski theories [103,106,107] . In the current theory, the conditions for the absence of ghosts and Laplacian instabilities are(2.12) q ˜ t > 0 , (2.13) c ˜ t 2 > 0 , where q ˜ t and c ˜ t 2 are defined by Eqs. (2.9) and (2.10) , respectively. Note that q ˜ t determines the sign of a kinetic term of h i j , while c ˜ t 2 corresponds to the propagation speed squared of tensor perturbations. For the scalar sector, we choose the perturbed line element d s 2 = − ( 1 + 2 α ) d t 2 + 2 ∂ i B d t d x i + a 2 ( t ) δ i j d x i d x j in the flat gauge, where α and B are scalar metric perturbations. There is also a scalar-field perturbation δϕ besides the matter perturbation δρ and the fluid velocity potential v . After deriving the quadratic-order action of scalar perturbations, we can eliminate nondynamical variables α , B , and v from the action. Then, we are left with the two dynamical perturbations δϕ and δρ in the second-order action. In the short-wavelength limit, there is neither ghost nor Laplacian instability for δϕ under the conditions [103,106,107] (2.14) q ˜ s = 2 ( η q ˜ t + 96 H 4 ξ , ϕ 2 ) > 0 , (2.15) c ˜ s 2 = η q ˜ t − 32 ( 2 + c ˜ t 2 + 6 w eff ) H 4 ξ , ϕ 2 η q ˜ t + 96 H 4 ξ , ϕ 2 > 0 , where c ˜ s corresponds to the propagation speed of δϕ , and w eff is the cosmological effective equation of state defined by(2.16) w eff ≡ − 1 − 2 H ˙ 3 H 2 . The stability conditions for δρ are given by ρ + P > 0 and c m 2 > 0 , where c m 2 is the matter sound speed squared. Under the stability condition (2.12) with η > 0 , the scalar no-ghost condition (2.14) is satisfied. Let us consider the case in which contributions of the scalar-GB coupling are suppressed, such that(2.17) { | ξ , ϕ H ϕ ˙ | , | ξ , ϕ ϕ ¨ | , | ξ , ϕ ϕ ϕ ˙ 2 | } ≪ M Pl 2 , H 4 ξ , ϕ 2 ≪ η q ˜ t . Then, it follows that q ˜ t ≃ M Pl 2 , c ˜ t 2 ≃ 1 , q ˜ s ≃ 2 η M Pl 2 , and c ˜ s 2 ≃ 1 . In such cases, provided that η > 0 , all the stability conditions are consistently satisfied. If the scalar-GB coupling contributes to the late-time cosmological dynamics, there is an observational bound on c ˜ t constrained from the GW170817 event together with the electromagnetic counterpart, i.e., − 3 × 10 − 15 ≤ c ˜ t − 1 ≤ 7 × 10 − 16 [108] for the redshift z ≤ 0.009 . This translates to the limit(2.18) | ξ , ϕ ϕ ¨ + ξ , ϕ ϕ ϕ ˙ 2 − ξ , ϕ H ϕ ˙ | ≲ 10 − 15 M Pl 2 , which gives a tight constraint on the amplitude of ξ ( ϕ ) . In this case, contributions of the scalar-GB coupling to the background Eqs. (2.5) and (2.6) are highly suppressed relative to the field density ρ ϕ = η ϕ ˙ 2 / 2 + V ( ϕ ) and the matter density. The bound (2.18) is not applied to early cosmological epochs including inflation, radiation, and matter eras. We note, however, that the dominance of the scalar-GB coupling to the background equations prevents the successful cosmic expansion history. This can also give rise to the violation of either of the stability conditions (2.12) -(2.15) . Provided the scalar-GB coupling is suppressed in such a way that inequalities (2.17) hold, the linear stabilities are ensured for both tensor and scalar perturbations. 3 f ( ϕ , G ) gravityWe extend ξ ( ϕ ) G gravity to more general theories in which a canonical scalar field ϕ with a potential V ( ϕ ) is coupled to the GB term of the form f ( ϕ , G ) . The action in such theories is given by(3.1) S = ∫ d 4 x − g [ M Pl 2 2 R + η X − V ( ϕ ) + f ( ϕ , G ) ] + S m ( g μ ν , Ψ m ) , where a matter field Ψ m is minimally coupled to gravity. It is more practical to introduce a scalar field χ and resort to the following action(3.2) S = ∫ d 4 x − g [ M Pl 2 2 R + η X − U ( ϕ , χ ) + ξ ( ϕ , χ ) G ] + S m ( g μ ν , Ψ m ) , where(3.3) U ( ϕ , χ ) ≡ V ( ϕ ) − f ( ϕ , χ ) + χ ξ ( ϕ , χ ) , ξ ( ϕ , χ ) ≡ f , χ ( ϕ , χ ) , with the notation f , χ = ∂ f / ∂ χ . Varying the action (3.2) with respect to χ , it follows that(3.4) ( χ − G ) ξ , χ = 0 . So long as ξ , χ ≠ 0 , we obtain χ = G . In this case, the action (3.2) reduces to (3.1) . Thus, the equivalence of (3.2) with (3.1) holds for(3.5) f , G G ≠ 0 , under which there is a new scalar DOF χ arising from the gravitational sector. Theories with f , G G = 0 correspond to the coupling f = ξ ( ϕ ) G , in which case the cosmological stability conditions were already discussed in Sec. 2 . In ξ ( ϕ ) G gravity, we do not have the additional scalar DOF χ arising from G , so the term ξ , χ in Eq. (3.4) does not have the meaning of f , G G . Thus, the action (3.2) with the new dynamical DOF χ does not reproduce the action (2.1) in ξ ( ϕ ) G gravity. In the following, we will focus on theories with f , G G ≠ 0 , i.e., those containing the nonlinear dependence of G in f . For the matter field Ψ m , we incorporate a perfect fluid without a dynamical vector DOF. This matter sector is described by the Schutz-Sorkin action [97–99] (3.6) S m = − ∫ d 4 x [ − g ρ ( n ) + J μ ∂ μ ℓ ] , where the fluid density ρ is a function of its number density n . The vector field J μ is related to n according to the relation n = J μ J ν g μ ν / g , where u μ = J μ / ( n − g ) is the fluid four velocity. A scalar quantity ℓ in S m is a Lagrange multiplier, with the notation of a partial derivative ∂ μ ℓ = ∂ ℓ / ∂ x μ . Varying the matter action (3.6) with respect to ℓ and J μ , respectively, we obtain(3.7) ∂ μ J μ = 0 , (3.8) ∂ μ ℓ = u μ ρ , n , where ρ , n = d ρ / d n . 3.1 Background equations We derive the background equations of motion on the spatially flat FLRW background given by the line element(3.9) d s 2 = − N 2 ( t ) d t 2 + a 2 ( t ) δ i j d x i d x j , where N ( t ) is a lapse function. Since the fluid four velocity in its rest frame is given by u μ = ( N − 1 , 0 , 0 , 0 ) , the vector field J μ has components J μ = ( n a 3 , 0 , 0 , 0 ) . From Eq. (3.7) , we obtain(3.10) N 0 ≡ n a 3 = constant , which means that the total fluid number N 0 is conserved. This translates to the differential equation n ˙ + 3 H n = 0 , which can be expressed as a form of the continuity equation(3.11) ρ ˙ + 3 H ( ρ + P ) = 0 , where P is a fluid pressure defined by P = n ρ , n − ρ . On the background (3.9) , the total action (3.2) is expressed in the form(3.12) S = ∫ d t d 3 x [ η a 3 ϕ ˙ 2 2 N − 3 M Pl 2 a a ˙ 2 N − N a 3 U ( ϕ , χ ) − 8 a ˙ 3 ξ ˙ ( ϕ , χ ) N 3 − N a 3 ρ − n a 3 ℓ ˙ ] . From Eq. (3.8) , we obtain the following relation(3.13) ℓ ˙ = − N ρ , n . Varying the action (3.12) with respect to N , a , ϕ , χ respectively and setting N = 1 at the end, we obtain the background equations of motion(3.14) 3 q t H 2 = 1 2 η ϕ ˙ 2 + U ( ϕ , χ ) + ρ , (3.15) 2 q t H ˙ = − η ϕ ˙ 2 − H 2 q t ( c t 2 − 1 ) − ρ − P , (3.16) η ( ϕ ¨ + 3 H ϕ ˙ ) + V , ϕ − f , ϕ = 0 , (3.17) χ = G = 24 H 2 ( H 2 + H ˙ ) , where(3.18) q t = M Pl 2 + 8 H ( ξ , ϕ ϕ ˙ + ξ , χ χ ˙ ) , (3.19) c t 2 = M Pl 2 + 8 ( ξ , ϕ ϕ ¨ + ξ , ϕ ϕ ϕ ˙ 2 + ξ , χ χ ¨ + ξ , χ χ χ ˙ 2 + 2 ξ , ϕ χ ϕ ˙ χ ˙ ) M Pl 2 + 8 H ( ξ , ϕ ϕ ˙ + ξ , χ χ ˙ ) . We recall that the perfect fluid obeys the continuity Eq. (3.11) . We notice that Eqs. (3.14) -(3.16) are of similar forms to Eqs. (2.5) -(2.7) in ξ ( ϕ ) G gravity, but the expressions of q t and c t 2 are different from q ˜ t and c ˜ t 2 , respectively, because of the appearance of time derivatives of χ . These χ derivatives do not vanish for ξ , χ ≠ 0 , i.e., for f , G G ≠ 0 . As we will show in Sec. 4 , nonlinearities of G in f are responsible for the appearance of a new scalar propagating DOF δχ . 3.2 Stabilities in the tensor sector We proceed to the derivation of stability conditions for tensor perturbations in theories given by the action (3.2) . The perturbed line element including the tensor perturbation h i j is(3.20) d s 2 = − d t 2 + a 2 ( t ) ( δ i j + h i j ) d x i d x j , where we impose the traceless and divergence-free gauge conditions h i i = 0 and ∂ i h i j = 0 . For the gravitational wave propagating along the z direction, nonvanishing components of h i j are expressed in the form(3.21) h 11 = h 1 ( t , z ) , h 22 = − h 1 ( t , z ) , h 12 = h 21 = h 2 ( t , z ) , where the two polarized modes h 1 and h 2 are functions of t and z . The second-order action arising from the matter action (3.6) can be expressed as(3.22) ( S m ( 2 ) ) t = − ∫ d t d 3 x ∑ i = 1 2 1 2 a 3 P h i 2 , where P can be eliminated by using the background Eq. (3.15) . Expanding the total action (3.2) up to quadratic order in tensor perturbations and integrating it by parts, the resulting second-order action reduces to(3.23) S t ( 2 ) = ∫ d t d 3 x ∑ i = 1 2 a 3 4 q t [ h ˙ i 2 − c t 2 a 2 ( ∂ h i ) 2 ] , where ( ∂ h i ) 2 = ( ∂ h i / ∂ z ) 2 . We recall that q t and c t 2 are given by Eqs. (3.18) and (3.19) , respectively. To avoid the ghost and Laplacian instabilities in the tensor sector, we require the two conditions q t > 0 and c t 2 > 0 , which translate to(3.24) M Pl 2 + 8 H ( ξ , ϕ ϕ ˙ + ξ , χ χ ˙ ) > 0 , (3.25) M Pl 2 + 8 ( ξ , ϕ ϕ ¨ + ξ , ϕ ϕ ϕ ˙ 2 + ξ , χ χ ¨ + ξ , χ χ χ ˙ 2 + 2 ξ , ϕ χ ϕ ˙ χ ˙ ) > 0 . In f ( G ) gravity without the scalar field ϕ , tensor stability conditions can be obtained by setting ϕ ˙ = 0 and ϕ ¨ = 0 in Eqs. (3.24) and (3.25) . We vary the action (3.23) with respect to h i (with i = 1 , 2 ) in Fourier space with a comoving wavenumber k . Then, we obtain the tensor perturbation equation of motion(3.26) h ¨ i + ( 3 H + q ˙ t q t ) h ˙ i + c t 2 k 2 a 2 h i = 0 , where k = | k | . Since ξ = f , χ = f , G , the G dependence in f leads to the modified evolution of gravitational waves in comparison to GR. If the energy densities of ϕ and χ are relevant to the late-time cosmological dynamics after the matter dominance, the observational constraint on the tensor propagation speed c t arising from the GW170817 event [108] (| c t − 1 | ≲ 10 − 15 ) gives a tight bound on the scalar-GB coupling f ( ϕ , G ) . Such a stringent limit is not applied to the cosmological dynamics in the early Universe, but the conditions (3.24) and (3.25) need to be still satisfied. 4 Stabilities of f ( ϕ , G ) gravity in the scalar sector In this section, we will derive conditions for the absence of scalar ghosts and Laplacian instabilities in theories given by the action (3.2) . A perturbed line element containing scalar perturbations α , B , ζ , and E is of the form(4.1) d s 2 = − ( 1 + 2 α ) d t 2 + 2 ∂ i B d t d x i + a 2 ( t ) [ ( 1 + 2 ζ ) δ i j + 2 ∂ i ∂ j E ] d x i d x j . For the scalar fields ϕ and χ , we consider perturbations δϕ and δχ on the background values ϕ ¯ ( t ) and χ ¯ ( t ) , respectively, such that(4.2) ϕ = ϕ ¯ ( t ) + δ ϕ ( t , x ) , χ = χ ¯ ( t ) + δ χ ( t , x ) , where we will omit a bar from the background quantities in the following. In the matter sector, the temporal and spatial components of J μ are decomposed into the background and perturbed parts as(4.3) J 0 = N 0 + δ J , J i = 1 a 2 ( t ) δ i k ∂ k δ j , where δJ and δj are scalar perturbations. In terms of the velocity potential v , the spatial component of fluid four velocity is expressed as u i = − ∂ i v . From Eq. (3.8) , the scalar quantity ℓ has a background part obeying the relation ℓ ˙ = − ρ , n besides a perturbation − ρ , n v . Then, we have(4.4) ℓ = − ∫ t ρ , n ( t ˜ ) d t ˜ − ρ , n v . Defining the matter density perturbation(4.5) δ ρ ≡ ρ , n a 3 [ δ J − N 0 ( 3 ζ + ∂ 2 E ) ] , the fluid number density n has a perturbation [107,109] (4.6) δ n = δ ρ ρ , n − ( N 0 ∂ χ + ∂ δ j ) 2 2 N 0 a 5 − ( 3 ζ + ∂ 2 E ) δ ρ ρ , n − N 0 ( ζ + ∂ 2 E ) ( 3 ζ − ∂ 2 E ) 2 a 3 , up to second order. The matter sound speed squared is given by(4.7) c m 2 = P , n ρ , n = n ρ , n n ρ , n . Expanding (3.6) up to quadratic order in perturbations, we obtain the second-order matter action same as that derived in Refs. [107,109] . Varying this matter action with respect to δj leads to(4.8) ∂ δ j = − a 3 n ( ∂ v + ∂ B ) , whose relation will be used to eliminate δj . In the following, we choose the gauge(4.9) E = 0 , under which a scalar quantity ξ associated with the spatial gauge transformation x i → x i + δ i j ∂ j ξ is fixed. A scalar quantity ξ 0 associated with the temporal part of the gauge transformation t → t + ξ 0 can be fixed by choosing a flat gauge (ζ = 0 ) or a unitary gauge (δ ϕ = 0 ). We do not specify the temporal gauge condition in deriving the second-order action, but we will do so at the end. Expanding the total action (3.2) up to quadratic order in scalar perturbations and integrating it by parts, the resulting second-order action is given by(4.10) S s ( 2 ) = ∫ d t d 3 x ( L flat + L ζ ) , where(4.11) L flat = a 3 [ η 2 δ ϕ ˙ 2 − η 2 ( ∂ δ ϕ ) 2 a 2 + 1 2 ( f , ϕ ϕ − V , ϕ ϕ ) δ ϕ 2 + { 1 2 η ϕ ˙ 2 − 3 H 2 ( 2 q t − M Pl 2 ) } α 2 − H ( 3 q t − M Pl 2 ) ∂ 2 B a 2 α − C 4 16 H 2 δ χ 2 + { C 1 δ ϕ ˙ + C 2 δ ϕ − C 3 ∂ 2 δ ϕ a 2 + 3 H C 4 δ χ ˙ − C 4 ∂ 2 δ χ a 2 + 3 ( H C 5 − H ˙ C 4 ) δ χ } α + ( C 3 δ ϕ ˙ + C 6 δ ϕ + C 4 δ χ ˙ + C 5 δ χ ) ∂ 2 B a 2 + ( ρ + P ) v ∂ 2 B a 2 − v δ ρ ˙ − 3 H ( 1 + c m 2 ) v δ ρ − 1 2 ( ρ + P ) ( ∂ v ) 2 a 2 − c m 2 2 ( ρ + P ) δ ρ 2 − α δ ρ ] , (4.12) L ζ = a 3 [ { 3 H ( 3 q t − M Pl 2 ) α − 3 ( C 3 δ ϕ ˙ + C 6 δ ϕ + C 4 δ χ ˙ + C 5 δ χ ) − 3 ( ρ + P ) v + 2 q t ∂ 2 B a 2 } ζ ˙ − 3 q t ζ ˙ 2 + q t c t 2 ( ∂ ζ ) 2 a 2 − 2 { q t α + ( 1 + H ˙ H 2 ) ( C 3 δ ϕ + C 4 δ χ ) } ∂ 2 ζ a 2 ] , where q t and c t 2 are given by Eqs. (3.18) and (3.19) , respectively, and(4.13) C 1 = 24 H 3 ξ , ϕ − η ϕ ˙ , C 2 = − 24 H 2 [ ( H 2 + H ˙ ) ξ , ϕ − H ( ξ , ϕ ϕ ϕ ˙ + ξ , ϕ χ χ ˙ ) ] − V , ϕ + f , ϕ , C 3 = 8 H 2 ξ , ϕ , C 4 = 8 H 2 ξ , χ , C 5 = − 8 H 2 ( H ξ , χ − ξ , ϕ χ ϕ ˙ − ξ , χ χ χ ˙ ) , C 6 = η ϕ ˙ − 8 H 2 ( H ξ , ϕ − ξ , ϕ ϕ ϕ ˙ − ξ , ϕ χ χ ˙ ) . Now, we switch to the Fourier space with a comoving wavenumber k . Varying the total action (4.10) with respect to α , B , and v , respectively, we obtain(4.14) C 1 δ ϕ ˙ + C 2 δ ϕ + 3 H C 4 δ χ ˙ − 3 ( H ˙ C 4 − H C 5 ) δ χ + 3 ( 3 q t − M Pl 2 ) H ζ ˙ + [ η ϕ ˙ 2 − 6 H 2 ( 2 q t − M Pl 2 ) ] α + k 2 a 2 [ 2 q t ζ + H ( 3 q t − M Pl 2 ) B + C 3 δ ϕ + C 4 δ χ ] − δ ρ = 0 , (4.15) C 3 δ ϕ ˙ + C 6 δ ϕ + 2 q t ζ ˙ + C 4 δ χ ˙ + C 5 δ χ − H ( 3 q t − M Pl 2 ) α + ( ρ + P ) v = 0 , (4.16) δ ρ ˙ + 3 H ( 1 + c m 2 ) δ ρ + 3 ( ρ + P ) ζ ˙ + k 2 a 2 ( ρ + P ) ( v + B ) = 0 . In the following, we choose the flat gauge given by(4.17) ζ = 0 , to obtain stability conditions for scalar perturbations. We will discuss the two cases: (A) f ( ϕ , G ) gravity and (B) f ( G ) gravity in turn. 4.1 f ( ϕ , G ) gravityIn f ( ϕ , G ) gravity with f , G G ≠ 0 , we can construct gauge-invariant scalar perturbations δ ϕ f = δ ϕ − ϕ ˙ ζ / H , δ χ f = δ χ − χ ˙ ζ / H , and δ ρ f = δ ρ − ρ ˙ ζ / H . For the gauge choice (4.17) , they reduce, respectively, to δϕ , δχ , and δρ , which correspond to the dynamical scalar DOFs. Note that the perturbation δ χ = δ G arises from nonlinearities in the GB term. We solve Eqs. (4.14) -(4.16) for α , B , v and substitute them into Eq. (4.10) . Then, the resulting quadratic-order action in Fourier space is expressed in the form(4.18) S s ( 2 ) = ∫ d t d 3 x a 3 ( X → ˙ t K X → ˙ − k 2 a 2 X → t G X → − X → t M X → − X → t B X → ˙ ) , where K , G , M , B are 3 × 3 matrices, and(4.19) X → t = ( δ ϕ , δ χ , δ ρ / k ) . The leading-order contributions to M and B are of order k 0 . Taking the small-scale limit k → ∞ , nonvanishing components of the symmetric matrices K and G are(4.20) K 11 = η [ C 3 ϕ ˙ − H ( 3 q t − M Pl 2 ) ] 2 + 6 C 3 2 H 2 q t 2 H 2 ( 3 q t − M Pl 2 ) 2 , K 22 = C 4 2 ( η ϕ ˙ 2 + 6 H 2 q t ) 2 H 2 ( 3 q t − M Pl 2 ) 2 , K 12 = K 21 = C 4 [ C 3 ( η ϕ ˙ 2 + 6 H 2 q t ) − η H ϕ ˙ ( 3 q t − M Pl 2 ) ] 2 H 2 ( 3 q t − M Pl 2 ) 2 , K 33 = a 2 2 ( ρ + P ) , and(4.21) G 11 = − η H ( 3 q t − M Pl 2 ) [ 2 C 3 ϕ ˙ − H ( 3 q t − M Pl 2 ) ] + C 3 2 [ ρ + P − 6 q t H ˙ + 3 H 2 q t ( c t 2 − 3 ) ] 2 H 2 ( 3 q t − M Pl 2 ) 2 , G 22 = C 4 2 [ 3 H 2 q t ( 3 − c t 2 ) + 6 q t H ˙ − ρ − P ] 2 H 2 ( 3 q t − M Pl 2 ) 2 , G 12 = G 21 = − C 4 [ η H ϕ ˙ ( 3 q t − M Pl 2 ) + C 3 { ρ + P − 6 q t H ˙ + 3 H 2 q t ( c t 2 − 3 ) } ] 2 H 2 ( 3 q t − M Pl 2 ) 2 , G 33 = a 2 c m 2 2 ( ρ + P ) . To derive these coefficients, we have absorbed k 2 -dependent terms present in B into the components of G and used the relation C 1 = 3 H C 3 − η ϕ ˙ , and(4.22) C ˙ 3 = C 6 + C 3 ( H + 2 H ˙ H ) − η ϕ ˙ , C ˙ 4 = C 4 ( H + 2 H ˙ H ) + C 5 , q ˙ t = H q t ( c t 2 − 1 ) + ( H + H ˙ H ) ( q t − M Pl 2 ) . The scalar ghosts are absent under the following three conditions(4.23) K 33 = a 2 2 ( ρ + P ) > 0 , (4.24) K 11 K 22 − K 12 2 = 3 C 4 2 η q t 2 ( 3 q t − M Pl 2 ) 2 > 0 , (4.25) det K = 3 C 4 2 η q t a 2 4 ( ρ + P ) ( 3 q t − M Pl 2 ) 2 > 0 . Under the no-ghost condition q t > 0 of tensor perturbations, inequalities (4.23) -(4.25) are satisfied for(4.26) ρ + P > 0 , (4.27) η > 0 . In the limit of large k , dominant contributions to the second-order action (4.18) arise from K and G . Then, the dispersion relation can be expressed in the form(4.28) det ( c s 2 K − G ) = 0 , where c s is the scalar propagation speed. Solving Eq. (4.28) for c s 2 , we obtain the following three solutions(4.29) c s 1 2 = 1 , (4.30) c s 2 2 = − η ϕ ˙ 2 + ρ + P + 3 q t [ ( c t 2 − 3 ) H 2 − 2 H ˙ ] 6 H 2 q t , (4.31) c s 3 2 = c m 2 , which correspond to the squared propagation speeds of δϕ , δχ , and δρ , respectively. The scalar perturbation δϕ has a luminal propagation speed, so it satisfies the Laplacian stability condition. For c m 2 > 0 , the matter perturbation δρ is free from Laplacian instability. On using the background Eq. (3.15) , the sound speed squared (4.30) can be expressed as1 1 If we eliminate c t 2 by using Eq. (3.15) , we can express Eq. (4.30) in the form(4.32) c s 2 2 = 1 + 2 H ˙ H 2 + η ϕ ˙ 2 + ρ + P 3 q t H 2 . From this expression, it seems that the existence of the last term can lead to c s 2 2 > 0 even in the decelerating Universe. In the absence of matter (ρ = 0 = P ), this possibility was suggested in Ref. [93] . Eliminating q t instead of c t 2 from Eq. (4.30) , it is clear that this possibility is forbidden even in the presence of matter. (4.33) c s 2 2 = 1 3 ( 4 − c t 2 + 4 H ˙ H 2 ) = − 1 3 ( 2 + c t 2 + 6 w eff ) , where w eff is the effective equation of state defined by Eq. (2.16) . The Laplacian stability of δχ is ensured for c s 2 2 > 0 , i.e.,(4.34) w eff < − 1 6 ( 2 + c t 2 ) . Since we need the condition c t 2 > 0 for the absence of Laplacian instability in the tensor sector, w eff must be in the range w eff < − 1 / 3 . This translates to the condition H ˙ + H 2 = a ¨ / a > 0 , so the Laplacian stability of δχ requires that the Universe is accelerating. In decelerating cosmological epochs, the condition (4.34) is always violated for c t 2 > 0 . During the radiation-dominated (w eff = 1 / 3 ) and matter-dominated (w eff = 0 ) eras, we have c s 2 2 = − ( 4 + c t 2 ) / 3 and c s 2 2 = − ( 2 + c t 2 ) / 3 , respectively, which are both negative for c t 2 > 0 . We thus showed that, for scalar-GB couplings f ( ϕ , G ) containing nonlinear functions of G , δχ is prone to the Laplacian instability during the radiation and matter eras. Hence nonlinear functions of G should not be present in decelerating cosmological epochs. Even if c s 2 2 is positive in the inflationary epoch, c s 2 2 changes its sign during the transition to a reheating epoch (in which w eff ≃ 0 for a standard reheating scenario). During the epoch of late-time cosmic acceleration, c s 2 2 can be positive, but it changes the sign as we go back to the matter era. Since δχ is coupled to δϕ and δρ , the instability of δχ leads to the growth of δϕ and δρ for perturbations deep inside the Hubble radius. This violates the successful background evolution during the decelerating cosmological epochs. The squared propagation speeds (4.29) -(4.31) have been derived by choosing the flat gauge (4.17) , but they are independent of the gauge choices. Indeed, we will show in Appendix A that the same values of c s 1 2 , c s 2 2 , and c s 3 2 can be obtained by choosing the unitary gauge. We also note that the scalar propagation speed squared (2.15) in ξ ( ϕ ) G gravity is not equivalent to the value (4.29) . As we observe in Eq. (2.15) , the propagation of ϕ is affected by the coupling ξ ( ϕ ) with the linear GB term G . In f ( ϕ , G ) theory with f , G G ≠ 0 , the new scalar field χ plays a role of the dynamical DOF arising from the nonlinear GB term. In this latter case, the propagation of the other field ϕ does not practically acquire the effect of a coupling with the GB term and hence c s 1 reduces to the luminal value. 4.2 f ( G ) gravityFinally, we also study the stability of scalar perturbations in f ( G ) gravity given by the action(4.35) S = ∫ d 4 x − g [ M Pl 2 2 R + f ( G ) ] + S m ( g μ ν , Ψ m ) . In this case, there is no scalar field ϕ coupled to the GB term. The action (4.35) is equivalent to Eq. (3.2) with ϕ = 0 , X = 0 , V ( ϕ ) = 0 , U = − f ( χ ) + χ ξ ( χ ) , and ξ = f , χ ( χ ) . As shown in Ref. [103] , this theory belongs to a subclass of Horndeski theories with one scalar DOF χ besides a matter fluid. In f ( G ) gravity, the second-order action of scalar perturbations is obtained by setting ϕ , δϕ , and their derivatives 0 in Eqs. (4.11) and (4.12) . We choose the flat gauge (4.17) and eliminate α , B , v from the action by using Eqs. (4.14) -(4.16) . Then, the second-order scalar action reduces to the form (4.18) with 2 × 2 matrices K , G , M , B and two dynamical perturbations(4.36) X → t = ( δ χ , δ ρ / k ) . In the small-scale limit, nonvanishing components of K and G are given by(4.37) K 11 = 3 C 4 2 q t ( 3 q t − M Pl 2 ) 2 , K 22 = a 2 2 ( ρ + P ) , (4.38) G 11 = − C 4 2 [ ρ + P + 3 q t [ ( c t 2 − 3 ) H 2 − 2 H ˙ ] 2 H 2 ( 3 q t − M Pl 2 ) 2 , G 22 = a 2 c m 2 2 ( ρ + P ) . The no-ghost conditions correspond to K 11 > 0 and K 22 > 0 , which are satisfied for q t > 0 and ρ + P > 0 . The propagation speed squared for δχ is(4.39) c s 1 2 = G 11 K 11 = − ρ + P + 3 q t [ ( c t 2 − 3 ) H 2 − 2 H ˙ ] 6 H 2 q t = − 1 3 ( 2 + c t 2 + 6 w eff ) , where, in the last equality, we used the background Eq. (3.15) with ϕ ˙ = 0 . The other matter propagation speed squared is given by c s 2 2 = G 22 / K 22 = c m 2 . Since the last expression of Eq. (4.39) is of the same form as Eq. (4.33) , the Laplacian instability of δχ is present in decelerating cosmological epochs. In Ref. [100] , violent growth of matter perturbations was found during the radiation and matter eras for f ( G ) models of late-time cosmic acceleration. This is attributed to the Laplacian instability of δχ coupled to δρ , which inevitably occurs for nonlinear functions of f ( G ) . 5 Conclusions In this letter, we studied the stability of cosmological perturbations on the spatially flat FLRW background in scalar-GB theories given by the action (3.1) . Provided that f , G G ≠ 0 , the action (3.1) is equivalent to (3.2) with a new scalar DOF χ arising from nonlinear GB terms. Theories with f , G G = 0 correspond to a linear GB term coupled to a scalar field ϕ of the form ξ ( ϕ ) G , which belongs to a subclass of Horndeski theories. To make a comparison with the scalar-GB coupling f ( ϕ , G ) containing nonlinear functions of G , we first revisited stabilities of cosmological perturbations in ξ ( ϕ ) G gravity in Sec. 2 . In this latter theory, provided that the scalar-GB coupling is subdominant to the background equations of motion, the stability conditions of tensor and scalar perturbations can be consistently satisfied. In Sec. 3 , we derived the background equations and stability conditions of tensor perturbations for the scalar-GB coupling f ( ϕ , G ) with f , G G ≠ 0 . Besides a canonical scalar field ϕ with the kinetic term ηX and the potential V ( ϕ ) , we incorporate a perfect fluid given by the Schutz-Sorkin action (3.6) . The absence of ghosts and Laplacian instabilities requires that the quantities q t and c t 2 defined by Eqs. (3.18) and (3.19) are both positive. In terms of q t and c t 2 , the background equations of motion in the gravitational sector can be expressed in a simple manner as Eqs. (3.14) and (3.15) , where the latter is used to simplify a scalar sound speed later. In Sec. 4 , we expanded the action in f ( ϕ , G ) gravity with f , G G ≠ 0 up to quadratic order in scalar perturbations. After eliminating nondynamical variables α , B , and v , the second-order action is of the form (4.18) with three dynamical perturbations (4.19) . With the no-ghost condition q t > 0 of tensor perturbations, the scalar ghosts are absent for η > 0 and ρ + P > 0 . The sound speeds of perturbations δϕ and δρ have the standard values 1 and c m , respectively. However, the squared propagation speed of δχ , which arises from nonlinear GB functions in f , has a nontrivial value c s 2 2 = − ( 2 + c t 2 + 6 w eff ) / 3 . Since the positivity of c s 2 2 requires that w eff < − ( 2 + c t 2 ) / 6 , we have w eff < − 1 / 3 under the absence of Laplacian instability in the tensor sector (c t 2 > 0 ). This means that the scalar perturbation associated with nonlinearities of the GB term is subject to Laplacian instability during decelerating cosmological epochs including radiation and matter eras. The same property also holds for f ( G ) gravity with f , G G ≠ 0 . We thus showed that a canonical scalar field ϕ coupled to a nonlinear GB term does not modify the property of negative values of c s 2 2 in the decelerating Universe. During inflation or the epoch of late-time cosmic acceleration, it is possible to avoid Laplacian instability of the perturbation δχ in f ( ϕ , G ) gravity with f , G G ≠ 0 . However, in the subsequent reheating period after inflation or in the preceding matter era before dark energy dominance, the Laplacian instability inevitably emerges to violate the successful background cosmological evolution. We have shown this for a canonical scalar field ϕ , but it may be interesting to see whether the same property persists for the scalar field ϕ arising in Horndeski theories and its extensions like DHOST theories [110,111] . While we focused on the analysis on the FLRW background, it will be also of interest to study whether some instabilities are present for perturbations on a static and spherically symmetric background in f ( ϕ , G ) gravity with f , G G ≠ 0 . The latter is important for the construction of stable hairy black hole or neutron star solutions in theories beyond the scalar-GB coupling ξ ( ϕ ) G . These issues are left for future works. Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgements ST is supported by the Grant-in-Aid for Scientific Research Fund of the JSPS Nos. 19K03854 and 22K03642 . Appendix A Stability conditions in f ( ϕ , G ) gravity in unitary gauge In this Appendix, we derive stability conditions of scalar perturbations in f ( ϕ , G ) gravity by choosing the unitary gauge(A.1) δ ϕ = 0 . Then, the gauge-invariant perturbations R = ζ − H δ ϕ / ϕ ˙ , δ χ u = δ χ − χ ˙ δ ϕ / ϕ ˙ , and δ ρ u = δ ρ − ρ ˙ δ ϕ / ϕ ˙ reduce, respectively, to ζ , δχ , and δρ . After the elimination of nondynamical variables α , B , v from Eq. (4.10) , the second-order action reduces to the form (4.18) with the dynamical perturbations(A.2) X → t = ( ζ , δ χ , δ ρ / k ) , where nonvanishing matrix components of K and G are K 11 , K 22 , K 12 = K 21 , K 33 and G 11 , G 22 , G 12 = G 21 , G 33 . In the short-wavelength limit, the ghosts are absent for(A.3) K 33 = a 2 2 ( ρ + P ) > 0 , (A.4) K 11 K 22 − K 12 2 = 3 C 4 2 η q t ϕ ˙ 2 2 H 2 ( 3 q t − M Pl 2 ) 2 > 0 , (A.5) det K = 3 C 4 2 η q t a 2 ϕ ˙ 2 4 ( ρ + P ) H 2 ( 3 q t − M Pl 2 ) 2 > 0 . Under the tensor no-ghost condition q t > 0 , inequalities (A.3) -(A.5) are satisfied for ρ + P > 0 and η > 0 . These conditions are the same as those derived by choosing the flat gauge. The scalar propagation speed squared c s 2 can be derived by solving the dispersion relation (4.28) . On using the background Eq. (3.15) , we obtain the three values of c s 2 exactly the same as Eqs. (4.29) -(4.31) . 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+PLB 137751 137751 S0370-2693(23)00085-0 10.1016/j.physletb.2023.137751 The Author(s) Astrophysics and Cosmology Cosmological stability in f ( ϕ , G ) gravity Shinji Tsujikawa tsujikawa@waseda.jp Department of Physics, Waseda University, 3-4-1 Okubo, Shinjuku, Tokyo 169-8555, Japan Department of Physics Waseda University 3-4-1 Okubo Tokyo Shinjuku 169-8555 Japan Department of Physics, Waseda University, 3-4-1 Okubo, Shinjuku, Tokyo 169-8555, Japan Editor: M. Trodden Abstract In gravitational theories where a canonical scalar field ϕ with a potential V ( ϕ ) is coupled to a Gauss-Bonnet (GB) term G with the Lagrangian f ( ϕ , G ) , we study the cosmological stability of tensor and scalar perturbations in the presence of a perfect fluid. We show that, in decelerating cosmological epochs with a positive tensor propagation speed squared, the existence of nonlinear functions of G in f always induces Laplacian instability of a dynamical scalar perturbation associated with the GB term. This is also the case for f ( G ) gravity, where the presence of nonlinear GB functions f ( G ) is not allowed during the radiation- and matter-dominated epochs. A linearly coupled GB term with ϕ of the form ξ ( ϕ ) G can be consistent with all the stability conditions, provided that the scalar-GB coupling is subdominant to the background cosmological dynamics. Data availability No data was used for the research described in the article. 1 Introduction General Relativity (GR) is a fundamental theory of gravity whose validity has been probed in Solar System experiments [1] and submillimeter laboratory tests [2,3] . Despite the success of GR describing gravitational interactions in the Solar System, there have been long-standing cosmological problems such as the origins of inflation, dark energy, and dark matter. To address these problems, one typically introduces additional degrees of freedom (DOFs) beyond those appearing in GR [4–10] . One of such new DOFs is a canonical scalar field ϕ with a potential V ( ϕ ) [11–22] . If the scalar field evolves slowly along the potential, it is possible to realize cosmic acceleration responsible for inflation or dark energy. An oscillating scalar field around the potential minimum can be also the source for dark matter. The other way of introducing a new dynamical DOF is to modify the gravitational sector from GR. The Lagrangian in GR is given by an Einstein-Hilbert term M Pl 2 R / 2 , where M Pl is the reduced Planck mass and R is the Ricci scalar. If we consider theories containing nonlinear functions of R of the form f ( R ) , there is one scalar DOF arising from the modification of gravity [23,24] . One well known example is the Starobinsky's model, in which the presence of a quadratic curvature term R 2 drives cosmic acceleration [25] . It is also possible to construct f ( R ) models of late-time cosmic acceleration [26–32] , while being consistent with local gravity constraints. The Einstein tensor G μ ν obtained by varying the Einstein-Hilbert action satisfies the conserved relation ∇ μ G μ ν = 0 (∇ μ is a covariant derivative operator), with the property of second-order field equations of motion in metrics. If we demand such conserved and second-order properties for 2-rank symmetric tensors, GR is the unique theory of gravity in 4 dimensions [33] . In spacetime dimensions higher than 4, there is a particular combination known as a Gauss-Bonnet (GB) term G consistent with those demands [34] . In 4 dimensions, the GB term is a topological surface term and hence it does not contribute to the field equations of motion. In the presence of a coupling between a scalar field ϕ and G of the form ξ ( ϕ ) G , the spacetime dynamics is modified by the time or spatial variation of ϕ . Indeed, this type of scalar-GB coupling appears in the context of low energy effective string theory [35–37] . The cosmological application of the coupling ξ ( ϕ ) G has been extensively performed in the literature [38–63] . Moreover, it is known that the same coupling gives rise to spherically symmetric solutions of hairy black holes and neutron stars [64–80] . The Lagrangian f ( G ) containing nonlinear functions of G also generates nontrivial contributions to the spacetime dynamics [81–90] . In Ref. [91] , De Felice and Suyama studied the stability of scalar perturbations in f ( R , G ) gravity on a spatially flat Friedmann-Lemaître-Robertson-Walker (FLRW) background. In theories with f , R G 2 − f , R R f , G G ≠ 0 , where f , R G = ∂ 2 f / ∂ G ∂ R , f , R R = ∂ 2 f / ∂ R 2 , and f , G G = ∂ 2 f / ∂ G 2 , there is an unusual scale-dependent sound speed which propagates superluminally in the short-wavelength limit, unless the vacuum is in a de Sitter state (see also Ref. [92] for the analysis in an anisotropic cosmological background). We note that this problem does not arise for f ( R ) gravity or M Pl 2 R / 2 + f ( G ) gravity. In Ref. [93] , the same authors extended the analysis to a more general Lagrangian f ( ϕ , R , G ) with a canonical scalar field ϕ and showed that the property of the scale-dependent sound speed is not modified by the presence of ϕ . Taking a perfect fluid (radiation or nonrelativistic matter) into account in f ( R , G ) gravity, the cosmological stability and evolution of matter perturbations were studied in Refs. [94–96] . In Einstein-scalar-GB gravity given by the Lagrangian M Pl 2 R / 2 + f ( ϕ , G ) , where ϕ is a canonical scalar field, the problem of scale-dependent sound speeds mentioned above is not present. In this theory, the propagation of scalar perturbations on the flat FLRW background was studied in Ref. [93] without taking into account matter. While the sound speed associated with the field ϕ is luminal for theories with f , G G ≠ 0 , the propagation speed squared c s 2 arising from a nonlinear GB term deviates from that of light and it can be even negative. In Ref. [93] , the authors discussed the possibility for satisfying the Laplacian stability condition c s 2 > 0 . In the presence of matter, however, the stability conditions are subject to modifications from those in the vacuum. To understand what happens for the dynamics of cosmological perturbations during radiation- and matter-dominated epochs, we need to study their stabilities by incorporating radiation or nonrelativistic matter. In this letter, we will derive general conditions for the absence of ghosts and Laplacian instabilities in M Pl 2 R / 2 + f ( ϕ , G ) gravity, where ϕ is a canonical scalar field with a potential V ( ϕ ) . In theories where the scalar field ϕ is coupled to the linear GB term, i.e., f ( ϕ , G ) = ξ ( ϕ ) G , there is only one dynamical scalar DOF ϕ . In theories with f , G G ≠ 0 , the Lagrangian f ( ϕ , G ) can be expressed in terms of two scalar fields ϕ and χ coupled to the linear GB term, where χ arises from the nonlinearity in G . Hence the latter theory has two dynamical scalar DOFs. To study the cosmological stability of f ( ϕ , G ) theories with f , G G ≠ 0 , we take a perfect fluid into account as a form of the Schutz-Sorkin action [97–99] . We will show that the squared sound speed arising from nonlinear functions of G is negative during decelerating cosmological epochs including radiation and matter eras. To reach this conclusion, we exploit the fact that the propagation speed squared c t 2 of tensor perturbations must be positive to avoid Laplacian instability of gravitational waves. The same Laplacian instability of scalar perturbations is also present in M Pl 2 R / 2 + f ( G ) gravity with any nonlinear function of G in f . We note that, in f ( G ) models of late-time cosmic acceleration, violent instabilities of matter density perturbations during the radiation and matter eras were reported in Ref. [100] . This can be regarded as the consequence of a negative sound speed squared of the scalar perturbation δ G arising from the nonlinearity of G in f . Since δ G is coupled to the matter perturbation δρ , the background cosmological evolution during the radiation and matter eras is spoiled by the rapid growth of δρ . Our analysis in this letter shows that similar catastrophic instabilities persist for more general scalar-GB couplings f ( ϕ , G ) with f , G G ≠ 0 . This letter is organized as follows. In Sec. 2 , we revisit cosmological stability conditions in M Pl 2 R / 2 + ξ ( ϕ ) G gravity with a canonical scalar field ϕ , which can be accommodated in a subclass of Horndeski theories with a single scalar DOF [101–104] . This is an exceptional case satisfying the condition f , G G = 0 , under which the Laplacian instability of scalar perturbations can be avoided. In Sec. 3 , we derive the background equations and stability conditions of tensor perturbations in M Pl 2 R / 2 + f ( ϕ , G ) gravity with f , G G ≠ 0 by incorporating a perfect fluid. In Sec. 4 , we proceed to the derivation of a second-order action of scalar perturbations and obtain conditions for the absence of ghosts and Laplacian instabilities in the scalar sector. In particular, we show that an effective cosmological equation of state w eff needs to be in the range w eff < − ( 2 + c t 2 ) / 6 to ensure Laplacian stabilities of the perturbation δ G . Sec. 5 is devoted to conclusions. 2 ξ ( ϕ ) G gravityWe first briefly revisit the cosmological stability in ξ ( ϕ ) G gravity given by the action(2.1) S = ∫ d 4 x − g [ M Pl 2 2 R + η X − V ( ϕ ) + ξ ( ϕ ) G ] + S m ( g μ ν , Ψ m ) , where g is a determinant of the metric tensor g μ ν , η is a constant, X = − ( 1 / 2 ) g μ ν ∇ μ ϕ ∇ ν ϕ is a kinetic term of the scalar field ϕ , V ( ϕ ) and ξ ( ϕ ) are functions of ϕ , and G is a GB term defined by(2.2) G ≡ R 2 − 4 R μ ν R μ ν + R μ ν ρ σ R μ ν ρ σ , with R μ ν and R μ ν ρ σ being the Ricci and Riemann tensors, respectively. For the matter action S m , we consider a perfect fluid minimally coupled to gravity. The action (2.1) contains one scalar DOF ϕ besides the matter field Ψ m . If we consider Horndeski theories [101] given by the action(2.3) S = ∫ d 4 x − g [ G 2 ( ϕ , X ) − G 3 ( ϕ , X ) □ ϕ + G 4 ( ϕ , X ) R + G 4 , X ( ϕ , X ) { ( □ ϕ ) 2 − ( ∇ μ ∇ ν ϕ ) ( ∇ μ ∇ ν ϕ ) } + G 5 ( ϕ , X ) G μ ν ∇ μ ∇ ν ϕ − 1 6 G 5 , X ( ϕ , X ) { ( □ ϕ ) 3 − 3 ( □ ϕ ) ( ∇ μ ∇ ν ϕ ) ( ∇ μ ∇ ν ϕ ) + 2 ( ∇ μ ∇ α ϕ ) ( ∇ α ∇ β ϕ ) ( ∇ β ∇ μ ϕ ) } ] + S m ( g μ ν , Ψ m ) , then the theory (2.1) can be accommodated by choosing the coupling functions [103] (2.4) G 2 ( ϕ , X ) = η X − V ( ϕ ) + 8 ξ , ϕ ϕ ϕ ϕ ( ϕ ) X 2 ( 3 − ln | X | ) , G 3 ( ϕ , X ) = 4 ξ , ϕ ϕ ϕ ( ϕ ) X ( 7 − 3 ln | X | ) , G 4 ( ϕ , X ) = M Pl 2 2 + 4 ξ , ϕ ϕ ( ϕ ) X ( 2 − ln | X | ) , G 5 ( ϕ , X ) = − 4 ξ , ϕ ( ϕ ) ln | X | , where we use the notations F , X = ∂ F / ∂ X and F , ϕ = ∂ F / ∂ ϕ for any arbitrary function F . Let us consider a spatially flat FLRW background given by the line element d s 2 = − d t 2 + a 2 ( t ) δ i j d x i d x j , where a ( t ) is a time-dependent scale factor. The perfect fluid has a density ρ and pressure P . The background equations as well as the perturbation equations in full Horndeski theories were derived in Refs. [103,105–107] . On using the correspondence (2.4) , the background equations of motion in theories given by the action (2.1) are(2.5) 3 q ˜ t H 2 = 1 2 η ϕ ˙ 2 + V ( ϕ ) + ρ , (2.6) 2 q ˜ t H ˙ = − η ϕ ˙ 2 − H 2 q ˜ t ( c ˜ t 2 − 1 ) − ρ − P , (2.7) η ( ϕ ¨ + 3 H ϕ ˙ ) + V , ϕ − ξ , ϕ G = 0 , (2.8) ρ ˙ + 3 H ( ρ + P ) = 0 , where H = a ˙ / a is the Hubble expansion rate, a dot represents the derivative with respect to t , and(2.9) q ˜ t = M Pl 2 + 8 ξ , ϕ H ϕ ˙ , (2.10) c ˜ t 2 = M Pl 2 + 8 ( ξ , ϕ ϕ ¨ + ξ , ϕ ϕ ϕ ˙ 2 ) M Pl 2 + 8 ξ , ϕ H ϕ ˙ , (2.11) G = 24 H 2 ( H 2 + H ˙ ) . In the presence of tensor perturbations h i j with the perturbed line element d s 2 = − d t 2 + a 2 ( t ) ( δ i j + h i j ) d x i d x j , the second-order action of traceless and divergence-free modes of h i j was already derived in full Horndeski theories [103,106,107] . In the current theory, the conditions for the absence of ghosts and Laplacian instabilities are(2.12) q ˜ t > 0 , (2.13) c ˜ t 2 > 0 , where q ˜ t and c ˜ t 2 are defined by Eqs. (2.9) and (2.10) , respectively. Note that q ˜ t determines the sign of a kinetic term of h i j , while c ˜ t 2 corresponds to the propagation speed squared of tensor perturbations. For the scalar sector, we choose the perturbed line element d s 2 = − ( 1 + 2 α ) d t 2 + 2 ∂ i B d t d x i + a 2 ( t ) δ i j d x i d x j in the flat gauge, where α and B are scalar metric perturbations. There is also a scalar-field perturbation δϕ besides the matter perturbation δρ and the fluid velocity potential v . After deriving the quadratic-order action of scalar perturbations, we can eliminate nondynamical variables α , B , and v from the action. Then, we are left with the two dynamical perturbations δϕ and δρ in the second-order action. In the short-wavelength limit, there is neither ghost nor Laplacian instability for δϕ under the conditions [103,106,107] (2.14) q ˜ s = 2 ( η q ˜ t + 96 H 4 ξ , ϕ 2 ) > 0 , (2.15) c ˜ s 2 = η q ˜ t − 32 ( 2 + c ˜ t 2 + 6 w eff ) H 4 ξ , ϕ 2 η q ˜ t + 96 H 4 ξ , ϕ 2 > 0 , where c ˜ s corresponds to the propagation speed of δϕ , and w eff is the cosmological effective equation of state defined by(2.16) w eff ≡ − 1 − 2 H ˙ 3 H 2 . The stability conditions for δρ are given by ρ + P > 0 and c m 2 > 0 , where c m 2 is the matter sound speed squared. Under the stability condition (2.12) with η > 0 , the scalar no-ghost condition (2.14) is satisfied. Let us consider the case in which contributions of the scalar-GB coupling are suppressed, such that(2.17) { | ξ , ϕ H ϕ ˙ | , | ξ , ϕ ϕ ¨ | , | ξ , ϕ ϕ ϕ ˙ 2 | } ≪ M Pl 2 , H 4 ξ , ϕ 2 ≪ η q ˜ t . Then, it follows that q ˜ t ≃ M Pl 2 , c ˜ t 2 ≃ 1 , q ˜ s ≃ 2 η M Pl 2 , and c ˜ s 2 ≃ 1 . In such cases, provided that η > 0 , all the stability conditions are consistently satisfied. If the scalar-GB coupling contributes to the late-time cosmological dynamics, there is an observational bound on c ˜ t constrained from the GW170817 event together with the electromagnetic counterpart, i.e., − 3 × 10 − 15 ≤ c ˜ t − 1 ≤ 7 × 10 − 16 [108] for the redshift z ≤ 0.009 . This translates to the limit(2.18) | ξ , ϕ ϕ ¨ + ξ , ϕ ϕ ϕ ˙ 2 − ξ , ϕ H ϕ ˙ | ≲ 10 − 15 M Pl 2 , which gives a tight constraint on the amplitude of ξ ( ϕ ) . In this case, contributions of the scalar-GB coupling to the background Eqs. (2.5) and (2.6) are highly suppressed relative to the field density ρ ϕ = η ϕ ˙ 2 / 2 + V ( ϕ ) and the matter density. The bound (2.18) is not applied to early cosmological epochs including inflation, radiation, and matter eras. We note, however, that the dominance of the scalar-GB coupling to the background equations prevents the successful cosmic expansion history. This can also give rise to the violation of either of the stability conditions (2.12) -(2.15) . Provided the scalar-GB coupling is suppressed in such a way that inequalities (2.17) hold, the linear stabilities are ensured for both tensor and scalar perturbations. 3 f ( ϕ , G ) gravityWe extend ξ ( ϕ ) G gravity to more general theories in which a canonical scalar field ϕ with a potential V ( ϕ ) is coupled to the GB term of the form f ( ϕ , G ) . The action in such theories is given by(3.1) S = ∫ d 4 x − g [ M Pl 2 2 R + η X − V ( ϕ ) + f ( ϕ , G ) ] + S m ( g μ ν , Ψ m ) , where a matter field Ψ m is minimally coupled to gravity. It is more practical to introduce a scalar field χ and resort to the following action(3.2) S = ∫ d 4 x − g [ M Pl 2 2 R + η X − U ( ϕ , χ ) + ξ ( ϕ , χ ) G ] + S m ( g μ ν , Ψ m ) , where(3.3) U ( ϕ , χ ) ≡ V ( ϕ ) − f ( ϕ , χ ) + χ ξ ( ϕ , χ ) , ξ ( ϕ , χ ) ≡ f , χ ( ϕ , χ ) , with the notation f , χ = ∂ f / ∂ χ . Varying the action (3.2) with respect to χ , it follows that(3.4) ( χ − G ) ξ , χ = 0 . So long as ξ , χ ≠ 0 , we obtain χ = G . In this case, the action (3.2) reduces to (3.1) . Thus, the equivalence of (3.2) with (3.1) holds for(3.5) f , G G ≠ 0 , under which there is a new scalar DOF χ arising from the gravitational sector. Theories with f , G G = 0 correspond to the coupling f = ξ ( ϕ ) G , in which case the cosmological stability conditions were already discussed in Sec. 2 . In ξ ( ϕ ) G gravity, we do not have the additional scalar DOF χ arising from G , so the term ξ , χ in Eq. (3.4) does not have the meaning of f , G G . Thus, the action (3.2) with the new dynamical DOF χ does not reproduce the action (2.1) in ξ ( ϕ ) G gravity. In the following, we will focus on theories with f , G G ≠ 0 , i.e., those containing the nonlinear dependence of G in f . For the matter field Ψ m , we incorporate a perfect fluid without a dynamical vector DOF. This matter sector is described by the Schutz-Sorkin action [97–99] (3.6) S m = − ∫ d 4 x [ − g ρ ( n ) + J μ ∂ μ ℓ ] , where the fluid density ρ is a function of its number density n . The vector field J μ is related to n according to the relation n = J μ J ν g μ ν / g , where u μ = J μ / ( n − g ) is the fluid four velocity. A scalar quantity ℓ in S m is a Lagrange multiplier, with the notation of a partial derivative ∂ μ ℓ = ∂ ℓ / ∂ x μ . Varying the matter action (3.6) with respect to ℓ and J μ , respectively, we obtain(3.7) ∂ μ J μ = 0 , (3.8) ∂ μ ℓ = u μ ρ , n , where ρ , n = d ρ / d n . 3.1 Background equations We derive the background equations of motion on the spatially flat FLRW background given by the line element(3.9) d s 2 = − N 2 ( t ) d t 2 + a 2 ( t ) δ i j d x i d x j , where N ( t ) is a lapse function. Since the fluid four velocity in its rest frame is given by u μ = ( N − 1 , 0 , 0 , 0 ) , the vector field J μ has components J μ = ( n a 3 , 0 , 0 , 0 ) . From Eq. (3.7) , we obtain(3.10) N 0 ≡ n a 3 = constant , which means that the total fluid number N 0 is conserved. This translates to the differential equation n ˙ + 3 H n = 0 , which can be expressed as a form of the continuity equation(3.11) ρ ˙ + 3 H ( ρ + P ) = 0 , where P is a fluid pressure defined by P = n ρ , n − ρ . On the background (3.9) , the total action (3.2) is expressed in the form(3.12) S = ∫ d t d 3 x [ η a 3 ϕ ˙ 2 2 N − 3 M Pl 2 a a ˙ 2 N − N a 3 U ( ϕ , χ ) − 8 a ˙ 3 ξ ˙ ( ϕ , χ ) N 3 − N a 3 ρ − n a 3 ℓ ˙ ] . From Eq. (3.8) , we obtain the following relation(3.13) ℓ ˙ = − N ρ , n . Varying the action (3.12) with respect to N , a , ϕ , χ respectively and setting N = 1 at the end, we obtain the background equations of motion(3.14) 3 q t H 2 = 1 2 η ϕ ˙ 2 + U ( ϕ , χ ) + ρ , (3.15) 2 q t H ˙ = − η ϕ ˙ 2 − H 2 q t ( c t 2 − 1 ) − ρ − P , (3.16) η ( ϕ ¨ + 3 H ϕ ˙ ) + V , ϕ − f , ϕ = 0 , (3.17) χ = G = 24 H 2 ( H 2 + H ˙ ) , where(3.18) q t = M Pl 2 + 8 H ( ξ , ϕ ϕ ˙ + ξ , χ χ ˙ ) , (3.19) c t 2 = M Pl 2 + 8 ( ξ , ϕ ϕ ¨ + ξ , ϕ ϕ ϕ ˙ 2 + ξ , χ χ ¨ + ξ , χ χ χ ˙ 2 + 2 ξ , ϕ χ ϕ ˙ χ ˙ ) M Pl 2 + 8 H ( ξ , ϕ ϕ ˙ + ξ , χ χ ˙ ) . We recall that the perfect fluid obeys the continuity Eq. (3.11) . We notice that Eqs. (3.14) -(3.16) are of similar forms to Eqs. (2.5) -(2.7) in ξ ( ϕ ) G gravity, but the expressions of q t and c t 2 are different from q ˜ t and c ˜ t 2 , respectively, because of the appearance of time derivatives of χ . These χ derivatives do not vanish for ξ , χ ≠ 0 , i.e., for f , G G ≠ 0 . As we will show in Sec. 4 , nonlinearities of G in f are responsible for the appearance of a new scalar propagating DOF δχ . 3.2 Stabilities in the tensor sector We proceed to the derivation of stability conditions for tensor perturbations in theories given by the action (3.2) . The perturbed line element including the tensor perturbation h i j is(3.20) d s 2 = − d t 2 + a 2 ( t ) ( δ i j + h i j ) d x i d x j , where we impose the traceless and divergence-free gauge conditions h i i = 0 and ∂ i h i j = 0 . For the gravitational wave propagating along the z direction, nonvanishing components of h i j are expressed in the form(3.21) h 11 = h 1 ( t , z ) , h 22 = − h 1 ( t , z ) , h 12 = h 21 = h 2 ( t , z ) , where the two polarized modes h 1 and h 2 are functions of t and z . The second-order action arising from the matter action (3.6) can be expressed as(3.22) ( S m ( 2 ) ) t = − ∫ d t d 3 x ∑ i = 1 2 1 2 a 3 P h i 2 , where P can be eliminated by using the background Eq. (3.15) . Expanding the total action (3.2) up to quadratic order in tensor perturbations and integrating it by parts, the resulting second-order action reduces to(3.23) S t ( 2 ) = ∫ d t d 3 x ∑ i = 1 2 a 3 4 q t [ h ˙ i 2 − c t 2 a 2 ( ∂ h i ) 2 ] , where ( ∂ h i ) 2 = ( ∂ h i / ∂ z ) 2 . We recall that q t and c t 2 are given by Eqs. (3.18) and (3.19) , respectively. To avoid the ghost and Laplacian instabilities in the tensor sector, we require the two conditions q t > 0 and c t 2 > 0 , which translate to(3.24) M Pl 2 + 8 H ( ξ , ϕ ϕ ˙ + ξ , χ χ ˙ ) > 0 , (3.25) M Pl 2 + 8 ( ξ , ϕ ϕ ¨ + ξ , ϕ ϕ ϕ ˙ 2 + ξ , χ χ ¨ + ξ , χ χ χ ˙ 2 + 2 ξ , ϕ χ ϕ ˙ χ ˙ ) > 0 . In f ( G ) gravity without the scalar field ϕ , tensor stability conditions can be obtained by setting ϕ ˙ = 0 and ϕ ¨ = 0 in Eqs. (3.24) and (3.25) . We vary the action (3.23) with respect to h i (with i = 1 , 2 ) in Fourier space with a comoving wavenumber k . Then, we obtain the tensor perturbation equation of motion(3.26) h ¨ i + ( 3 H + q ˙ t q t ) h ˙ i + c t 2 k 2 a 2 h i = 0 , where k = | k | . Since ξ = f , χ = f , G , the G dependence in f leads to the modified evolution of gravitational waves in comparison to GR. If the energy densities of ϕ and χ are relevant to the late-time cosmological dynamics after the matter dominance, the observational constraint on the tensor propagation speed c t arising from the GW170817 event [108] (| c t − 1 | ≲ 10 − 15 ) gives a tight bound on the scalar-GB coupling f ( ϕ , G ) . Such a stringent limit is not applied to the cosmological dynamics in the early Universe, but the conditions (3.24) and (3.25) need to be still satisfied. 4 Stabilities of f ( ϕ , G ) gravity in the scalar sector In this section, we will derive conditions for the absence of scalar ghosts and Laplacian instabilities in theories given by the action (3.2) . A perturbed line element containing scalar perturbations α , B , ζ , and E is of the form(4.1) d s 2 = − ( 1 + 2 α ) d t 2 + 2 ∂ i B d t d x i + a 2 ( t ) [ ( 1 + 2 ζ ) δ i j + 2 ∂ i ∂ j E ] d x i d x j . For the scalar fields ϕ and χ , we consider perturbations δϕ and δχ on the background values ϕ ¯ ( t ) and χ ¯ ( t ) , respectively, such that(4.2) ϕ = ϕ ¯ ( t ) + δ ϕ ( t , x ) , χ = χ ¯ ( t ) + δ χ ( t , x ) , where we will omit a bar from the background quantities in the following. In the matter sector, the temporal and spatial components of J μ are decomposed into the background and perturbed parts as(4.3) J 0 = N 0 + δ J , J i = 1 a 2 ( t ) δ i k ∂ k δ j , where δJ and δj are scalar perturbations. In terms of the velocity potential v , the spatial component of fluid four velocity is expressed as u i = − ∂ i v . From Eq. (3.8) , the scalar quantity ℓ has a background part obeying the relation ℓ ˙ = − ρ , n besides a perturbation − ρ , n v . Then, we have(4.4) ℓ = − ∫ t ρ , n ( t ˜ ) d t ˜ − ρ , n v . Defining the matter density perturbation(4.5) δ ρ ≡ ρ , n a 3 [ δ J − N 0 ( 3 ζ + ∂ 2 E ) ] , the fluid number density n has a perturbation [107,109] (4.6) δ n = δ ρ ρ , n − ( N 0 ∂ χ + ∂ δ j ) 2 2 N 0 a 5 − ( 3 ζ + ∂ 2 E ) δ ρ ρ , n − N 0 ( ζ + ∂ 2 E ) ( 3 ζ − ∂ 2 E ) 2 a 3 , up to second order. The matter sound speed squared is given by(4.7) c m 2 = P , n ρ , n = n ρ , n n ρ , n . Expanding (3.6) up to quadratic order in perturbations, we obtain the second-order matter action same as that derived in Refs. [107,109] . Varying this matter action with respect to δj leads to(4.8) ∂ δ j = − a 3 n ( ∂ v + ∂ B ) , whose relation will be used to eliminate δj . In the following, we choose the gauge(4.9) E = 0 , under which a scalar quantity ξ associated with the spatial gauge transformation x i → x i + δ i j ∂ j ξ is fixed. A scalar quantity ξ 0 associated with the temporal part of the gauge transformation t → t + ξ 0 can be fixed by choosing a flat gauge (ζ = 0 ) or a unitary gauge (δ ϕ = 0 ). We do not specify the temporal gauge condition in deriving the second-order action, but we will do so at the end. Expanding the total action (3.2) up to quadratic order in scalar perturbations and integrating it by parts, the resulting second-order action is given by(4.10) S s ( 2 ) = ∫ d t d 3 x ( L flat + L ζ ) , where(4.11) L flat = a 3 [ η 2 δ ϕ ˙ 2 − η 2 ( ∂ δ ϕ ) 2 a 2 + 1 2 ( f , ϕ ϕ − V , ϕ ϕ ) δ ϕ 2 + { 1 2 η ϕ ˙ 2 − 3 H 2 ( 2 q t − M Pl 2 ) } α 2 − H ( 3 q t − M Pl 2 ) ∂ 2 B a 2 α − C 4 16 H 2 δ χ 2 + { C 1 δ ϕ ˙ + C 2 δ ϕ − C 3 ∂ 2 δ ϕ a 2 + 3 H C 4 δ χ ˙ − C 4 ∂ 2 δ χ a 2 + 3 ( H C 5 − H ˙ C 4 ) δ χ } α + ( C 3 δ ϕ ˙ + C 6 δ ϕ + C 4 δ χ ˙ + C 5 δ χ ) ∂ 2 B a 2 + ( ρ + P ) v ∂ 2 B a 2 − v δ ρ ˙ − 3 H ( 1 + c m 2 ) v δ ρ − 1 2 ( ρ + P ) ( ∂ v ) 2 a 2 − c m 2 2 ( ρ + P ) δ ρ 2 − α δ ρ ] , (4.12) L ζ = a 3 [ { 3 H ( 3 q t − M Pl 2 ) α − 3 ( C 3 δ ϕ ˙ + C 6 δ ϕ + C 4 δ χ ˙ + C 5 δ χ ) − 3 ( ρ + P ) v + 2 q t ∂ 2 B a 2 } ζ ˙ − 3 q t ζ ˙ 2 + q t c t 2 ( ∂ ζ ) 2 a 2 − 2 { q t α + ( 1 + H ˙ H 2 ) ( C 3 δ ϕ + C 4 δ χ ) } ∂ 2 ζ a 2 ] , where q t and c t 2 are given by Eqs. (3.18) and (3.19) , respectively, and(4.13) C 1 = 24 H 3 ξ , ϕ − η ϕ ˙ , C 2 = − 24 H 2 [ ( H 2 + H ˙ ) ξ , ϕ − H ( ξ , ϕ ϕ ϕ ˙ + ξ , ϕ χ χ ˙ ) ] − V , ϕ + f , ϕ , C 3 = 8 H 2 ξ , ϕ , C 4 = 8 H 2 ξ , χ , C 5 = − 8 H 2 ( H ξ , χ − ξ , ϕ χ ϕ ˙ − ξ , χ χ χ ˙ ) , C 6 = η ϕ ˙ − 8 H 2 ( H ξ , ϕ − ξ , ϕ ϕ ϕ ˙ − ξ , ϕ χ χ ˙ ) . Now, we switch to the Fourier space with a comoving wavenumber k . Varying the total action (4.10) with respect to α , B , and v , respectively, we obtain(4.14) C 1 δ ϕ ˙ + C 2 δ ϕ + 3 H C 4 δ χ ˙ − 3 ( H ˙ C 4 − H C 5 ) δ χ + 3 ( 3 q t − M Pl 2 ) H ζ ˙ + [ η ϕ ˙ 2 − 6 H 2 ( 2 q t − M Pl 2 ) ] α + k 2 a 2 [ 2 q t ζ + H ( 3 q t − M Pl 2 ) B + C 3 δ ϕ + C 4 δ χ ] − δ ρ = 0 , (4.15) C 3 δ ϕ ˙ + C 6 δ ϕ + 2 q t ζ ˙ + C 4 δ χ ˙ + C 5 δ χ − H ( 3 q t − M Pl 2 ) α + ( ρ + P ) v = 0 , (4.16) δ ρ ˙ + 3 H ( 1 + c m 2 ) δ ρ + 3 ( ρ + P ) ζ ˙ + k 2 a 2 ( ρ + P ) ( v + B ) = 0 . In the following, we choose the flat gauge given by(4.17) ζ = 0 , to obtain stability conditions for scalar perturbations. We will discuss the two cases: (A) f ( ϕ , G ) gravity and (B) f ( G ) gravity in turn. 4.1 f ( ϕ , G ) gravityIn f ( ϕ , G ) gravity with f , G G ≠ 0 , we can construct gauge-invariant scalar perturbations δ ϕ f = δ ϕ − ϕ ˙ ζ / H , δ χ f = δ χ − χ ˙ ζ / H , and δ ρ f = δ ρ − ρ ˙ ζ / H . For the gauge choice (4.17) , they reduce, respectively, to δϕ , δχ , and δρ , which correspond to the dynamical scalar DOFs. Note that the perturbation δ χ = δ G arises from nonlinearities in the GB term. We solve Eqs. (4.14) -(4.16) for α , B , v and substitute them into Eq. (4.10) . Then, the resulting quadratic-order action in Fourier space is expressed in the form(4.18) S s ( 2 ) = ∫ d t d 3 x a 3 ( X → ˙ t K X → ˙ − k 2 a 2 X → t G X → − X → t M X → − X → t B X → ˙ ) , where K , G , M , B are 3 × 3 matrices, and(4.19) X → t = ( δ ϕ , δ χ , δ ρ / k ) . The leading-order contributions to M and B are of order k 0 . Taking the small-scale limit k → ∞ , nonvanishing components of the symmetric matrices K and G are(4.20) K 11 = η [ C 3 ϕ ˙ − H ( 3 q t − M Pl 2 ) ] 2 + 6 C 3 2 H 2 q t 2 H 2 ( 3 q t − M Pl 2 ) 2 , K 22 = C 4 2 ( η ϕ ˙ 2 + 6 H 2 q t ) 2 H 2 ( 3 q t − M Pl 2 ) 2 , K 12 = K 21 = C 4 [ C 3 ( η ϕ ˙ 2 + 6 H 2 q t ) − η H ϕ ˙ ( 3 q t − M Pl 2 ) ] 2 H 2 ( 3 q t − M Pl 2 ) 2 , K 33 = a 2 2 ( ρ + P ) , and(4.21) G 11 = − η H ( 3 q t − M Pl 2 ) [ 2 C 3 ϕ ˙ − H ( 3 q t − M Pl 2 ) ] + C 3 2 [ ρ + P − 6 q t H ˙ + 3 H 2 q t ( c t 2 − 3 ) ] 2 H 2 ( 3 q t − M Pl 2 ) 2 , G 22 = C 4 2 [ 3 H 2 q t ( 3 − c t 2 ) + 6 q t H ˙ − ρ − P ] 2 H 2 ( 3 q t − M Pl 2 ) 2 , G 12 = G 21 = − C 4 [ η H ϕ ˙ ( 3 q t − M Pl 2 ) + C 3 { ρ + P − 6 q t H ˙ + 3 H 2 q t ( c t 2 − 3 ) } ] 2 H 2 ( 3 q t − M Pl 2 ) 2 , G 33 = a 2 c m 2 2 ( ρ + P ) . To derive these coefficients, we have absorbed k 2 -dependent terms present in B into the components of G and used the relation C 1 = 3 H C 3 − η ϕ ˙ , and(4.22) C ˙ 3 = C 6 + C 3 ( H + 2 H ˙ H ) − η ϕ ˙ , C ˙ 4 = C 4 ( H + 2 H ˙ H ) + C 5 , q ˙ t = H q t ( c t 2 − 1 ) + ( H + H ˙ H ) ( q t − M Pl 2 ) . The scalar ghosts are absent under the following three conditions(4.23) K 33 = a 2 2 ( ρ + P ) > 0 , (4.24) K 11 K 22 − K 12 2 = 3 C 4 2 η q t 2 ( 3 q t − M Pl 2 ) 2 > 0 , (4.25) det K = 3 C 4 2 η q t a 2 4 ( ρ + P ) ( 3 q t − M Pl 2 ) 2 > 0 . Under the no-ghost condition q t > 0 of tensor perturbations, inequalities (4.23) -(4.25) are satisfied for(4.26) ρ + P > 0 , (4.27) η > 0 . In the limit of large k , dominant contributions to the second-order action (4.18) arise from K and G . Then, the dispersion relation can be expressed in the form(4.28) det ( c s 2 K − G ) = 0 , where c s is the scalar propagation speed. Solving Eq. (4.28) for c s 2 , we obtain the following three solutions(4.29) c s 1 2 = 1 , (4.30) c s 2 2 = − η ϕ ˙ 2 + ρ + P + 3 q t [ ( c t 2 − 3 ) H 2 − 2 H ˙ ] 6 H 2 q t , (4.31) c s 3 2 = c m 2 , which correspond to the squared propagation speeds of δϕ , δχ , and δρ , respectively. The scalar perturbation δϕ has a luminal propagation speed, so it satisfies the Laplacian stability condition. For c m 2 > 0 , the matter perturbation δρ is free from Laplacian instability. On using the background Eq. (3.15) , the sound speed squared (4.30) can be expressed as1 1 If we eliminate c t 2 by using Eq. (3.15) , we can express Eq. (4.30) in the form(4.32) c s 2 2 = 1 + 2 H ˙ H 2 + η ϕ ˙ 2 + ρ + P 3 q t H 2 . From this expression, it seems that the existence of the last term can lead to c s 2 2 > 0 even in the decelerating Universe. In the absence of matter (ρ = 0 = P ), this possibility was suggested in Ref. [93] . Eliminating q t instead of c t 2 from Eq. (4.30) , it is clear that this possibility is forbidden even in the presence of matter. (4.33) c s 2 2 = 1 3 ( 4 − c t 2 + 4 H ˙ H 2 ) = − 1 3 ( 2 + c t 2 + 6 w eff ) , where w eff is the effective equation of state defined by Eq. (2.16) . The Laplacian stability of δχ is ensured for c s 2 2 > 0 , i.e.,(4.34) w eff < − 1 6 ( 2 + c t 2 ) . Since we need the condition c t 2 > 0 for the absence of Laplacian instability in the tensor sector, w eff must be in the range w eff < − 1 / 3 . This translates to the condition H ˙ + H 2 = a ¨ / a > 0 , so the Laplacian stability of δχ requires that the Universe is accelerating. In decelerating cosmological epochs, the condition (4.34) is always violated for c t 2 > 0 . During the radiation-dominated (w eff = 1 / 3 ) and matter-dominated (w eff = 0 ) eras, we have c s 2 2 = − ( 4 + c t 2 ) / 3 and c s 2 2 = − ( 2 + c t 2 ) / 3 , respectively, which are both negative for c t 2 > 0 . We thus showed that, for scalar-GB couplings f ( ϕ , G ) containing nonlinear functions of G , δχ is prone to the Laplacian instability during the radiation and matter eras. Hence nonlinear functions of G should not be present in decelerating cosmological epochs. Even if c s 2 2 is positive in the inflationary epoch, c s 2 2 changes its sign during the transition to a reheating epoch (in which w eff ≃ 0 for a standard reheating scenario). During the epoch of late-time cosmic acceleration, c s 2 2 can be positive, but it changes the sign as we go back to the matter era. Since δχ is coupled to δϕ and δρ , the instability of δχ leads to the growth of δϕ and δρ for perturbations deep inside the Hubble radius. This violates the successful background evolution during the decelerating cosmological epochs. The squared propagation speeds (4.29) -(4.31) have been derived by choosing the flat gauge (4.17) , but they are independent of the gauge choices. Indeed, we will show in Appendix A that the same values of c s 1 2 , c s 2 2 , and c s 3 2 can be obtained by choosing the unitary gauge. We also note that the scalar propagation speed squared (2.15) in ξ ( ϕ ) G gravity is not equivalent to the value (4.29) . As we observe in Eq. (2.15) , the propagation of ϕ is affected by the coupling ξ ( ϕ ) with the linear GB term G . In f ( ϕ , G ) theory with f , G G ≠ 0 , the new scalar field χ plays a role of the dynamical DOF arising from the nonlinear GB term. In this latter case, the propagation of the other field ϕ does not practically acquire the effect of a coupling with the GB term and hence c s 1 reduces to the luminal value. 4.2 f ( G ) gravityFinally, we also study the stability of scalar perturbations in f ( G ) gravity given by the action(4.35) S = ∫ d 4 x − g [ M Pl 2 2 R + f ( G ) ] + S m ( g μ ν , Ψ m ) . In this case, there is no scalar field ϕ coupled to the GB term. The action (4.35) is equivalent to Eq. (3.2) with ϕ = 0 , X = 0 , V ( ϕ ) = 0 , U = − f ( χ ) + χ ξ ( χ ) , and ξ = f , χ ( χ ) . As shown in Ref. [103] , this theory belongs to a subclass of Horndeski theories with one scalar DOF χ besides a matter fluid. In f ( G ) gravity, the second-order action of scalar perturbations is obtained by setting ϕ , δϕ , and their derivatives 0 in Eqs. (4.11) and (4.12) . We choose the flat gauge (4.17) and eliminate α , B , v from the action by using Eqs. (4.14) -(4.16) . Then, the second-order scalar action reduces to the form (4.18) with 2 × 2 matrices K , G , M , B and two dynamical perturbations(4.36) X → t = ( δ χ , δ ρ / k ) . In the small-scale limit, nonvanishing components of K and G are given by(4.37) K 11 = 3 C 4 2 q t ( 3 q t − M Pl 2 ) 2 , K 22 = a 2 2 ( ρ + P ) , (4.38) G 11 = − C 4 2 [ ρ + P + 3 q t [ ( c t 2 − 3 ) H 2 − 2 H ˙ ] 2 H 2 ( 3 q t − M Pl 2 ) 2 , G 22 = a 2 c m 2 2 ( ρ + P ) . The no-ghost conditions correspond to K 11 > 0 and K 22 > 0 , which are satisfied for q t > 0 and ρ + P > 0 . The propagation speed squared for δχ is(4.39) c s 1 2 = G 11 K 11 = − ρ + P + 3 q t [ ( c t 2 − 3 ) H 2 − 2 H ˙ ] 6 H 2 q t = − 1 3 ( 2 + c t 2 + 6 w eff ) , where, in the last equality, we used the background Eq. (3.15) with ϕ ˙ = 0 . The other matter propagation speed squared is given by c s 2 2 = G 22 / K 22 = c m 2 . Since the last expression of Eq. (4.39) is of the same form as Eq. (4.33) , the Laplacian instability of δχ is present in decelerating cosmological epochs. In Ref. [100] , violent growth of matter perturbations was found during the radiation and matter eras for f ( G ) models of late-time cosmic acceleration. This is attributed to the Laplacian instability of δχ coupled to δρ , which inevitably occurs for nonlinear functions of f ( G ) . 5 Conclusions In this letter, we studied the stability of cosmological perturbations on the spatially flat FLRW background in scalar-GB theories given by the action (3.1) . Provided that f , G G ≠ 0 , the action (3.1) is equivalent to (3.2) with a new scalar DOF χ arising from nonlinear GB terms. Theories with f , G G = 0 correspond to a linear GB term coupled to a scalar field ϕ of the form ξ ( ϕ ) G , which belongs to a subclass of Horndeski theories. To make a comparison with the scalar-GB coupling f ( ϕ , G ) containing nonlinear functions of G , we first revisited stabilities of cosmological perturbations in ξ ( ϕ ) G gravity in Sec. 2 . In this latter theory, provided that the scalar-GB coupling is subdominant to the background equations of motion, the stability conditions of tensor and scalar perturbations can be consistently satisfied. In Sec. 3 , we derived the background equations and stability conditions of tensor perturbations for the scalar-GB coupling f ( ϕ , G ) with f , G G ≠ 0 . Besides a canonical scalar field ϕ with the kinetic term ηX and the potential V ( ϕ ) , we incorporate a perfect fluid given by the Schutz-Sorkin action (3.6) . The absence of ghosts and Laplacian instabilities requires that the quantities q t and c t 2 defined by Eqs. (3.18) and (3.19) are both positive. In terms of q t and c t 2 , the background equations of motion in the gravitational sector can be expressed in a simple manner as Eqs. (3.14) and (3.15) , where the latter is used to simplify a scalar sound speed later. In Sec. 4 , we expanded the action in f ( ϕ , G ) gravity with f , G G ≠ 0 up to quadratic order in scalar perturbations. After eliminating nondynamical variables α , B , and v , the second-order action is of the form (4.18) with three dynamical perturbations (4.19) . With the no-ghost condition q t > 0 of tensor perturbations, the scalar ghosts are absent for η > 0 and ρ + P > 0 . The sound speeds of perturbations δϕ and δρ have the standard values 1 and c m , respectively. However, the squared propagation speed of δχ , which arises from nonlinear GB functions in f , has a nontrivial value c s 2 2 = − ( 2 + c t 2 + 6 w eff ) / 3 . Since the positivity of c s 2 2 requires that w eff < − ( 2 + c t 2 ) / 6 , we have w eff < − 1 / 3 under the absence of Laplacian instability in the tensor sector (c t 2 > 0 ). This means that the scalar perturbation associated with nonlinearities of the GB term is subject to Laplacian instability during decelerating cosmological epochs including radiation and matter eras. The same property also holds for f ( G ) gravity with f , G G ≠ 0 . We thus showed that a canonical scalar field ϕ coupled to a nonlinear GB term does not modify the property of negative values of c s 2 2 in the decelerating Universe. During inflation or the epoch of late-time cosmic acceleration, it is possible to avoid Laplacian instability of the perturbation δχ in f ( ϕ , G ) gravity with f , G G ≠ 0 . However, in the subsequent reheating period after inflation or in the preceding matter era before dark energy dominance, the Laplacian instability inevitably emerges to violate the successful background cosmological evolution. We have shown this for a canonical scalar field ϕ , but it may be interesting to see whether the same property persists for the scalar field ϕ arising in Horndeski theories and its extensions like DHOST theories [110,111] . While we focused on the analysis on the FLRW background, it will be also of interest to study whether some instabilities are present for perturbations on a static and spherically symmetric background in f ( ϕ , G ) gravity with f , G G ≠ 0 . The latter is important for the construction of stable hairy black hole or neutron star solutions in theories beyond the scalar-GB coupling ξ ( ϕ ) G . These issues are left for future works. Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgements ST is supported by the Grant-in-Aid for Scientific Research Fund of the JSPS Nos. 19K03854 and 22K03642 . Appendix A Stability conditions in f ( ϕ , G ) gravity in unitary gauge In this Appendix, we derive stability conditions of scalar perturbations in f ( ϕ , G ) gravity by choosing the unitary gauge(A.1) δ ϕ = 0 . Then, the gauge-invariant perturbations R = ζ − H δ ϕ / ϕ ˙ , δ χ u = δ χ − χ ˙ δ ϕ / ϕ ˙ , and δ ρ u = δ ρ − ρ ˙ δ ϕ / ϕ ˙ reduce, respectively, to ζ , δχ , and δρ . After the elimination of nondynamical variables α , B , v from Eq. (4.10) , the second-order action reduces to the form (4.18) with the dynamical perturbations(A.2) X → t = ( ζ , δ χ , δ ρ / k ) , where nonvanishing matrix components of K and G are K 11 , K 22 , K 12 = K 21 , K 33 and G 11 , G 22 , G 12 = G 21 , G 33 . In the short-wavelength limit, the ghosts are absent for(A.3) K 33 = a 2 2 ( ρ + P ) > 0 , (A.4) K 11 K 22 − K 12 2 = 3 C 4 2 η q t ϕ ˙ 2 2 H 2 ( 3 q t − M Pl 2 ) 2 > 0 , (A.5) det K = 3 C 4 2 η q t a 2 ϕ ˙ 2 4 ( ρ + P ) H 2 ( 3 q t − M Pl 2 ) 2 > 0 . Under the tensor no-ghost condition q t > 0 , inequalities (A.3) -(A.5) are satisfied for ρ + P > 0 and η > 0 . These conditions are the same as those derived by choosing the flat gauge. The scalar propagation speed squared c s 2 can be derived by solving the dispersion relation (4.28) . On using the background Eq. (3.15) , we obtain the three values of c s 2 exactly the same as Eqs. (4.29) -(4.31) . 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-NUPHB 116106 116106 S0550-3213(23)00035-4 10.1016/j.nuclphysb.2023.116106 The Author(s) High Energy Physics – Phenomenology The formal seesaw mechanism of Majorana neutrinos with unbroken gauge symmetry Zhi-zhong Xing Conceptualization Investigation Methodology Writing – original draft Writing – review & editing a b ⁎ xingzz@ihep.ac.cn a Institute of High Energy Physics and School of Physical Sciences, University of Chinese Academy of Sciences, Beijing 100049, China Institute of High Energy Physics School of Physical Sciences University of Chinese Academy of Sciences Beijing 100049 China Institute of High Energy Physics and School of Physical Sciences, University of Chinese Academy of Sciences, Beijing 100049, China b Center of High Energy Physics, Peking University, Beijing 100871, China Center of High Energy Physics Peking University Beijing 100871 China Center of High Energy Physics, Peking University, Beijing 100871, China ⁎ Correspondence to: Institute of High Energy Physics and School of Physical Sciences, University of Chinese Academy of Sciences, Beijing 100049, China. Institute of High Energy Physics School of Physical Sciences University of Chinese Academy of Sciences Beijing 100049 China Editor: Tommy Ohlsson Abstract We reformulate the canonical seesaw mechanism in the case that the electroweak gauge symmetry is unbroken, and show that it can formally work and allow us to derive an exact seesaw formula for the light and heavy Majorana neutrinos. We elucidate the reason why there is a mismatch between the mass eigenstates of heavy Majorana neutrinos associated with thermal leptogenesis and those associated with the seesaw framework, and establish the exact and explicit relations between the original and derivational seesaw parameters by using an Euler-like parametrization of the 6 × 6 active-sterile flavor mixing matrix. Data availability No data was used for the research described in the article. 1 Motivation Among all the proposed mechanisms toward deeply understanding the true origin of tiny masses of the three known neutrinos ν i (for i = 1 , 2 , 3 ), whose flavor eigenstates are commonly denoted as ν α (for α = e , μ , τ ), the canonical seesaw mechanism [1–5] stands out as being most economical and most natural. The simplicity of this mechanism lies in two aspects: (a) it just takes into account the right-handed neutrino fields N α R , the chiral counterparts of the left-handed neutrino fields ν α L (for α = e , μ , τ ), which were originally ignored from the particle content of the standard model (SM) [6] ; (b) it simply allows for lepton number violation or the Majorana nature of massive neutrinos [7] , which is completely harmless to the theoretical framework of the SM itself. The naturalness of this mechanism is reflected in its attributing the small masses of ν i to the existence of three heavy Majorana neutrinos N i (for i = 1 , 2 , 3 ), whose masses are expected to be far above the fulcrum of the seesaw — presumably the electroweak symmetry breaking scale of the SM characterized by the vacuum expectation value of the Higgs field. On the other hand, the seesaw mechanism offers a big bonus to cosmology: the CP-violating and out-of-equilibrium decays of heavy Majorana neutrinos may give rise to a net lepton-antilepton asymmetry in the early Universe, and such a leptogenesis mechanism [8] can finally lead to baryogenesis as a natural interpretation of the observed baryon-antibaryon asymmetry in today's Universe [9] . In this sense the seesaw mechanism is the very stone that can kill two fundamental birds in particle physics and cosmology. Note that the seesaw mechanism is expected to take effect at a superhigh energy scale Λ which is essentially of the order of the heavy Majorana neutrino masses. But the SU ( 2 ) L × U ( 1 ) Y electroweak gauge symmetry has been unbroken until the Higgs field develops a nonzero vacuum expectation value v of O ( 10 2 ) GeV . In this situation the three active neutrinos are actually impossible to acquire their true masses of O ( v 2 / Λ ) at the seesaw scale Λ due to the absence of a real fulcrum of the seesaw. On the other hand, thermal leptogenesis can be realized via the lepton-number-violating decays of heavy Majorana neutrinos into the leptonic and Higgs doublets at Λ. So we are well motivated to ask a conceptually important question: how can the seesaw mechanism formally survive with the unbroken electroweak gauge symmetry and work together with the leptogenesis mechanism? If the answer to this question is affirmative, we wonder whether the mass eigenstates of heavy Majorana neutrinos associated with thermal leptogenesis are exactly the same as those associated with the seesaw mechanism itself.1 1 A mismatch of this kind has been observed and discussed in the seesaw framework after spontaneous electroweak symmetry breaking and in an approximate way (see, e.g., Refs. [10–14] ). Here we shall take a new look at it before electroweak symmetry breaking and in an exact way at the tree level. In case that there exists a mismatch between these two sets of mass bases, then the question becomes how small this mismatch is likely to be. To answer the above questions and clarify some conceptual ambiguities that have never been taken seriously, we are going to study how to make the seesaw mechanism formally work before spontaneous electroweak symmetry breaking. We show that an exact seesaw relation between the light and heavy Majorana neutrinos can be established far above the electroweak scale, and it becomes the realistic seesaw relation after the Higgs field develops its vacuum expectation value. In this way it is straightforward to elucidate the reason why there is a mismatch between the mass eigenstates of heavy Majorana neutrinos associated with thermal leptogenesis and those associated with the seesaw mechanism. With the help of a full Euler-like parametrization of the flavor structure in the seesaw framework, we illuminate such a mismatch in a more specific way. The exact and explicit relations between the original and derivational parameters of massive Majorana neutrinos are obtained as a by-product, and they are expected to be useful in determining or constraining some of the original seesaw parameters from the low-energy neutrino experiments. 2 A formal seesaw mechanism? 2.1 The leptonic Yukawa interactions Let us begin with the gauge-invariant leptonic Yukawa interactions and the SU ( 2 ) L -singlet Majorana neutrino mass term of the canonical seesaw mechanism at Λ2 2 Throughout this paper, our discussions are subject to the minimal extension of the SM with three right-handed neutrino fields and lepton number violation at zero temperature, so as to make our key point clear and avoid possible complications (e.g., thermal corrections to the masses of heavy Majorana neutrinos [14] ). (1) − L Λ = ℓ L ‾ Y l H l R + ℓ L ‾ Y ν H ˜ N R + 1 2 ( N R ) c ‾ M R N R + h . c . , where ℓ L = ( ν L l L ) T denotes the leptonic SU ( 2 ) L doublet of the SM with ν L = ( ν e L ν μ L ν τ L ) T and l L = ( l e L l μ L l τ L ) T standing respectively for the column vectors of the left-handed neutrino and charged lepton fields, H ˜ ≡ i σ 2 H ⁎ with H = ( ϕ + ϕ 0 ) T being the Higgs doublet of the SM and σ 2 being the second Pauli matrix, l R = ( l e R l μ R l τ R ) T and N R = ( N e R N μ R N τ R ) T stand respectively for the column vectors of the right-handed charged lepton and neutrino fields which are the SU ( 2 ) L singlets, ( N R ) c ≡ C N R ‾ T with C being the charge-conjugation matrix and satisfying C − 1 = C † = C T = − C , Y l and Y ν represent the respective Yukawa coupling matrices of charged leptons and neutrinos, and M R is the symmetric right-handed neutrino mass matrix. In Eq. (1) the hypercharges of ℓ L , l R , N R , H and H ˜ are − 1 / 2 , −1, 0, + 1 / 2 and − 1 / 2 , respectively. Since ν L ‾ Y ν N R is a Lorentz scalar and can be transformed into(2) ν L ‾ Y ν N R = [ ν L ‾ Y ν N R ] T = ( N R ) c ‾ Y ν T ( ν L ) c , where ( ν L ) c ≡ C ν L ‾ T is the charge-conjugated counterpart of ν L , one may easily rewrite Eq. (1) as(3) − L Λ = l L ‾ Y l l R ϕ 0 + 1 2 [ ν L ( N R ) c ] ‾ ( 0 Y ν ϕ 0 ⁎ Y ν T ϕ 0 ⁎ M R ) [ ( ν L ) c N R ] + ν L ‾ Y l l R ϕ + − l L ‾ Y ν N R ϕ − + h . c . . This expression is highly nontrivial in the sense that it clearly shows a direct correlation between the left- and right-handed neutrino fields via their Yukawa couplings to the neutral component of the Higgs doublet even though the SU ( 2 ) L × U ( 1 ) Y gauge symmetry is perfect at the seesaw scale Λ. In this situation the 3 × 3 Yukawa coupling matrix Y ν can be regarded as a “virtual” fulcrum of the seesaw before spontaneous electroweak symmetry breaking. Note that both the scalar field ϕ 0 and its charge-conjugated counterpart ϕ 0 ⁎ have the mass dimension and act like two complex numbers in Eq. (3) . But of course they possess the respective hypercharges + 1 / 2 and − 1 / 2 as ϕ ± do. After spontaneous symmetry breaking ϕ 0 and ϕ 0 ⁎ will acquire the same vacuum expectation value 〈 ϕ 0 〉 = 〈 ϕ 0 ⁎ 〉 = v / 2 with v ≃ 246 GeV , together with 〈 ϕ − 〉 = 〈 ϕ + 〉 = 0 , as in the SM. Then the formal seesaw will acquire a real fulcrum which allows one to naturally attribute the smallness of three active Majorana neutrino masses to the existence of three heavy Majorana neutrinos, as can be seen later on. 2.2 The leptogenesis-associated basis Now that all the SM particles are exactly massless in the early Universe when the temperature is far above the electroweak scale, a realization of thermal leptogenesis at Λ ≫ v only needs to calculate the lepton-number-violating decays of heavy Majorana neutrinos into the leptonic doublet and the Higgs doublet at the one-loop level by simply starting from Eq. (1) instead of Eq. (3) (see, e.g., Refs. [8,15–18] ). In this case the column vector of the mass eigenstates of three heavy Majorana neutrinos, denoted as N ′ = ( N 1 N 2 N 3 ) T , can easily be obtained by making the Autonne-Takagi transformation [19,20] as follows:(4) U 0 ′ † M R U 0 ′ ⁎ = D N , N R ′ = U 0 ′ T N R , where U 0 ′ is a unitary matrix, and D N ≡ Diag { M 1 , M 2 , M 3 } with M i being the masses of N i (for i = 1 , 2 , 3 ). As a result, the Lagrangian L Λ in Eq. (1) becomes(5) − L Λ = ℓ L ‾ Y l H l R + ℓ L ‾ Y ν H ˜ N R ′ + 1 2 ( N R ′ ) c ‾ D N N R ′ + h . c . , where Y ν ≡ Y ν U 0 ′ ⁎ is defined for the sake of simplicity. The rates of N i decaying into ℓ L and H or their CP-conjugated states are therefore determined by M i and Y ν , so are the corresponding CP-violating asymmetries associated closely with thermal leptogenesis [15–18] .3 3 Here we have used some calligraphic characters to denote the relevant physical quantities in the basis where M R is diagonalized by the unitary transformation made in Eq. (4) . This basis is associated with N i decays and thermal leptogenesis, and it is conceptually different from the basis taken for the seesaw mechanism as can be seen below. To be more specific, the flavor-dependent CP-violating asymmetries of N i decays are given by(6) ε i α ≡ Γ ( N i → ℓ α + H ) − Γ ( N i → ℓ α ‾ + H ‾ ) ∑ α [ Γ ( N i → ℓ α + H ) + Γ ( N i → ℓ α ‾ + H ‾ ) ] = 1 8 π ( Y ν † Y ν ) i i ∑ j ≠ i { Im [ ( Y ν ⁎ ) α i ( Y ν ) α j ( Y ν † Y ν ) i j ξ ( x j i ) + ( Y ν ⁎ ) α i ( Y ν ) α j ( Y ν † Y ν ) i j ⁎ ζ ( x j i ) ] } , where the Latin and Greek subscripts run respectively over ( 1 , 2 , 3 ) and ( e , μ , τ ) , x j i ≡ M j 2 / M i 2 are defined, ξ ( x j i ) = x j i { 1 + 1 / ( 1 − x j i ) + ( 1 + x j i ) ln [ x j i / ( 1 + x j i ) ] } and ζ ( x j i ) = 1 / ( 1 − x j i ) are the loop functions. A net lepton-antilepton asymmetry can therefore result from ε i α in the early Universe, and later on it can be partly converted into a net baryon-antibaryon asymmetry via the sphaleron interactions (see Ref. [21] for a recent review). At this point it is worth remarking that the right-handed neutrino fields N α R have zero weak isospin and hypercharge, and hence they have no coupling with the charged and neutral gauge bosons of the SM. As a consequence, the mass eigenstates of heavy Majorana neutrinos obtained from Eq. (4) do not participate in the weak charged-current interactions of the SM,(7) − L cc = g 2 ℓ L ‾ γ μ ( σ 1 W μ 1 + σ 2 W μ 2 ) ℓ L = g 2 l L ‾ γ μ W μ − ν L + h . c . , where g denotes the weak gauge coupling constant, σ 1 , 2 represent the first and second Pauli matrices, W 1 , 2 μ are two of the original SU ( 2 ) L gauge fields, and W μ ± ≡ ( W μ 1 ∓ i W μ 2 ) / 2 stand for the fields of the physical charged gauge bosons W ± . But in the seesaw framework we shall see that the expression of L cc in Eq. (7) will get modified, and the corresponding mass eigenstates of three heavy Majorana neutrinos can definitely take part in the weak charged-current interactions. 2.3 The seesaw-associated basis We proceed to show that the canonical seesaw mechanism can “formally” work before spontaneous electroweak symmetry breaking but the corresponding mass eigenstates of three heavy Majorana neutrinos are not exactly the same as N i (for i = 1 , 2 , 3 ) obtained above for the neutrino decays and thermal leptogenesis. To clarify this important point, let us diagonalize the symmetric 6 × 6 matrix in Eq. (3) in the following Autonne-Takagi way:(8) U † ( 0 Y ν ϕ 0 ⁎ Y ν T ϕ 0 ⁎ M R ) U ⁎ = ( D ν 0 0 D N ) , where U is a 6 × 6 unitary matrix, and the diagonal and real matrices D ν and D N are defined as D ν ≡ Diag { m 1 , m 2 , m 3 } and D N ≡ Diag { M 1 , M 2 , M 3 } . Meanwhile, the column vectors of left- and right-handed neutrino fields [ ν L ( N R ) c ] T and [ ( ν L ) c N R ] T undergo the transformations(9) [ ν L ( N R ) c ] ⟶ U † [ ν L ( N R ) c ] , [ ( ν L ) c N R ] ⟶ U T [ ( ν L ) c N R ] , such that the Lagrangian L Λ in Eq. (3) keeps unchanged and thus its overall gauge symmetry is unbroken. Now that Y ν is dimensionless and ϕ 0 has the same mass dimension as M R , one may argue that m i should be the “working” or “virtual” mass parameters of three light Majorana neutrinos as the electroweak gauge symmetry is unbroken at the seesaw scale Λ. In comparison, M i are essentially the true masses of three heavy Majorana neutrinos in the existence of the ϕ 0 ( ⁎ ) -mediated neutrino Yukawa interactions. Along this line of thought, we find that it is useful to decompose U into the product of three matrices,(10) U = ( I 0 0 U 0 ′ ) ( A R S B ) ( U 0 0 0 I ) , where the 3 × 3 unitary matrix U 0 ′ has been defined in Eq. (4) to primarily describe flavor mixing in the sterile (heavy) neutrino sector, U 0 denotes the other 3 × 3 unitary matrix that is mainly responsible for flavor mixing in the active (light) neutrino sector, while the 3 × 3 matrices A , B , R and S signify the interplay between these two sectors [22–24] . The unitarity of U assures(11) A A † + R R † = B B † + S S † = I , A S † + R B † = A † R + S † B = 0 , A † A + S † S = B † B + R † R = I . On the other hand, the arbitrary charged-lepton Yukawa coupling matrix Y l in Eq. (3) can be diagonalized by a bi-unitary transformation:(12) U l † ( Y l ϕ 0 ) V l = D l , l L ′ = U l † l L , l R ′ = V l l R , where U l and V l are unitary, D l ≡ Diag { m e , m μ , m τ } stands for the “working” or “virtual” masses of three charged leptons before spontaneous electroweak symmetry breaking,4 4 Note that the scalar field ϕ 0 in Eq. (12) carries a hypercharge, and hence D l cannot be simply understood as a diagonal “mass” matrix. The physical meaning of D l is actually vague in our calculations which are mathematically exact and clear, so is the physical meaning of D ν in Eq. (8) . But this vagueness will automatically disappear after spontaneous electroweak symmetry breaking, as can be subsequently seen. and l ′ = ( e μ τ ) T is defined as the column vector of the mass eigenstates of three charged leptons versus the column vector of their flavor eigenstates l = ( l e l μ l τ ) T . Substituting Eqs. (8) —(10) and (12) into Eq. (3) , we immediately arrive at(13) − L Λ = l L ′ ‾ D l l R ′ + 1 2 ν L ′ ‾ D ν ( ν L ′ ) c + 1 2 ( N R ′ ) c ‾ D N N R ′ + ν L ‾ Y l l R ϕ + − l L ‾ Y ν N R ϕ − + h . c . , where ν ′ = ( ν 1 ν 2 ν 3 ) T denotes the column vector of the working mass eigenstates of three light Majorana neutrinos far above the electroweak scale, and N ′ = ( N 1 N 2 N 3 ) T stands for the column vectors of the mass eigenstates of three heavy Majorana neutrinos relevant to the seesaw mechanism at Λ ≫ v . In this case the flavor eigenstates ν L and N R can be expressed in terms of the mass eigenstates ν L ′ and N R ′ or their charge-conjugated states as follows:(14) ν L = U ν L ′ + R ( N R ′ ) c , N R = S ′ ⁎ ( ν L ′ ) c + U ′ ⁎ N R ′ , where U ≡ A U 0 , U ′ ≡ U 0 ′ B and S ′ ≡ U 0 ′ S U 0 are defined. Taking account of the Majorana property of ν i and N i (i.e., ν i c = ν i and N i c = N i [7] for i = 1 , 2 , 3 ), one simply obtains ( N R ′ ) c = ( N ′ c ) L = N L ′ and ( ν L ′ ) c = ( ν ′ c ) R = ν R ′ . One may then substitute the expression of l L in Eq. (12) and that of ν L in Eq. (14) into the standard form of L cc in Eq. (7) and get at(15) − L cc = g 2 ( e μ τ ) L ‾ γ μ [ U PMNS ( ν 1 ν 2 ν 3 ) L + R PMNS ( N 1 N 2 N 3 ) L ] W μ − + h . c . , where U PMNS = U l † U is just the Pontecorvo-Maki-Nakagawa-Sakata (PMNS) lepton flavor mixing matrix [25–27] used to describe the flavor oscillations of three active neutrinos, and R PMNS = U l † R is an analogue of U PMNS in the seesaw mechanism which characterizes the strengths of weak charged-current interactions for three heavy Majorana neutrinos. Without loss of generality, one may choose a convenient flavor basis in which the mass eigenstates of three charged leptons are identified with their corresponding flavor eigenstates (i.e., l L = l L ′ , or equivalently U l = I ). In this case we are simply left with U PMNS = U and R PMNS = R , namely the effects of lepton flavor mixing originate purely from the active and sterile Majorana neutrino sectors and from the interplay between these two sectors. We shall take advantage of this flavor basis in the following discussions unless otherwise specified. 2.4 Mismatch between the two bases Before discussing a mismatch between the mass eigenstates of heavy Majorana neutrinos associated with thermal leptogenesis and those associated with the seesaw mechanism, let us take a look at the flavor structures of active and sterile neutrinos in the case that the electroweak gauge symmetry is unbroken at Λ. First of all, a combination of Eqs. (8) and (10) allows us to immediately derive the exact seesaw relation between the working masses of three light Majorana neutrinos and the real masses of three heavy Majorana neutrinos:(16) U D ν U T + R D N R T = 0 , in which U and R are also correlated with each other via the unitarity condition U U † + R R † = I . Note that U = A U 0 holds, where the unitary matrix U 0 is primarily responsible for flavor mixing of the three active neutrinos. So we find it useful to rewrite Eq. (16) as(17) U 0 D ν U 0 T = ( i A − 1 R ) D N ( i A − 1 R ) T , whose left- and right-hand sides are composed of the derivational and original seesaw parameters, respectively. This point will become more obvious when a complete Euler-like parametrization of the 6 × 6 unitary matrix U in Eq. (10) is adopted, as can be seen in section 3 . Needless to say, the active-sterile flavor mixing matrix R essentially plays the role of the neutrino Yukawa coupling matrix Y ν in the canonical seesaw framework,(18) Y ν ϕ 0 ⁎ = R D N [ I − ( B − 1 S A − 1 R ) T ] U ′ T ; and the right-handed Majorana neutrino mass matrix M R can be reconstructed into the form(19) M R = U ′ [ D N − ( B − 1 S A − 1 R ) D N ( B − 1 S A − 1 R ) T ] U ′ T . Note that all the quantities in Eqs. (18) and (19) belong to the original seesaw parameters in the sense that they have nothing to do with D ν and U 0 — the working masses and the primary flavor mixing matrix of three light Majorana neutrinos which are derived from the seesaw mechanism. Now we turn to an unavoidable mismatch between the mass eigenstates of three heavy Majorana neutrinos associated with the seesaw and leptogenesis mechanisms. Eq. (14) tells us that the mass eigenstates N R ′ in the seesaw basis can be expressed as(20) N R ′ = ( U ′ ⁎ ) − 1 [ N R − S ′ ⁎ ( ν L ′ ) c ] = ( B ⁎ ) − 1 [ N R ′ − U 0 ′ T S ′ ⁎ ( ν L ′ ) c ] , where Eq. (4) has been used to link N R ′ to N R ′ . To be more explicit, Eq. (20) means(21) ( N 1 N 2 N 3 ) = ( B ⁎ ) − 1 [ ( N 1 N 2 N 3 ) − U 0 ′ T S ′ ⁎ ( ν 1 ν 2 ν 3 ) ] , from which the differences between N i in the seesaw basis and N i (for i = 1 , 2 , 3 ) in the thermal leptogenesis basis can be clearly seen. Similarly, a combination of Eqs. (4) and (19) leads us to(22) D N = B [ D N − ( B − 1 S A − 1 R ) D N ( B − 1 S A − 1 R ) T ] B T , from which one may easily see the difference between D N and D N . Although N R ′ (or D N ) and N R ′ (or D N ) would exactly coincide with each other if the Yukawa coupling matrix Y ν (or equivalently, R or S ) were switched off, such a coincidence would make no sense because both the seesaw and leptogenesis mechanisms would fail in this special case. In the presence of the neutrino Yukawa interactions, thermal leptogenesis may take effect via the CP-violating and out-of-equilibrium decays of heavy Majorana neutrinos into the leptonic and Higgs doublets, while the seesaw mechanism can “formally” work with the help of an interplay between the active and sterile neutrino fields coupled only to the neutral component of the Higgs doublet. That is the key reason why there is an inevitable mismatch between the seesaw- and leptogenesis-associated bases for heavy Majorana neutrinos before spontaneous electroweak symmetry breaking. 2.5 After gauge symmetry breaking So far we have made some proper transformations of the charged lepton and neutrino fields in the flavor space to obtain their respective working or true mass eigenstates. All such unitary flavor basis transformations are completely reversible, and hence they do not affect the gauge invariance of L Λ at the seesaw scale. As already shown in Eqs. (20) and (22) , a seeable mismatch between N R ′ and N R ′ or between D N and D N results from the fact that the working seesaw mechanism itself is only associated with the neutral component of the Higgs doublet while the heavy Majorana neutrino decays and thermal leptogenesis at the seesaw scale Λ are associated with the whole Higgs doublet. This unavoidable mismatch deserves to be conceptually clarified as we have done, because it is an intrinsic issue of the seesaw and leptogenesis mechanisms. It is now straightforward to prove that the formal seesaw mechanism far above the electroweak scale will become real after the Higgs potential of the SM is minimized at 〈 H 〉 ≡ 〈 0 | H | 0 〉 = v / 2 with a special direction characterized by 〈 ϕ ± 〉 = 0 and 〈 ϕ 0 〉 = v / 2 , by which the electroweak gauge symmetry is spontaneously broken and thus all the particles coupled to the Higgs field acquire their nonzero masses. In this case the Lagrangian in Eq. (3) can be simplified to a more popular form,(23) − L Λ ′ = l L ‾ M l l R + 1 2 [ ν L ( N R ) c ] ‾ ( 0 M D M D T M R ) [ ( ν L ) c N R ] + h . c . , where M l ≡ Y l 〈 ϕ 0 〉 = Y l v / 2 denotes the charged lepton mass matrix, and M D ≡ Y ν 〈 ϕ 0 〉 = Y ν v / 2 is usually referred to as the Dirac neutrino mass matrix. The expression of M D in terms of the seesaw parameters can be directly read off from Eq. (18) , namely(24) M D = R D N [ I − ( B − 1 S A − 1 R ) T ] U ′ T . We find that the exact seesaw formula obtained in Eq. (16) and the analytical results obtained in Eqs. (19) —(22) formally keep unchanged after spontaneous gauge symmetry breaking, but they are now subject to the electroweak scale. In other words, the electroweak symmetry breaking itself does not really affect the flavor structures of the seesaw mechanism. This observation implies that it is possible to determine or constrain some of the original seesaw-associated flavor parameters in some low-energy neutrino experiments, after the radiative corrections to such parameters are properly taken into account with the help of the relevant renormaliztion-group equations (RGEs) between a superhigh seesaw scale and the electroweak scale [28] . Note that the exact seesaw formula obtained in Eq. (16) can be simplified to the more popular form in the leading-order approximations of Eqs. (19) and (24) . That is, M R ≃ U ′ D N U ′ T and M D ≃ R D N U ′ T , so the effective mass matrix for three active Majorana neutrinos is given by(25) M ν ≡ U 0 D ν U 0 T ≃ − R D N R T ≃ − M D M R − 1 M D T , where A ≃ B ≃ I has been assumed (i.e., U ≃ U 0 holds in the neglect of the non-unitary effects characterized by A ≠ I ). In this approximation the effective Majorana mass term for three active neutrinos at low energies turns out to be(26) − L ν = 1 2 ν L ‾ M ν ( ν L ) c + h . c . = 1 2 ν L ′ ‾ D ν ( ν L ′ ) c + h . c . , where the column vector of the light neutrino mass eigenstates ν L ′ has already been defined below Eq. (13) , and the physical meaning of D ν as the diagonal Majorana neutrino mass matrix becomes definite and obvious. 3 How small is the mismatch? 3.1 An Euler-like parametrization To clearly see how small the difference between N R ′ (or D N ) and N R ′ (or D N ) is expected to be, let us follow Refs. [22–24] to make an Euler-like parametrization of the 6 × 6 unitary matrix U in Eq. (10) . First of all we introduce fifteen 6 × 6 Euler-like unitary matrices of the form O i j (for 1 ≤ i < j ≤ 6 ): its ( i , i ) and ( j , j ) entries are both identical to c i j ≡ cos θ i j with θ i j being a flavor mixing angle and lying in the first quadrant, its other four diagonal elements are all equal to one, its ( i , j ) and ( j , i ) entries are given respectively by s ˆ i j ⁎ ≡ e − i δ i j sin θ i j and − s ˆ i j ≡ − e i δ i j sin θ i j with δ i j being a CP-violating phase, and its other off-diagonal elements are all equal to zero. These matrices are then grouped in the following way to respectively describe the active flavor sector, the sterile flavor sector and the interplay between these two sectors:(27) ( U 0 0 0 I ) = O 23 O 13 O 12 , ( I 0 0 U 0 ′ ) = O 56 O 46 O 45 , ( A R S B ) = O 36 O 26 O 16 O 35 O 25 O 15 O 34 O 24 O 14 , where the pattern of U 0 is quite similar to the standard parametrization of a unitary PMNS matrix as advocated by the Particle Data Group [9] ,5 5 When U 0 is applied to the phenomenology of neutrino physics in the basis of U l = I , it is the phase parameter δ ≡ δ 13 − δ 12 − δ 23 that characterizes the strength of CP violation in neutrino oscillations. (28) U 0 = ( c 12 c 13 s ˆ 12 ⁎ c 13 s ˆ 13 ⁎ − s ˆ 12 c 23 − c 12 s ˆ 13 s ˆ 23 ⁎ c 12 c 23 − s ˆ 12 ⁎ s ˆ 13 s ˆ 23 ⁎ c 13 s ˆ 23 ⁎ s ˆ 12 s ˆ 23 − c 12 s ˆ 13 c 23 − c 12 s ˆ 23 − s ˆ 12 ⁎ s ˆ 13 c 23 c 13 c 23 ) , and the expression of U 0 ′ can be directly read off from that of U 0 with the subscript replacements 12 ↔ 45 , 13 ↔ 46 and 23 ↔ 56 for the three rotation angles and three CP-violating phases. The explicit expressions of A , B , R and S in terms of c i j and s ˆ i j (for i = 1 , 2 , 3 and j = 4 , 5 , 6 ) are rather lengthy, and hence they are listed in Eqs. (A.1) and (A.2) in Appendix A for the same of simplicity. Among the four active-sterile flavor mixing matrices, only A and R affect the physical processes in which the light and heavy Majorana neutrinos take part, as can be seen from Eq. (15) . As both U = A U 0 and R appear in L cc in the chosen flavor basis (i.e., U l = I ), three of the nice CP-violating phases (or their combinations) of A and R can always be rotated away by properly redefining the phases of three charged lepton fields [29,30] . The PMNS matrix U is obviously non-unitary because of U U † = A A † = I − R R † ≠ I , but its deviation from exact unitarity (i.e., from U 0 ) is found to be very small. A detailed and careful analysis of currently available electroweak precision measurements and neutrino oscillation data has put a stringent constraint on the non-unitarity of U — the latter is below or far below O ( 10 − 2 ) [31–35] . This result implies that the deviation of A A † from I ought to be smaller than O ( 10 − 2 ) , and thus the nine active-sterile flavor mixing angles in R should be smaller than O ( 10 − 1 ) . The advantage of such a phenomenological observation is that U ≃ U 0 can be a quite reliable approximation in most cases, but its disadvantage is that an experimental exploration of the seesaw-induced non-unitary effects of U at low energies will be rather challenging. 3.2 Smallness of the mismatch Eq. (20) tells us that a difference between the mass eigenstates of three heavy Majorana neutrinos associated with the seesaw mechanism (i.e., N R ′ ) and those associated with thermal leptogenesis (i.e., N R ′ ) is mainly characterized by the deviation of ( B ⁎ ) − 1 from the identity matrix I . With the help of Eq. (A.2) , we arrive at(29) ( B ⁎ ) − 1 = ( c 14 − 1 c 24 − 1 c 34 − 1 0 0 + t ˆ 14 c 24 − 1 c 34 − 1 t ˆ 15 ⁎ + t ˆ 24 c 34 − 1 c 24 − 1 t ˆ 25 ⁎ + t ˆ 34 c 15 − 1 c 25 − 1 t ˆ 35 ⁎ c 15 − 1 c 25 − 1 c 35 − 1 0 + t ˆ 14 c 24 − 1 c 34 − 1 c 24 − 1 t ˆ 16 ⁎ + t ˆ 24 c 34 − 1 t ˆ 15 t ˆ 25 ⁎ t ˆ 16 ⁎ + t ˆ 24 c 34 − 1 c 25 − 1 c 16 − 1 t ˆ 26 ⁎ + t ˆ 34 t ˆ 15 c 25 − 1 t ˆ 35 ⁎ t ˆ 16 ⁎ + t ˆ 34 c 35 − 1 c 16 − 1 c 26 − 1 t ˆ 36 ⁎ + t ˆ 34 t ˆ 25 t ˆ 35 ⁎ c 16 − 1 t ˆ 26 ⁎ + t ˆ 15 c 25 − 1 c 35 − 1 t ˆ 16 ⁎ + t ˆ 25 c 35 − 1 c 16 − 1 t ˆ 26 ⁎ + t ˆ 35 c 16 − 1 c 26 − 1 t ˆ 36 ⁎ c 16 − 1 c 26 − 1 c 36 − 1 ) ≃ I + ( 1 2 ( s 14 2 + s 24 2 + s 34 2 ) 0 0 s ˆ 14 s ˆ 15 ⁎ + s ˆ 24 s ˆ 25 ⁎ + s ˆ 34 s ˆ 35 ⁎ 1 2 ( s 15 2 + s 25 2 + s 35 2 ) 0 s ˆ 14 s ˆ 16 ⁎ + s ˆ 24 s ˆ 26 ⁎ + s ˆ 34 s ˆ 36 ⁎ s ˆ 15 s ˆ 16 ⁎ + s ˆ 25 s ˆ 26 ⁎ + s ˆ 35 s ˆ 36 ⁎ 1 2 ( s 16 2 + s 26 2 + s 36 2 ) ) , where t ˆ i j ≡ e i δ i j tan θ i j is defined, and all the terms of O ( s i j 4 ) or smaller have been omitted from the second equation as an excellent approximation due to the smallness of θ i j (for i = 1 , 2 , 3 and j = 4 , 5 , 6 ). We see that ( B ⁎ ) − 1 is also a lower triangular matrix like B itself. On the other hand, the factor U 0 ′ T S ′ ⁎ appearing in Eq. (20) can be explicitly expressed as follows:(30) U 0 ′ T S ′ ⁎ = B T S ⁎ U 0 ⁎ ≃ − ( s ˆ 14 ⁎ s ˆ 24 ⁎ s ˆ 34 ⁎ s ˆ 15 ⁎ s ˆ 25 ⁎ s ˆ 35 ⁎ s ˆ 16 ⁎ s ˆ 26 ⁎ s ˆ 36 ⁎ ) U 0 ⁎ , where Eq. (A.2) has been used, and the terms of O ( s i j 3 ) or smaller have been omitted from the second equation as a very good approximation. Now we conclude that the heavy Majorana neutrino mass basis N R ′ is identical to N R ′ up to the accuracy of O ( s i j 2 ) , but it contains a small contribution of O ( s i j ) from the light Majorana neutrino mass basis ( ν L ′ ) c in the seesaw framework. Since the magnitudes of θ i j (for i = 1 , 2 , 3 and j = 4 , 5 , 6 ) are highly suppressed in a realistic seesaw model with little fine-tuning, the mismatch between N R ′ and N R ′ is expected to be negligible in most cases. Let us proceed to examine how small the difference between D N and D N in Eq. (21) can be. First of all, Eq. (A.2) allows us to make the approximation(31) B ≃ I − ( 1 2 ( s 14 2 + s 24 2 + s 34 2 ) 0 0 s ˆ 14 ⁎ s ˆ 15 + s ˆ 24 ⁎ s ˆ 25 + s ˆ 34 ⁎ s ˆ 35 1 2 ( s 15 2 + s 25 2 + s 35 2 ) 0 s ˆ 14 ⁎ s ˆ 16 + s ˆ 24 ⁎ s ˆ 26 + s ˆ 34 ⁎ s ˆ 36 s ˆ 15 ⁎ s ˆ 16 + s ˆ 25 ⁎ s ˆ 26 + s ˆ 35 ⁎ s ˆ 36 1 2 ( s 16 2 + s 26 2 + s 36 2 ) ) , where the terms of O ( s i j 4 ) or smaller have been omitted. Secondly, we obtain(32) A − 1 R = ( t ˆ 14 ⁎ c 14 − 1 t ˆ 15 ⁎ c 14 − 1 c 15 − 1 t ˆ 16 ⁎ c 14 − 1 t ˆ 24 ⁎ t ˆ 14 t ˆ 15 ⁎ t ˆ 24 ⁎ + c 15 − 1 c 24 − 1 t ˆ 25 ⁎ + t ˆ 14 c 15 − 1 t ˆ 16 ⁎ t ˆ 24 ⁎ + t ˆ 15 t ˆ 16 ⁎ c 24 − 1 t ˆ 25 ⁎ + c 16 − 1 c 24 − 1 c 25 − 1 t ˆ 26 ⁎ c 14 − 1 c 24 − 1 t ˆ 34 ⁎ + t ˆ 14 t ˆ 15 ⁎ c 24 − 1 t ˆ 34 ⁎ + c 15 − 1 t ˆ 24 t ˆ 25 ⁎ t ˆ 34 ⁎ + c 15 − 1 c 25 − 1 c 34 − 1 t ˆ 35 ⁎ + t ˆ 14 c 15 − 1 t ˆ 16 ⁎ c 24 − 1 t ˆ 34 ⁎ + t ˆ 15 t ˆ 16 ⁎ t ˆ 24 t ˆ 25 ⁎ t ˆ 34 ⁎ + t ˆ 15 t ˆ 16 ⁎ c 25 − 1 c 34 − 1 t ˆ 35 ⁎ + c 16 − 1 t ˆ 24 c 25 − 1 t ˆ 26 ⁎ t ˆ 34 ⁎ + c 16 − 1 t ˆ 25 t ˆ 26 ⁎ c 34 − 1 t ˆ 35 ⁎ + c 16 − 1 c 26 − 1 c 34 − 1 c 35 − 1 t ˆ 36 ⁎ ) ≃ ( s ˆ 14 ⁎ s ˆ 15 ⁎ s ˆ 16 ⁎ s ˆ 24 ⁎ s ˆ 25 ⁎ s ˆ 26 ⁎ s ˆ 34 ⁎ s ˆ 35 ⁎ s ˆ 36 ⁎ ) from Eq. (A.1) , where the terms of O ( s i j 3 ) or smaller have been neglected in the second equation as a reasonably good approximation. The exact expression of B − 1 S can be directly read off from that of − ( A − 1 R ) ⁎ with the help of Eq. (32) by making the subscript replacements 15 ↔ 24 , 16 ↔ 34 and 26 ↔ 35 , so can its approximate expression. As a consequence,(33) B − 1 S A − 1 R ≃ − ( s 14 2 + s 24 2 + s 34 2 s ˆ 14 s ˆ 15 ⁎ + s ˆ 24 s ˆ 25 ⁎ + s ˆ 34 s ˆ 35 ⁎ s ˆ 14 s ˆ 16 ⁎ + s ˆ 24 s ˆ 26 ⁎ + s ˆ 34 s ˆ 36 ⁎ s ˆ 14 ⁎ s ˆ 15 + s ˆ 24 ⁎ s ˆ 25 + s ˆ 34 ⁎ s ˆ 35 s 15 2 + s 25 2 + s 35 2 s ˆ 15 s ˆ 16 ⁎ + s ˆ 25 s ˆ 26 ⁎ + s ˆ 35 s ˆ 36 ⁎ s ˆ 14 ⁎ s ˆ 16 + s ˆ 24 ⁎ s ˆ 26 + s ˆ 34 ⁎ s ˆ 36 s ˆ 15 ⁎ s ˆ 16 + s ˆ 25 ⁎ s ˆ 26 + s ˆ 35 ⁎ s ˆ 36 s 16 2 + s 26 2 + s 36 2 ) holds in the same approximation as made above. This result implies that D N and D N are identical to each other up to the accuracy of O ( s i j 2 ) , simply because on the right-hand side of Eq. (22) the second term is suppressed in magnitude to O ( s i j 4 ) as compared with the first term. It is worth remarking that our above analytical approximations are more or less subject to the canonical seesaw mechanism at an energy scale far above the electroweak scale, and thus the mismatch between N R ′ (or D N ) and N R ′ (or D N ) is very small. This situation will change when the low-scale seesaw and leptogenesis scenarios, in which a mismatch between the two sets of mass bases for heavy Majorana neutrinos is crucial, are taken into account (see, e.g., Refs. [11,12] ). 3.3 Determination of D ν and U 0 As already shown in Eq. (17) , the nine effective flavor parameters of three light Majorana neutrinos in D ν and U 0 (i.e., three effective masses, three flavor mixing angles and three CP-violating phases) can be expressed in terms of the eighteen seesaw parameters hidden in A , R and D N (i.e., three heavy Majorana neutrino masses, nine active-sterile flavor mixing angles and six CP-violating phases). It is obvious that all the derivational seesaw parameters on the left-hand side of Eq. (17) would vanish if R ∝ Y ν were switched off. So this equation provides an unambiguous way to determine the light degrees of freedom from the heavy degrees of freedom in the seesaw framework. To be more specific, the six independent elements of the effective Majorana neutrino mass matrix M ν ≡ U 0 D ν U 0 T are given as follows:(34) ( M ν ) 11 = m 1 c 12 2 c 13 2 + m 2 s ˆ 12 ⁎ 2 c 13 2 + m 3 s ˆ 13 ⁎ 2 , ( M ν ) 12 = − m 1 c 12 c 13 ( s ˆ 12 c 23 + c 12 s ˆ 13 s ˆ 23 ⁎ ) + m 2 s ˆ 12 ⁎ c 13 ( c 12 c 23 − s ˆ 12 ⁎ s ˆ 13 s ˆ 23 ⁎ ) + m 3 c 13 s ˆ 13 ⁎ s ˆ 23 ⁎ , ( M ν ) 13 = m 1 c 12 c 13 ( s ˆ 12 s ˆ 23 − c 12 s ˆ 13 c 23 ) − m 2 s ˆ 12 ⁎ c 13 ( c 12 s ˆ 23 + s ˆ 12 ⁎ s ˆ 13 c 23 ) + m 3 c 13 s ˆ 13 ⁎ c 23 , ( M ν ) 22 = m 1 ( s ˆ 12 c 23 + c 12 s ˆ 13 s ˆ 23 ⁎ ) 2 + m 2 ( c 12 c 23 − s ˆ 12 ⁎ s ˆ 13 s ˆ 23 ⁎ ) 2 + m 3 c 13 2 s ˆ 23 ⁎ 2 , ( M ν ) 23 = − m 1 ( s ˆ 12 c 23 + c 12 s ˆ 13 s ˆ 23 ⁎ ) ( s ˆ 12 s ˆ 23 − c 12 s ˆ 13 c 23 ) − m 2 ( c 12 c 23 − s ˆ 12 ⁎ s ˆ 13 s ˆ 23 ⁎ ) ( c 12 s ˆ 23 + s ˆ 12 ⁎ s ˆ 13 c 23 ) + m 3 c 13 2 c 23 s ˆ 23 ⁎ , ( M ν ) 33 = m 1 ( s ˆ 12 s ˆ 23 − c 12 s ˆ 13 c 23 ) 2 + m 2 ( c 12 s ˆ 23 + s ˆ 12 ⁎ s ˆ 13 c 23 ) 2 + m 3 c 13 2 c 23 2 . On the other hand, Eq. (17) tells us that these six matrix elements can originally be determined by M ν = − ( A − 1 R ) D N ( A − 1 R ) T thanks to the exact seesaw relation bridging the big gap between the light and heavy Majorana neutrinos. With the help of the explicit expression of A − 1 R given in Eq. (32) , it is straightforward to obtain the expressions for the elements of M ν in terms of M i , θ i j and δ i j (for i = 1 , 2 , 3 and j = 4 , 5 , 6 ). Instead of presenting the exact analytical results, which are rather lengthy and hence less instructive, here we make the leading-order approximations for the expressions of A and R given in Eq. (A.1) and then arrive at(35) ( M ν ) 11 ≃ − [ M 1 s ˆ 14 ⁎ 2 + M 2 s ˆ 15 ⁎ 2 + M 3 s ˆ 16 ⁎ 2 ] , ( M ν ) 12 ≃ − [ M 1 s ˆ 14 ⁎ s ˆ 24 ⁎ + M 2 s ˆ 15 ⁎ s ˆ 25 ⁎ + M 3 s ˆ 16 ⁎ s ˆ 26 ⁎ ] , ( M ν ) 13 ≃ − [ M 1 s ˆ 14 ⁎ s ˆ 34 ⁎ + M 2 s ˆ 15 ⁎ s ˆ 35 ⁎ + M 3 s ˆ 16 ⁎ s ˆ 36 ⁎ ] , ( M ν ) 22 ≃ − [ M 1 s ˆ 24 ⁎ 2 + M 2 s ˆ 25 ⁎ 2 + M 3 s ˆ 26 ⁎ 2 ] , ( M ν ) 23 ≃ − [ M 1 s ˆ 24 ⁎ s ˆ 34 ⁎ + M 2 s ˆ 25 ⁎ s ˆ 35 ⁎ + M 3 s ˆ 26 ⁎ s ˆ 36 ⁎ ] , ( M ν ) 33 ≃ − [ M 1 s ˆ 34 ⁎ 2 + M 2 s ˆ 35 ⁎ 2 + M 3 s ˆ 36 ⁎ 2 ] . Let us emphasize that there appear nine CP-violating phases in Eq. (35) , but three of them (or their combinations) are redundant and can always be removed by rephasing the charged lepton fields in a proper way.6 6 A straightforward way to remove the three redundant phase parameters of A and R is just to switch off three of the nine phases in the nine active-sterile flavor mixing matrices O i j (for i = 1 , 2 , 3 and j = 4 , 5 , 6 ) in Eq. (27) from the very beginning. As there are many options in doing so, we do not go into details here. A combination of Eqs. (34) and (35) allows us to establish the direct relations between the nine derivational and eighteen original seesaw parameters. So the former can in principle be determined from the latter for a given seesaw model (a top-down approach), and the latter may be partly probed or constrained from the former with the help of some low-energy neutrino experiments (a bottom-up approach). A careful and detailed analysis of the parameter space along this line of thought will be made elsewhere. 4 Summary We have reformulated the canonical seesaw mechanism by considering the fact that the electroweak gauge symmetry is unbroken at the seesaw scale characterized by the masses of heavy Majorana neutrinos, and shown that it can formally work and allow us to derive an exact seesaw relation between the active (light) and sterile (heavy) Majorana neutrinos. In this way we have elucidated the reason why there is an unavoidable mismatch between the mass eigenstates of heavy Majorana neutrinos associated with the seesaw and thermal leptogenesis mechanisms. The smallness of this mismatch has been discussed with the help of a complete Euler-like parametrization of the flavor structure in the seesaw framework, and the exact and explicit relations between the original and derivational seesaw parameters have been established as a by-product. We hope that this work may help clarify some conceptual ambiguities associated with the validity of the seesaw mechanism before and after spontaneous electroweak symmetry breaking, because such ambiguities have never been taken serious in the literature. It should also be helpful to clarify the ambiguities associated with the RGE evolution between the “virtual” flavor parameters of Majorana neutrinos at the seesaw scale and those “real” ones at the electroweak scale, which is crucial to bridge the gap between a well-motivated UV-complete flavor theory including the seesaw mechanism and all the possible low-energy flavor experiments. CRediT authorship contribution statement Zhi-zhong Xing: Conceptualization, Investigation, Methodology, Writing – original draft, Writing – review & editing.Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgements I am greatly indebted to Di Zhang and Shun Zhou for numerous helpful discussions and comments about this paper, a work dedicated to the 50th birthday of my home institute, the Institute of High Energy Physics, which was founded on 1 February 1973. I would also like to thank Marco Drewes for very useful discussions and comments. My research is supported in part by the National Natural Science Foundation of China under grant No. 12075254 and grant No. 11835013 . Appendix A The expressions of A , B , R and S Given the Euler-like parametrization of the 6 × 6 unitary flavor mixing matrix U decomposed in Eq. (27) , the 3 × 3 active-sterile flavor mixing matrices A , B , R and S depend on the same nine rotation angles θ i j and nine phase angles δ i j (for i = 1 , 2 , 3 and j = 4 , 5 , 6 ). To be explicit [22,23] ,(A.1) A = ( c 14 c 15 c 16 0 0 − c 14 c 15 s ˆ 16 s ˆ 26 ⁎ − c 14 s ˆ 15 s ˆ 25 ⁎ c 26 − s ˆ 14 s ˆ 24 ⁎ c 25 c 26 c 24 c 25 c 26 0 − c 14 c 15 s ˆ 16 c 26 s ˆ 36 ⁎ + c 14 s ˆ 15 s ˆ 25 ⁎ s ˆ 26 s ˆ 36 ⁎ − c 14 s ˆ 15 c 25 s ˆ 35 ⁎ c 36 + s ˆ 14 s ˆ 24 ⁎ c 25 s ˆ 26 s ˆ 36 ⁎ + s ˆ 14 s ˆ 24 ⁎ s ˆ 25 s ˆ 35 ⁎ c 36 − s ˆ 14 c 24 s ˆ 34 ⁎ c 35 c 36 − c 24 c 25 s ˆ 26 s ˆ 36 ⁎ − c 24 s ˆ 25 s ˆ 35 ⁎ c 36 − s ˆ 24 s ˆ 34 ⁎ c 35 c 36 c 34 c 35 c 36 ) , R = ( s ˆ 14 ⁎ c 15 c 16 s ˆ 15 ⁎ c 16 s ˆ 16 ⁎ − s ˆ 14 ⁎ c 15 s ˆ 16 s ˆ 26 ⁎ − s ˆ 14 ⁎ s ˆ 15 s ˆ 25 ⁎ c 26 + c 14 s ˆ 24 ⁎ c 25 c 26 − s ˆ 15 ⁎ s ˆ 16 s ˆ 26 ⁎ + c 15 s ˆ 25 ⁎ c 26 c 16 s ˆ 26 ⁎ − s ˆ 14 ⁎ c 15 s ˆ 16 c 26 s ˆ 36 ⁎ + s ˆ 14 ⁎ s ˆ 15 s ˆ 25 ⁎ s ˆ 26 s ˆ 36 ⁎ − s ˆ 14 ⁎ s ˆ 15 c 25 s ˆ 35 ⁎ c 36 − c 14 s ˆ 24 ⁎ c 25 s ˆ 26 s ˆ 36 ⁎ − c 14 s ˆ 24 ⁎ s ˆ 25 s ˆ 35 ⁎ c 36 + c 14 c 24 s ˆ 34 ⁎ c 35 c 36 − s ˆ 15 ⁎ s ˆ 16 c 26 s ˆ 36 ⁎ − c 15 s ˆ 25 ⁎ s ˆ 26 s ˆ 36 ⁎ + c 15 c 25 s ˆ 35 ⁎ c 36 c 16 c 26 s ˆ 36 ⁎ ) ; and(A.2) B = ( c 14 c 24 c 34 0 0 − c 14 c 24 s ˆ 34 ⁎ s ˆ 35 − c 14 s ˆ 24 ⁎ s ˆ 25 c 35 − s ˆ 14 ⁎ s ˆ 15 c 25 c 35 c 15 c 25 c 35 0 − c 14 c 24 s ˆ 34 ⁎ c 35 s ˆ 36 + c 14 s ˆ 24 ⁎ s ˆ 25 s ˆ 35 ⁎ s ˆ 36 − c 14 s ˆ 24 ⁎ c 25 s ˆ 26 c 36 + s ˆ 14 ⁎ s ˆ 15 c 25 s ˆ 35 ⁎ s ˆ 36 + s ˆ 14 ⁎ s ˆ 15 s ˆ 25 ⁎ s ˆ 26 c 36 − s ˆ 14 ⁎ c 15 s ˆ 16 c 26 c 36 − c 15 c 25 s ˆ 35 ⁎ s ˆ 36 − c 15 s ˆ 25 ⁎ s ˆ 26 c 36 − s ˆ 15 ⁎ s ˆ 16 c 26 c 36 c 16 c 26 c 36 ) , S = ( − s ˆ 14 c 24 c 34 − s ˆ 24 c 34 − s ˆ 34 s ˆ 14 c 24 s ˆ 34 ⁎ s ˆ 35 + s ˆ 14 s ˆ 24 ⁎ s ˆ 25 c 35 − c 14 s ˆ 15 c 25 c 35 s ˆ 24 s ˆ 34 ⁎ s ˆ 35 − c 24 s ˆ 25 c 35 − c 34 s ˆ 35 s ˆ 14 c 24 s ˆ 34 ⁎ c 35 s ˆ 36 − s ˆ 14 s ˆ 24 ⁎ s ˆ 25 s ˆ 35 ⁎ s ˆ 36 + s ˆ 14 s ˆ 24 ⁎ c 25 s ˆ 26 c 36 + c 14 s ˆ 15 c 25 s ˆ 35 ⁎ s ˆ 36 + c 14 s ˆ 15 s ˆ 25 ⁎ s ˆ 26 c 36 − c 14 c 15 s ˆ 16 c 26 c 36 s ˆ 24 s ˆ 34 ⁎ c 35 s ˆ 36 + c 24 s ˆ 25 s ˆ 35 ⁎ s ˆ 36 − c 24 c 25 s ˆ 26 c 36 − c 34 c 35 s ˆ 36 ) . 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+NUPHB 116106 116106 S0550-3213(23)00035-4 10.1016/j.nuclphysb.2023.116106 The Author(s) High Energy Physics – Phenomenology The formal seesaw mechanism of Majorana neutrinos with unbroken gauge symmetry Zhi-zhong Xing Conceptualization Investigation Methodology Writing – original draft Writing – review & editing a b ⁎ xingzz@ihep.ac.cn a Institute of High Energy Physics and School of Physical Sciences, University of Chinese Academy of Sciences, Beijing 100049, China Institute of High Energy Physics School of Physical Sciences University of Chinese Academy of Sciences Beijing 100049 China Institute of High Energy Physics and School of Physical Sciences, University of Chinese Academy of Sciences, Beijing 100049, China b Center of High Energy Physics, Peking University, Beijing 100871, China Center of High Energy Physics Peking University Beijing 100871 China Center of High Energy Physics, Peking University, Beijing 100871, China ⁎ Correspondence to: Institute of High Energy Physics and School of Physical Sciences, University of Chinese Academy of Sciences, Beijing 100049, China. Institute of High Energy Physics School of Physical Sciences University of Chinese Academy of Sciences Beijing 100049 China Editor: Tommy Ohlsson Abstract We reformulate the canonical seesaw mechanism in the case that the electroweak gauge symmetry is unbroken, and show that it can formally work and allow us to derive an exact seesaw formula for the light and heavy Majorana neutrinos. We elucidate the reason why there is a mismatch between the mass eigenstates of heavy Majorana neutrinos associated with thermal leptogenesis and those associated with the seesaw framework, and establish the exact and explicit relations between the original and derivational seesaw parameters by using an Euler-like parametrization of the 6 × 6 active-sterile flavor mixing matrix. Data availability No data was used for the research described in the article. 1 Motivation Among all the proposed mechanisms toward deeply understanding the true origin of tiny masses of the three known neutrinos ν i (for i = 1 , 2 , 3 ), whose flavor eigenstates are commonly denoted as ν α (for α = e , μ , τ ), the canonical seesaw mechanism [1–5] stands out as being most economical and most natural. The simplicity of this mechanism lies in two aspects: (a) it just takes into account the right-handed neutrino fields N α R , the chiral counterparts of the left-handed neutrino fields ν α L (for α = e , μ , τ ), which were originally ignored from the particle content of the standard model (SM) [6] ; (b) it simply allows for lepton number violation or the Majorana nature of massive neutrinos [7] , which is completely harmless to the theoretical framework of the SM itself. The naturalness of this mechanism is reflected in its attributing the small masses of ν i to the existence of three heavy Majorana neutrinos N i (for i = 1 , 2 , 3 ), whose masses are expected to be far above the fulcrum of the seesaw — presumably the electroweak symmetry breaking scale of the SM characterized by the vacuum expectation value of the Higgs field. On the other hand, the seesaw mechanism offers a big bonus to cosmology: the CP-violating and out-of-equilibrium decays of heavy Majorana neutrinos may give rise to a net lepton-antilepton asymmetry in the early Universe, and such a leptogenesis mechanism [8] can finally lead to baryogenesis as a natural interpretation of the observed baryon-antibaryon asymmetry in today's Universe [9] . In this sense the seesaw mechanism is the very stone that can kill two fundamental birds in particle physics and cosmology. Note that the seesaw mechanism is expected to take effect at a superhigh energy scale Λ which is essentially of the order of the heavy Majorana neutrino masses. But the SU ( 2 ) L × U ( 1 ) Y electroweak gauge symmetry has been unbroken until the Higgs field develops a nonzero vacuum expectation value v of O ( 10 2 ) GeV . In this situation the three active neutrinos are actually impossible to acquire their true masses of O ( v 2 / Λ ) at the seesaw scale Λ due to the absence of a real fulcrum of the seesaw. On the other hand, thermal leptogenesis can be realized via the lepton-number-violating decays of heavy Majorana neutrinos into the leptonic and Higgs doublets at Λ. So we are well motivated to ask a conceptually important question: how can the seesaw mechanism formally survive with the unbroken electroweak gauge symmetry and work together with the leptogenesis mechanism? If the answer to this question is affirmative, we wonder whether the mass eigenstates of heavy Majorana neutrinos associated with thermal leptogenesis are exactly the same as those associated with the seesaw mechanism itself.1 1 A mismatch of this kind has been observed and discussed in the seesaw framework after spontaneous electroweak symmetry breaking and in an approximate way (see, e.g., Refs. [10–14] ). Here we shall take a new look at it before electroweak symmetry breaking and in an exact way at the tree level. In case that there exists a mismatch between these two sets of mass bases, then the question becomes how small this mismatch is likely to be. To answer the above questions and clarify some conceptual ambiguities that have never been taken seriously, we are going to study how to make the seesaw mechanism formally work before spontaneous electroweak symmetry breaking. We show that an exact seesaw relation between the light and heavy Majorana neutrinos can be established far above the electroweak scale, and it becomes the realistic seesaw relation after the Higgs field develops its vacuum expectation value. In this way it is straightforward to elucidate the reason why there is a mismatch between the mass eigenstates of heavy Majorana neutrinos associated with thermal leptogenesis and those associated with the seesaw mechanism. With the help of a full Euler-like parametrization of the flavor structure in the seesaw framework, we illuminate such a mismatch in a more specific way. The exact and explicit relations between the original and derivational parameters of massive Majorana neutrinos are obtained as a by-product, and they are expected to be useful in determining or constraining some of the original seesaw parameters from the low-energy neutrino experiments. 2 A formal seesaw mechanism? 2.1 The leptonic Yukawa interactions Let us begin with the gauge-invariant leptonic Yukawa interactions and the SU ( 2 ) L -singlet Majorana neutrino mass term of the canonical seesaw mechanism at Λ2 2 Throughout this paper, our discussions are subject to the minimal extension of the SM with three right-handed neutrino fields and lepton number violation at zero temperature, so as to make our key point clear and avoid possible complications (e.g., thermal corrections to the masses of heavy Majorana neutrinos [14] ). (1) − L Λ = ℓ L ‾ Y l H l R + ℓ L ‾ Y ν H ˜ N R + 1 2 ( N R ) c ‾ M R N R + h . c . , where ℓ L = ( ν L l L ) T denotes the leptonic SU ( 2 ) L doublet of the SM with ν L = ( ν e L ν μ L ν τ L ) T and l L = ( l e L l μ L l τ L ) T standing respectively for the column vectors of the left-handed neutrino and charged lepton fields, H ˜ ≡ i σ 2 H ⁎ with H = ( ϕ + ϕ 0 ) T being the Higgs doublet of the SM and σ 2 being the second Pauli matrix, l R = ( l e R l μ R l τ R ) T and N R = ( N e R N μ R N τ R ) T stand respectively for the column vectors of the right-handed charged lepton and neutrino fields which are the SU ( 2 ) L singlets, ( N R ) c ≡ C N R ‾ T with C being the charge-conjugation matrix and satisfying C − 1 = C † = C T = − C , Y l and Y ν represent the respective Yukawa coupling matrices of charged leptons and neutrinos, and M R is the symmetric right-handed neutrino mass matrix. In Eq. (1) the hypercharges of ℓ L , l R , N R , H and H ˜ are − 1 / 2 , −1, 0, + 1 / 2 and − 1 / 2 , respectively. Since ν L ‾ Y ν N R is a Lorentz scalar and can be transformed into(2) ν L ‾ Y ν N R = [ ν L ‾ Y ν N R ] T = ( N R ) c ‾ Y ν T ( ν L ) c , where ( ν L ) c ≡ C ν L ‾ T is the charge-conjugated counterpart of ν L , one may easily rewrite Eq. (1) as(3) − L Λ = l L ‾ Y l l R ϕ 0 + 1 2 [ ν L ( N R ) c ] ‾ ( 0 Y ν ϕ 0 ⁎ Y ν T ϕ 0 ⁎ M R ) [ ( ν L ) c N R ] + ν L ‾ Y l l R ϕ + − l L ‾ Y ν N R ϕ − + h . c . . This expression is highly nontrivial in the sense that it clearly shows a direct correlation between the left- and right-handed neutrino fields via their Yukawa couplings to the neutral component of the Higgs doublet even though the SU ( 2 ) L × U ( 1 ) Y gauge symmetry is perfect at the seesaw scale Λ. In this situation the 3 × 3 Yukawa coupling matrix Y ν can be regarded as a “virtual” fulcrum of the seesaw before spontaneous electroweak symmetry breaking. Note that both the scalar field ϕ 0 and its charge-conjugated counterpart ϕ 0 ⁎ have the mass dimension and act like two complex numbers in Eq. (3) . But of course they possess the respective hypercharges + 1 / 2 and − 1 / 2 as ϕ ± do. After spontaneous symmetry breaking ϕ 0 and ϕ 0 ⁎ will acquire the same vacuum expectation value 〈 ϕ 0 〉 = 〈 ϕ 0 ⁎ 〉 = v / 2 with v ≃ 246 GeV , together with 〈 ϕ − 〉 = 〈 ϕ + 〉 = 0 , as in the SM. Then the formal seesaw will acquire a real fulcrum which allows one to naturally attribute the smallness of three active Majorana neutrino masses to the existence of three heavy Majorana neutrinos, as can be seen later on. 2.2 The leptogenesis-associated basis Now that all the SM particles are exactly massless in the early Universe when the temperature is far above the electroweak scale, a realization of thermal leptogenesis at Λ ≫ v only needs to calculate the lepton-number-violating decays of heavy Majorana neutrinos into the leptonic doublet and the Higgs doublet at the one-loop level by simply starting from Eq. (1) instead of Eq. (3) (see, e.g., Refs. [8,15–18] ). In this case the column vector of the mass eigenstates of three heavy Majorana neutrinos, denoted as N ′ = ( N 1 N 2 N 3 ) T , can easily be obtained by making the Autonne-Takagi transformation [19,20] as follows:(4) U 0 ′ † M R U 0 ′ ⁎ = D N , N R ′ = U 0 ′ T N R , where U 0 ′ is a unitary matrix, and D N ≡ Diag { M 1 , M 2 , M 3 } with M i being the masses of N i (for i = 1 , 2 , 3 ). As a result, the Lagrangian L Λ in Eq. (1) becomes(5) − L Λ = ℓ L ‾ Y l H l R + ℓ L ‾ Y ν H ˜ N R ′ + 1 2 ( N R ′ ) c ‾ D N N R ′ + h . c . , where Y ν ≡ Y ν U 0 ′ ⁎ is defined for the sake of simplicity. The rates of N i decaying into ℓ L and H or their CP-conjugated states are therefore determined by M i and Y ν , so are the corresponding CP-violating asymmetries associated closely with thermal leptogenesis [15–18] .3 3 Here we have used some calligraphic characters to denote the relevant physical quantities in the basis where M R is diagonalized by the unitary transformation made in Eq. (4) . This basis is associated with N i decays and thermal leptogenesis, and it is conceptually different from the basis taken for the seesaw mechanism as can be seen below. To be more specific, the flavor-dependent CP-violating asymmetries of N i decays are given by(6) ε i α ≡ Γ ( N i → ℓ α + H ) − Γ ( N i → ℓ α ‾ + H ‾ ) ∑ α [ Γ ( N i → ℓ α + H ) + Γ ( N i → ℓ α ‾ + H ‾ ) ] = 1 8 π ( Y ν † Y ν ) i i ∑ j ≠ i { Im [ ( Y ν ⁎ ) α i ( Y ν ) α j ( Y ν † Y ν ) i j ξ ( x j i ) + ( Y ν ⁎ ) α i ( Y ν ) α j ( Y ν † Y ν ) i j ⁎ ζ ( x j i ) ] } , where the Latin and Greek subscripts run respectively over ( 1 , 2 , 3 ) and ( e , μ , τ ) , x j i ≡ M j 2 / M i 2 are defined, ξ ( x j i ) = x j i { 1 + 1 / ( 1 − x j i ) + ( 1 + x j i ) ln [ x j i / ( 1 + x j i ) ] } and ζ ( x j i ) = 1 / ( 1 − x j i ) are the loop functions. A net lepton-antilepton asymmetry can therefore result from ε i α in the early Universe, and later on it can be partly converted into a net baryon-antibaryon asymmetry via the sphaleron interactions (see Ref. [21] for a recent review). At this point it is worth remarking that the right-handed neutrino fields N α R have zero weak isospin and hypercharge, and hence they have no coupling with the charged and neutral gauge bosons of the SM. As a consequence, the mass eigenstates of heavy Majorana neutrinos obtained from Eq. (4) do not participate in the weak charged-current interactions of the SM,(7) − L cc = g 2 ℓ L ‾ γ μ ( σ 1 W μ 1 + σ 2 W μ 2 ) ℓ L = g 2 l L ‾ γ μ W μ − ν L + h . c . , where g denotes the weak gauge coupling constant, σ 1 , 2 represent the first and second Pauli matrices, W 1 , 2 μ are two of the original SU ( 2 ) L gauge fields, and W μ ± ≡ ( W μ 1 ∓ i W μ 2 ) / 2 stand for the fields of the physical charged gauge bosons W ± . But in the seesaw framework we shall see that the expression of L cc in Eq. (7) will get modified, and the corresponding mass eigenstates of three heavy Majorana neutrinos can definitely take part in the weak charged-current interactions. 2.3 The seesaw-associated basis We proceed to show that the canonical seesaw mechanism can “formally” work before spontaneous electroweak symmetry breaking but the corresponding mass eigenstates of three heavy Majorana neutrinos are not exactly the same as N i (for i = 1 , 2 , 3 ) obtained above for the neutrino decays and thermal leptogenesis. To clarify this important point, let us diagonalize the symmetric 6 × 6 matrix in Eq. (3) in the following Autonne-Takagi way:(8) U † ( 0 Y ν ϕ 0 ⁎ Y ν T ϕ 0 ⁎ M R ) U ⁎ = ( D ν 0 0 D N ) , where U is a 6 × 6 unitary matrix, and the diagonal and real matrices D ν and D N are defined as D ν ≡ Diag { m 1 , m 2 , m 3 } and D N ≡ Diag { M 1 , M 2 , M 3 } . Meanwhile, the column vectors of left- and right-handed neutrino fields [ ν L ( N R ) c ] T and [ ( ν L ) c N R ] T undergo the transformations(9) [ ν L ( N R ) c ] ⟶ U † [ ν L ( N R ) c ] , [ ( ν L ) c N R ] ⟶ U T [ ( ν L ) c N R ] , such that the Lagrangian L Λ in Eq. (3) keeps unchanged and thus its overall gauge symmetry is unbroken. Now that Y ν is dimensionless and ϕ 0 has the same mass dimension as M R , one may argue that m i should be the “working” or “virtual” mass parameters of three light Majorana neutrinos as the electroweak gauge symmetry is unbroken at the seesaw scale Λ. In comparison, M i are essentially the true masses of three heavy Majorana neutrinos in the existence of the ϕ 0 ( ⁎ ) -mediated neutrino Yukawa interactions. Along this line of thought, we find that it is useful to decompose U into the product of three matrices,(10) U = ( I 0 0 U 0 ′ ) ( A R S B ) ( U 0 0 0 I ) , where the 3 × 3 unitary matrix U 0 ′ has been defined in Eq. (4) to primarily describe flavor mixing in the sterile (heavy) neutrino sector, U 0 denotes the other 3 × 3 unitary matrix that is mainly responsible for flavor mixing in the active (light) neutrino sector, while the 3 × 3 matrices A , B , R and S signify the interplay between these two sectors [22–24] . The unitarity of U assures(11) A A † + R R † = B B † + S S † = I , A S † + R B † = A † R + S † B = 0 , A † A + S † S = B † B + R † R = I . On the other hand, the arbitrary charged-lepton Yukawa coupling matrix Y l in Eq. (3) can be diagonalized by a bi-unitary transformation:(12) U l † ( Y l ϕ 0 ) V l = D l , l L ′ = U l † l L , l R ′ = V l l R , where U l and V l are unitary, D l ≡ Diag { m e , m μ , m τ } stands for the “working” or “virtual” masses of three charged leptons before spontaneous electroweak symmetry breaking,4 4 Note that the scalar field ϕ 0 in Eq. (12) carries a hypercharge, and hence D l cannot be simply understood as a diagonal “mass” matrix. The physical meaning of D l is actually vague in our calculations which are mathematically exact and clear, so is the physical meaning of D ν in Eq. (8) . But this vagueness will automatically disappear after spontaneous electroweak symmetry breaking, as can be subsequently seen. and l ′ = ( e μ τ ) T is defined as the column vector of the mass eigenstates of three charged leptons versus the column vector of their flavor eigenstates l = ( l e l μ l τ ) T . Substituting Eqs. (8) —(10) and (12) into Eq. (3) , we immediately arrive at(13) − L Λ = l L ′ ‾ D l l R ′ + 1 2 ν L ′ ‾ D ν ( ν L ′ ) c + 1 2 ( N R ′ ) c ‾ D N N R ′ + ν L ‾ Y l l R ϕ + − l L ‾ Y ν N R ϕ − + h . c . , where ν ′ = ( ν 1 ν 2 ν 3 ) T denotes the column vector of the working mass eigenstates of three light Majorana neutrinos far above the electroweak scale, and N ′ = ( N 1 N 2 N 3 ) T stands for the column vectors of the mass eigenstates of three heavy Majorana neutrinos relevant to the seesaw mechanism at Λ ≫ v . In this case the flavor eigenstates ν L and N R can be expressed in terms of the mass eigenstates ν L ′ and N R ′ or their charge-conjugated states as follows:(14) ν L = U ν L ′ + R ( N R ′ ) c , N R = S ′ ⁎ ( ν L ′ ) c + U ′ ⁎ N R ′ , where U ≡ A U 0 , U ′ ≡ U 0 ′ B and S ′ ≡ U 0 ′ S U 0 are defined. Taking account of the Majorana property of ν i and N i (i.e., ν i c = ν i and N i c = N i [7] for i = 1 , 2 , 3 ), one simply obtains ( N R ′ ) c = ( N ′ c ) L = N L ′ and ( ν L ′ ) c = ( ν ′ c ) R = ν R ′ . One may then substitute the expression of l L in Eq. (12) and that of ν L in Eq. (14) into the standard form of L cc in Eq. (7) and get at(15) − L cc = g 2 ( e μ τ ) L ‾ γ μ [ U PMNS ( ν 1 ν 2 ν 3 ) L + R PMNS ( N 1 N 2 N 3 ) L ] W μ − + h . c . , where U PMNS = U l † U is just the Pontecorvo-Maki-Nakagawa-Sakata (PMNS) lepton flavor mixing matrix [25–27] used to describe the flavor oscillations of three active neutrinos, and R PMNS = U l † R is an analogue of U PMNS in the seesaw mechanism which characterizes the strengths of weak charged-current interactions for three heavy Majorana neutrinos. Without loss of generality, one may choose a convenient flavor basis in which the mass eigenstates of three charged leptons are identified with their corresponding flavor eigenstates (i.e., l L = l L ′ , or equivalently U l = I ). In this case we are simply left with U PMNS = U and R PMNS = R , namely the effects of lepton flavor mixing originate purely from the active and sterile Majorana neutrino sectors and from the interplay between these two sectors. We shall take advantage of this flavor basis in the following discussions unless otherwise specified. 2.4 Mismatch between the two bases Before discussing a mismatch between the mass eigenstates of heavy Majorana neutrinos associated with thermal leptogenesis and those associated with the seesaw mechanism, let us take a look at the flavor structures of active and sterile neutrinos in the case that the electroweak gauge symmetry is unbroken at Λ. First of all, a combination of Eqs. (8) and (10) allows us to immediately derive the exact seesaw relation between the working masses of three light Majorana neutrinos and the real masses of three heavy Majorana neutrinos:(16) U D ν U T + R D N R T = 0 , in which U and R are also correlated with each other via the unitarity condition U U † + R R † = I . Note that U = A U 0 holds, where the unitary matrix U 0 is primarily responsible for flavor mixing of the three active neutrinos. So we find it useful to rewrite Eq. (16) as(17) U 0 D ν U 0 T = ( i A − 1 R ) D N ( i A − 1 R ) T , whose left- and right-hand sides are composed of the derivational and original seesaw parameters, respectively. This point will become more obvious when a complete Euler-like parametrization of the 6 × 6 unitary matrix U in Eq. (10) is adopted, as can be seen in section 3 . Needless to say, the active-sterile flavor mixing matrix R essentially plays the role of the neutrino Yukawa coupling matrix Y ν in the canonical seesaw framework,(18) Y ν ϕ 0 ⁎ = R D N [ I − ( B − 1 S A − 1 R ) T ] U ′ T ; and the right-handed Majorana neutrino mass matrix M R can be reconstructed into the form(19) M R = U ′ [ D N − ( B − 1 S A − 1 R ) D N ( B − 1 S A − 1 R ) T ] U ′ T . Note that all the quantities in Eqs. (18) and (19) belong to the original seesaw parameters in the sense that they have nothing to do with D ν and U 0 — the working masses and the primary flavor mixing matrix of three light Majorana neutrinos which are derived from the seesaw mechanism. Now we turn to an unavoidable mismatch between the mass eigenstates of three heavy Majorana neutrinos associated with the seesaw and leptogenesis mechanisms. Eq. (14) tells us that the mass eigenstates N R ′ in the seesaw basis can be expressed as(20) N R ′ = ( U ′ ⁎ ) − 1 [ N R − S ′ ⁎ ( ν L ′ ) c ] = ( B ⁎ ) − 1 [ N R ′ − U 0 ′ T S ′ ⁎ ( ν L ′ ) c ] , where Eq. (4) has been used to link N R ′ to N R ′ . To be more explicit, Eq. (20) means(21) ( N 1 N 2 N 3 ) = ( B ⁎ ) − 1 [ ( N 1 N 2 N 3 ) − U 0 ′ T S ′ ⁎ ( ν 1 ν 2 ν 3 ) ] , from which the differences between N i in the seesaw basis and N i (for i = 1 , 2 , 3 ) in the thermal leptogenesis basis can be clearly seen. Similarly, a combination of Eqs. (4) and (19) leads us to(22) D N = B [ D N − ( B − 1 S A − 1 R ) D N ( B − 1 S A − 1 R ) T ] B T , from which one may easily see the difference between D N and D N . Although N R ′ (or D N ) and N R ′ (or D N ) would exactly coincide with each other if the Yukawa coupling matrix Y ν (or equivalently, R or S ) were switched off, such a coincidence would make no sense because both the seesaw and leptogenesis mechanisms would fail in this special case. In the presence of the neutrino Yukawa interactions, thermal leptogenesis may take effect via the CP-violating and out-of-equilibrium decays of heavy Majorana neutrinos into the leptonic and Higgs doublets, while the seesaw mechanism can “formally” work with the help of an interplay between the active and sterile neutrino fields coupled only to the neutral component of the Higgs doublet. That is the key reason why there is an inevitable mismatch between the seesaw- and leptogenesis-associated bases for heavy Majorana neutrinos before spontaneous electroweak symmetry breaking. 2.5 After gauge symmetry breaking So far we have made some proper transformations of the charged lepton and neutrino fields in the flavor space to obtain their respective working or true mass eigenstates. All such unitary flavor basis transformations are completely reversible, and hence they do not affect the gauge invariance of L Λ at the seesaw scale. As already shown in Eqs. (20) and (22) , a seeable mismatch between N R ′ and N R ′ or between D N and D N results from the fact that the working seesaw mechanism itself is only associated with the neutral component of the Higgs doublet while the heavy Majorana neutrino decays and thermal leptogenesis at the seesaw scale Λ are associated with the whole Higgs doublet. This unavoidable mismatch deserves to be conceptually clarified as we have done, because it is an intrinsic issue of the seesaw and leptogenesis mechanisms. It is now straightforward to prove that the formal seesaw mechanism far above the electroweak scale will become real after the Higgs potential of the SM is minimized at 〈 H 〉 ≡ 〈 0 | H | 0 〉 = v / 2 with a special direction characterized by 〈 ϕ ± 〉 = 0 and 〈 ϕ 0 〉 = v / 2 , by which the electroweak gauge symmetry is spontaneously broken and thus all the particles coupled to the Higgs field acquire their nonzero masses. In this case the Lagrangian in Eq. (3) can be simplified to a more popular form,(23) − L Λ ′ = l L ‾ M l l R + 1 2 [ ν L ( N R ) c ] ‾ ( 0 M D M D T M R ) [ ( ν L ) c N R ] + h . c . , where M l ≡ Y l 〈 ϕ 0 〉 = Y l v / 2 denotes the charged lepton mass matrix, and M D ≡ Y ν 〈 ϕ 0 〉 = Y ν v / 2 is usually referred to as the Dirac neutrino mass matrix. The expression of M D in terms of the seesaw parameters can be directly read off from Eq. (18) , namely(24) M D = R D N [ I − ( B − 1 S A − 1 R ) T ] U ′ T . We find that the exact seesaw formula obtained in Eq. (16) and the analytical results obtained in Eqs. (19) —(22) formally keep unchanged after spontaneous gauge symmetry breaking, but they are now subject to the electroweak scale. In other words, the electroweak symmetry breaking itself does not really affect the flavor structures of the seesaw mechanism. This observation implies that it is possible to determine or constrain some of the original seesaw-associated flavor parameters in some low-energy neutrino experiments, after the radiative corrections to such parameters are properly taken into account with the help of the relevant renormaliztion-group equations (RGEs) between a superhigh seesaw scale and the electroweak scale [28] . Note that the exact seesaw formula obtained in Eq. (16) can be simplified to the more popular form in the leading-order approximations of Eqs. (19) and (24) . That is, M R ≃ U ′ D N U ′ T and M D ≃ R D N U ′ T , so the effective mass matrix for three active Majorana neutrinos is given by(25) M ν ≡ U 0 D ν U 0 T ≃ − R D N R T ≃ − M D M R − 1 M D T , where A ≃ B ≃ I has been assumed (i.e., U ≃ U 0 holds in the neglect of the non-unitary effects characterized by A ≠ I ). In this approximation the effective Majorana mass term for three active neutrinos at low energies turns out to be(26) − L ν = 1 2 ν L ‾ M ν ( ν L ) c + h . c . = 1 2 ν L ′ ‾ D ν ( ν L ′ ) c + h . c . , where the column vector of the light neutrino mass eigenstates ν L ′ has already been defined below Eq. (13) , and the physical meaning of D ν as the diagonal Majorana neutrino mass matrix becomes definite and obvious. 3 How small is the mismatch? 3.1 An Euler-like parametrization To clearly see how small the difference between N R ′ (or D N ) and N R ′ (or D N ) is expected to be, let us follow Refs. [22–24] to make an Euler-like parametrization of the 6 × 6 unitary matrix U in Eq. (10) . First of all we introduce fifteen 6 × 6 Euler-like unitary matrices of the form O i j (for 1 ≤ i < j ≤ 6 ): its ( i , i ) and ( j , j ) entries are both identical to c i j ≡ cos θ i j with θ i j being a flavor mixing angle and lying in the first quadrant, its other four diagonal elements are all equal to one, its ( i , j ) and ( j , i ) entries are given respectively by s ˆ i j ⁎ ≡ e − i δ i j sin θ i j and − s ˆ i j ≡ − e i δ i j sin θ i j with δ i j being a CP-violating phase, and its other off-diagonal elements are all equal to zero. These matrices are then grouped in the following way to respectively describe the active flavor sector, the sterile flavor sector and the interplay between these two sectors:(27) ( U 0 0 0 I ) = O 23 O 13 O 12 , ( I 0 0 U 0 ′ ) = O 56 O 46 O 45 , ( A R S B ) = O 36 O 26 O 16 O 35 O 25 O 15 O 34 O 24 O 14 , where the pattern of U 0 is quite similar to the standard parametrization of a unitary PMNS matrix as advocated by the Particle Data Group [9] ,5 5 When U 0 is applied to the phenomenology of neutrino physics in the basis of U l = I , it is the phase parameter δ ≡ δ 13 − δ 12 − δ 23 that characterizes the strength of CP violation in neutrino oscillations. (28) U 0 = ( c 12 c 13 s ˆ 12 ⁎ c 13 s ˆ 13 ⁎ − s ˆ 12 c 23 − c 12 s ˆ 13 s ˆ 23 ⁎ c 12 c 23 − s ˆ 12 ⁎ s ˆ 13 s ˆ 23 ⁎ c 13 s ˆ 23 ⁎ s ˆ 12 s ˆ 23 − c 12 s ˆ 13 c 23 − c 12 s ˆ 23 − s ˆ 12 ⁎ s ˆ 13 c 23 c 13 c 23 ) , and the expression of U 0 ′ can be directly read off from that of U 0 with the subscript replacements 12 ↔ 45 , 13 ↔ 46 and 23 ↔ 56 for the three rotation angles and three CP-violating phases. The explicit expressions of A , B , R and S in terms of c i j and s ˆ i j (for i = 1 , 2 , 3 and j = 4 , 5 , 6 ) are rather lengthy, and hence they are listed in Eqs. (A.1) and (A.2) in Appendix A for the same of simplicity. Among the four active-sterile flavor mixing matrices, only A and R affect the physical processes in which the light and heavy Majorana neutrinos take part, as can be seen from Eq. (15) . As both U = A U 0 and R appear in L cc in the chosen flavor basis (i.e., U l = I ), three of the nice CP-violating phases (or their combinations) of A and R can always be rotated away by properly redefining the phases of three charged lepton fields [29,30] . The PMNS matrix U is obviously non-unitary because of U U † = A A † = I − R R † ≠ I , but its deviation from exact unitarity (i.e., from U 0 ) is found to be very small. A detailed and careful analysis of currently available electroweak precision measurements and neutrino oscillation data has put a stringent constraint on the non-unitarity of U — the latter is below or far below O ( 10 − 2 ) [31–35] . This result implies that the deviation of A A † from I ought to be smaller than O ( 10 − 2 ) , and thus the nine active-sterile flavor mixing angles in R should be smaller than O ( 10 − 1 ) . The advantage of such a phenomenological observation is that U ≃ U 0 can be a quite reliable approximation in most cases, but its disadvantage is that an experimental exploration of the seesaw-induced non-unitary effects of U at low energies will be rather challenging. 3.2 Smallness of the mismatch Eq. (20) tells us that a difference between the mass eigenstates of three heavy Majorana neutrinos associated with the seesaw mechanism (i.e., N R ′ ) and those associated with thermal leptogenesis (i.e., N R ′ ) is mainly characterized by the deviation of ( B ⁎ ) − 1 from the identity matrix I . With the help of Eq. (A.2) , we arrive at(29) ( B ⁎ ) − 1 = ( c 14 − 1 c 24 − 1 c 34 − 1 0 0 + t ˆ 14 c 24 − 1 c 34 − 1 t ˆ 15 ⁎ + t ˆ 24 c 34 − 1 c 24 − 1 t ˆ 25 ⁎ + t ˆ 34 c 15 − 1 c 25 − 1 t ˆ 35 ⁎ c 15 − 1 c 25 − 1 c 35 − 1 0 + t ˆ 14 c 24 − 1 c 34 − 1 c 24 − 1 t ˆ 16 ⁎ + t ˆ 24 c 34 − 1 t ˆ 15 t ˆ 25 ⁎ t ˆ 16 ⁎ + t ˆ 24 c 34 − 1 c 25 − 1 c 16 − 1 t ˆ 26 ⁎ + t ˆ 34 t ˆ 15 c 25 − 1 t ˆ 35 ⁎ t ˆ 16 ⁎ + t ˆ 34 c 35 − 1 c 16 − 1 c 26 − 1 t ˆ 36 ⁎ + t ˆ 34 t ˆ 25 t ˆ 35 ⁎ c 16 − 1 t ˆ 26 ⁎ + t ˆ 15 c 25 − 1 c 35 − 1 t ˆ 16 ⁎ + t ˆ 25 c 35 − 1 c 16 − 1 t ˆ 26 ⁎ + t ˆ 35 c 16 − 1 c 26 − 1 t ˆ 36 ⁎ c 16 − 1 c 26 − 1 c 36 − 1 ) ≃ I + ( 1 2 ( s 14 2 + s 24 2 + s 34 2 ) 0 0 s ˆ 14 s ˆ 15 ⁎ + s ˆ 24 s ˆ 25 ⁎ + s ˆ 34 s ˆ 35 ⁎ 1 2 ( s 15 2 + s 25 2 + s 35 2 ) 0 s ˆ 14 s ˆ 16 ⁎ + s ˆ 24 s ˆ 26 ⁎ + s ˆ 34 s ˆ 36 ⁎ s ˆ 15 s ˆ 16 ⁎ + s ˆ 25 s ˆ 26 ⁎ + s ˆ 35 s ˆ 36 ⁎ 1 2 ( s 16 2 + s 26 2 + s 36 2 ) ) , where t ˆ i j ≡ e i δ i j tan θ i j is defined, and all the terms of O ( s i j 4 ) or smaller have been omitted from the second equation as an excellent approximation due to the smallness of θ i j (for i = 1 , 2 , 3 and j = 4 , 5 , 6 ). We see that ( B ⁎ ) − 1 is also a lower triangular matrix like B itself. On the other hand, the factor U 0 ′ T S ′ ⁎ appearing in Eq. (20) can be explicitly expressed as follows:(30) U 0 ′ T S ′ ⁎ = B T S ⁎ U 0 ⁎ ≃ − ( s ˆ 14 ⁎ s ˆ 24 ⁎ s ˆ 34 ⁎ s ˆ 15 ⁎ s ˆ 25 ⁎ s ˆ 35 ⁎ s ˆ 16 ⁎ s ˆ 26 ⁎ s ˆ 36 ⁎ ) U 0 ⁎ , where Eq. (A.2) has been used, and the terms of O ( s i j 3 ) or smaller have been omitted from the second equation as a very good approximation. Now we conclude that the heavy Majorana neutrino mass basis N R ′ is identical to N R ′ up to the accuracy of O ( s i j 2 ) , but it contains a small contribution of O ( s i j ) from the light Majorana neutrino mass basis ( ν L ′ ) c in the seesaw framework. Since the magnitudes of θ i j (for i = 1 , 2 , 3 and j = 4 , 5 , 6 ) are highly suppressed in a realistic seesaw model with little fine-tuning, the mismatch between N R ′ and N R ′ is expected to be negligible in most cases. Let us proceed to examine how small the difference between D N and D N in Eq. (21) can be. First of all, Eq. (A.2) allows us to make the approximation(31) B ≃ I − ( 1 2 ( s 14 2 + s 24 2 + s 34 2 ) 0 0 s ˆ 14 ⁎ s ˆ 15 + s ˆ 24 ⁎ s ˆ 25 + s ˆ 34 ⁎ s ˆ 35 1 2 ( s 15 2 + s 25 2 + s 35 2 ) 0 s ˆ 14 ⁎ s ˆ 16 + s ˆ 24 ⁎ s ˆ 26 + s ˆ 34 ⁎ s ˆ 36 s ˆ 15 ⁎ s ˆ 16 + s ˆ 25 ⁎ s ˆ 26 + s ˆ 35 ⁎ s ˆ 36 1 2 ( s 16 2 + s 26 2 + s 36 2 ) ) , where the terms of O ( s i j 4 ) or smaller have been omitted. Secondly, we obtain(32) A − 1 R = ( t ˆ 14 ⁎ c 14 − 1 t ˆ 15 ⁎ c 14 − 1 c 15 − 1 t ˆ 16 ⁎ c 14 − 1 t ˆ 24 ⁎ t ˆ 14 t ˆ 15 ⁎ t ˆ 24 ⁎ + c 15 − 1 c 24 − 1 t ˆ 25 ⁎ + t ˆ 14 c 15 − 1 t ˆ 16 ⁎ t ˆ 24 ⁎ + t ˆ 15 t ˆ 16 ⁎ c 24 − 1 t ˆ 25 ⁎ + c 16 − 1 c 24 − 1 c 25 − 1 t ˆ 26 ⁎ c 14 − 1 c 24 − 1 t ˆ 34 ⁎ + t ˆ 14 t ˆ 15 ⁎ c 24 − 1 t ˆ 34 ⁎ + c 15 − 1 t ˆ 24 t ˆ 25 ⁎ t ˆ 34 ⁎ + c 15 − 1 c 25 − 1 c 34 − 1 t ˆ 35 ⁎ + t ˆ 14 c 15 − 1 t ˆ 16 ⁎ c 24 − 1 t ˆ 34 ⁎ + t ˆ 15 t ˆ 16 ⁎ t ˆ 24 t ˆ 25 ⁎ t ˆ 34 ⁎ + t ˆ 15 t ˆ 16 ⁎ c 25 − 1 c 34 − 1 t ˆ 35 ⁎ + c 16 − 1 t ˆ 24 c 25 − 1 t ˆ 26 ⁎ t ˆ 34 ⁎ + c 16 − 1 t ˆ 25 t ˆ 26 ⁎ c 34 − 1 t ˆ 35 ⁎ + c 16 − 1 c 26 − 1 c 34 − 1 c 35 − 1 t ˆ 36 ⁎ ) ≃ ( s ˆ 14 ⁎ s ˆ 15 ⁎ s ˆ 16 ⁎ s ˆ 24 ⁎ s ˆ 25 ⁎ s ˆ 26 ⁎ s ˆ 34 ⁎ s ˆ 35 ⁎ s ˆ 36 ⁎ ) from Eq. (A.1) , where the terms of O ( s i j 3 ) or smaller have been neglected in the second equation as a reasonably good approximation. The exact expression of B − 1 S can be directly read off from that of − ( A − 1 R ) ⁎ with the help of Eq. (32) by making the subscript replacements 15 ↔ 24 , 16 ↔ 34 and 26 ↔ 35 , so can its approximate expression. As a consequence,(33) B − 1 S A − 1 R ≃ − ( s 14 2 + s 24 2 + s 34 2 s ˆ 14 s ˆ 15 ⁎ + s ˆ 24 s ˆ 25 ⁎ + s ˆ 34 s ˆ 35 ⁎ s ˆ 14 s ˆ 16 ⁎ + s ˆ 24 s ˆ 26 ⁎ + s ˆ 34 s ˆ 36 ⁎ s ˆ 14 ⁎ s ˆ 15 + s ˆ 24 ⁎ s ˆ 25 + s ˆ 34 ⁎ s ˆ 35 s 15 2 + s 25 2 + s 35 2 s ˆ 15 s ˆ 16 ⁎ + s ˆ 25 s ˆ 26 ⁎ + s ˆ 35 s ˆ 36 ⁎ s ˆ 14 ⁎ s ˆ 16 + s ˆ 24 ⁎ s ˆ 26 + s ˆ 34 ⁎ s ˆ 36 s ˆ 15 ⁎ s ˆ 16 + s ˆ 25 ⁎ s ˆ 26 + s ˆ 35 ⁎ s ˆ 36 s 16 2 + s 26 2 + s 36 2 ) holds in the same approximation as made above. This result implies that D N and D N are identical to each other up to the accuracy of O ( s i j 2 ) , simply because on the right-hand side of Eq. (22) the second term is suppressed in magnitude to O ( s i j 4 ) as compared with the first term. It is worth remarking that our above analytical approximations are more or less subject to the canonical seesaw mechanism at an energy scale far above the electroweak scale, and thus the mismatch between N R ′ (or D N ) and N R ′ (or D N ) is very small. This situation will change when the low-scale seesaw and leptogenesis scenarios, in which a mismatch between the two sets of mass bases for heavy Majorana neutrinos is crucial, are taken into account (see, e.g., Refs. [11,12] ). 3.3 Determination of D ν and U 0 As already shown in Eq. (17) , the nine effective flavor parameters of three light Majorana neutrinos in D ν and U 0 (i.e., three effective masses, three flavor mixing angles and three CP-violating phases) can be expressed in terms of the eighteen seesaw parameters hidden in A , R and D N (i.e., three heavy Majorana neutrino masses, nine active-sterile flavor mixing angles and six CP-violating phases). It is obvious that all the derivational seesaw parameters on the left-hand side of Eq. (17) would vanish if R ∝ Y ν were switched off. So this equation provides an unambiguous way to determine the light degrees of freedom from the heavy degrees of freedom in the seesaw framework. To be more specific, the six independent elements of the effective Majorana neutrino mass matrix M ν ≡ U 0 D ν U 0 T are given as follows:(34) ( M ν ) 11 = m 1 c 12 2 c 13 2 + m 2 s ˆ 12 ⁎ 2 c 13 2 + m 3 s ˆ 13 ⁎ 2 , ( M ν ) 12 = − m 1 c 12 c 13 ( s ˆ 12 c 23 + c 12 s ˆ 13 s ˆ 23 ⁎ ) + m 2 s ˆ 12 ⁎ c 13 ( c 12 c 23 − s ˆ 12 ⁎ s ˆ 13 s ˆ 23 ⁎ ) + m 3 c 13 s ˆ 13 ⁎ s ˆ 23 ⁎ , ( M ν ) 13 = m 1 c 12 c 13 ( s ˆ 12 s ˆ 23 − c 12 s ˆ 13 c 23 ) − m 2 s ˆ 12 ⁎ c 13 ( c 12 s ˆ 23 + s ˆ 12 ⁎ s ˆ 13 c 23 ) + m 3 c 13 s ˆ 13 ⁎ c 23 , ( M ν ) 22 = m 1 ( s ˆ 12 c 23 + c 12 s ˆ 13 s ˆ 23 ⁎ ) 2 + m 2 ( c 12 c 23 − s ˆ 12 ⁎ s ˆ 13 s ˆ 23 ⁎ ) 2 + m 3 c 13 2 s ˆ 23 ⁎ 2 , ( M ν ) 23 = − m 1 ( s ˆ 12 c 23 + c 12 s ˆ 13 s ˆ 23 ⁎ ) ( s ˆ 12 s ˆ 23 − c 12 s ˆ 13 c 23 ) − m 2 ( c 12 c 23 − s ˆ 12 ⁎ s ˆ 13 s ˆ 23 ⁎ ) ( c 12 s ˆ 23 + s ˆ 12 ⁎ s ˆ 13 c 23 ) + m 3 c 13 2 c 23 s ˆ 23 ⁎ , ( M ν ) 33 = m 1 ( s ˆ 12 s ˆ 23 − c 12 s ˆ 13 c 23 ) 2 + m 2 ( c 12 s ˆ 23 + s ˆ 12 ⁎ s ˆ 13 c 23 ) 2 + m 3 c 13 2 c 23 2 . On the other hand, Eq. (17) tells us that these six matrix elements can originally be determined by M ν = − ( A − 1 R ) D N ( A − 1 R ) T thanks to the exact seesaw relation bridging the big gap between the light and heavy Majorana neutrinos. With the help of the explicit expression of A − 1 R given in Eq. (32) , it is straightforward to obtain the expressions for the elements of M ν in terms of M i , θ i j and δ i j (for i = 1 , 2 , 3 and j = 4 , 5 , 6 ). Instead of presenting the exact analytical results, which are rather lengthy and hence less instructive, here we make the leading-order approximations for the expressions of A and R given in Eq. (A.1) and then arrive at(35) ( M ν ) 11 ≃ − [ M 1 s ˆ 14 ⁎ 2 + M 2 s ˆ 15 ⁎ 2 + M 3 s ˆ 16 ⁎ 2 ] , ( M ν ) 12 ≃ − [ M 1 s ˆ 14 ⁎ s ˆ 24 ⁎ + M 2 s ˆ 15 ⁎ s ˆ 25 ⁎ + M 3 s ˆ 16 ⁎ s ˆ 26 ⁎ ] , ( M ν ) 13 ≃ − [ M 1 s ˆ 14 ⁎ s ˆ 34 ⁎ + M 2 s ˆ 15 ⁎ s ˆ 35 ⁎ + M 3 s ˆ 16 ⁎ s ˆ 36 ⁎ ] , ( M ν ) 22 ≃ − [ M 1 s ˆ 24 ⁎ 2 + M 2 s ˆ 25 ⁎ 2 + M 3 s ˆ 26 ⁎ 2 ] , ( M ν ) 23 ≃ − [ M 1 s ˆ 24 ⁎ s ˆ 34 ⁎ + M 2 s ˆ 25 ⁎ s ˆ 35 ⁎ + M 3 s ˆ 26 ⁎ s ˆ 36 ⁎ ] , ( M ν ) 33 ≃ − [ M 1 s ˆ 34 ⁎ 2 + M 2 s ˆ 35 ⁎ 2 + M 3 s ˆ 36 ⁎ 2 ] . Let us emphasize that there appear nine CP-violating phases in Eq. (35) , but three of them (or their combinations) are redundant and can always be removed by rephasing the charged lepton fields in a proper way.6 6 A straightforward way to remove the three redundant phase parameters of A and R is just to switch off three of the nine phases in the nine active-sterile flavor mixing matrices O i j (for i = 1 , 2 , 3 and j = 4 , 5 , 6 ) in Eq. (27) from the very beginning. As there are many options in doing so, we do not go into details here. A combination of Eqs. (34) and (35) allows us to establish the direct relations between the nine derivational and eighteen original seesaw parameters. So the former can in principle be determined from the latter for a given seesaw model (a top-down approach), and the latter may be partly probed or constrained from the former with the help of some low-energy neutrino experiments (a bottom-up approach). A careful and detailed analysis of the parameter space along this line of thought will be made elsewhere. 4 Summary We have reformulated the canonical seesaw mechanism by considering the fact that the electroweak gauge symmetry is unbroken at the seesaw scale characterized by the masses of heavy Majorana neutrinos, and shown that it can formally work and allow us to derive an exact seesaw relation between the active (light) and sterile (heavy) Majorana neutrinos. In this way we have elucidated the reason why there is an unavoidable mismatch between the mass eigenstates of heavy Majorana neutrinos associated with the seesaw and thermal leptogenesis mechanisms. The smallness of this mismatch has been discussed with the help of a complete Euler-like parametrization of the flavor structure in the seesaw framework, and the exact and explicit relations between the original and derivational seesaw parameters have been established as a by-product. We hope that this work may help clarify some conceptual ambiguities associated with the validity of the seesaw mechanism before and after spontaneous electroweak symmetry breaking, because such ambiguities have never been taken serious in the literature. It should also be helpful to clarify the ambiguities associated with the RGE evolution between the “virtual” flavor parameters of Majorana neutrinos at the seesaw scale and those “real” ones at the electroweak scale, which is crucial to bridge the gap between a well-motivated UV-complete flavor theory including the seesaw mechanism and all the possible low-energy flavor experiments. CRediT authorship contribution statement Zhi-zhong Xing: Conceptualization, Investigation, Methodology, Writing – original draft, Writing – review & editing.Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgements I am greatly indebted to Di Zhang and Shun Zhou for numerous helpful discussions and comments about this paper, a work dedicated to the 50th birthday of my home institute, the Institute of High Energy Physics, which was founded on 1 February 1973. I would also like to thank Marco Drewes for very useful discussions and comments. My research is supported in part by the National Natural Science Foundation of China under grant No. 12075254 and grant No. 11835013 . Appendix A The expressions of A , B , R and S Given the Euler-like parametrization of the 6 × 6 unitary flavor mixing matrix U decomposed in Eq. (27) , the 3 × 3 active-sterile flavor mixing matrices A , B , R and S depend on the same nine rotation angles θ i j and nine phase angles δ i j (for i = 1 , 2 , 3 and j = 4 , 5 , 6 ). To be explicit [22,23] ,(A.1) A = ( c 14 c 15 c 16 0 0 − c 14 c 15 s ˆ 16 s ˆ 26 ⁎ − c 14 s ˆ 15 s ˆ 25 ⁎ c 26 − s ˆ 14 s ˆ 24 ⁎ c 25 c 26 c 24 c 25 c 26 0 − c 14 c 15 s ˆ 16 c 26 s ˆ 36 ⁎ + c 14 s ˆ 15 s ˆ 25 ⁎ s ˆ 26 s ˆ 36 ⁎ − c 14 s ˆ 15 c 25 s ˆ 35 ⁎ c 36 + s ˆ 14 s ˆ 24 ⁎ c 25 s ˆ 26 s ˆ 36 ⁎ + s ˆ 14 s ˆ 24 ⁎ s ˆ 25 s ˆ 35 ⁎ c 36 − s ˆ 14 c 24 s ˆ 34 ⁎ c 35 c 36 − c 24 c 25 s ˆ 26 s ˆ 36 ⁎ − c 24 s ˆ 25 s ˆ 35 ⁎ c 36 − s ˆ 24 s ˆ 34 ⁎ c 35 c 36 c 34 c 35 c 36 ) , R = ( s ˆ 14 ⁎ c 15 c 16 s ˆ 15 ⁎ c 16 s ˆ 16 ⁎ − s ˆ 14 ⁎ c 15 s ˆ 16 s ˆ 26 ⁎ − s ˆ 14 ⁎ s ˆ 15 s ˆ 25 ⁎ c 26 + c 14 s ˆ 24 ⁎ c 25 c 26 − s ˆ 15 ⁎ s ˆ 16 s ˆ 26 ⁎ + c 15 s ˆ 25 ⁎ c 26 c 16 s ˆ 26 ⁎ − s ˆ 14 ⁎ c 15 s ˆ 16 c 26 s ˆ 36 ⁎ + s ˆ 14 ⁎ s ˆ 15 s ˆ 25 ⁎ s ˆ 26 s ˆ 36 ⁎ − s ˆ 14 ⁎ s ˆ 15 c 25 s ˆ 35 ⁎ c 36 − c 14 s ˆ 24 ⁎ c 25 s ˆ 26 s ˆ 36 ⁎ − c 14 s ˆ 24 ⁎ s ˆ 25 s ˆ 35 ⁎ c 36 + c 14 c 24 s ˆ 34 ⁎ c 35 c 36 − s ˆ 15 ⁎ s ˆ 16 c 26 s ˆ 36 ⁎ − c 15 s ˆ 25 ⁎ s ˆ 26 s ˆ 36 ⁎ + c 15 c 25 s ˆ 35 ⁎ c 36 c 16 c 26 s ˆ 36 ⁎ ) ; and(A.2) B = ( c 14 c 24 c 34 0 0 − c 14 c 24 s ˆ 34 ⁎ s ˆ 35 − c 14 s ˆ 24 ⁎ s ˆ 25 c 35 − s ˆ 14 ⁎ s ˆ 15 c 25 c 35 c 15 c 25 c 35 0 − c 14 c 24 s ˆ 34 ⁎ c 35 s ˆ 36 + c 14 s ˆ 24 ⁎ s ˆ 25 s ˆ 35 ⁎ s ˆ 36 − c 14 s ˆ 24 ⁎ c 25 s ˆ 26 c 36 + s ˆ 14 ⁎ s ˆ 15 c 25 s ˆ 35 ⁎ s ˆ 36 + s ˆ 14 ⁎ s ˆ 15 s ˆ 25 ⁎ s ˆ 26 c 36 − s ˆ 14 ⁎ c 15 s ˆ 16 c 26 c 36 − c 15 c 25 s ˆ 35 ⁎ s ˆ 36 − c 15 s ˆ 25 ⁎ s ˆ 26 c 36 − s ˆ 15 ⁎ s ˆ 16 c 26 c 36 c 16 c 26 c 36 ) , S = ( − s ˆ 14 c 24 c 34 − s ˆ 24 c 34 − s ˆ 34 s ˆ 14 c 24 s ˆ 34 ⁎ s ˆ 35 + s ˆ 14 s ˆ 24 ⁎ s ˆ 25 c 35 − c 14 s ˆ 15 c 25 c 35 s ˆ 24 s ˆ 34 ⁎ s ˆ 35 − c 24 s ˆ 25 c 35 − c 34 s ˆ 35 s ˆ 14 c 24 s ˆ 34 ⁎ c 35 s ˆ 36 − s ˆ 14 s ˆ 24 ⁎ s ˆ 25 s ˆ 35 ⁎ s ˆ 36 + s ˆ 14 s ˆ 24 ⁎ c 25 s ˆ 26 c 36 + c 14 s ˆ 15 c 25 s ˆ 35 ⁎ s ˆ 36 + c 14 s ˆ 15 s ˆ 25 ⁎ s ˆ 26 c 36 − c 14 c 15 s ˆ 16 c 26 c 36 s ˆ 24 s ˆ 34 ⁎ c 35 s ˆ 36 + c 24 s ˆ 25 s ˆ 35 ⁎ s ˆ 36 − c 24 c 25 s ˆ 26 c 36 − c 34 c 35 s ˆ 36 ) . 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-]>NUPHB 116107 116107 S0550-3213(23)00036-6 10.1016/j.nuclphysb.2023.116107 The Author(s) Quantum Field Theory and Statistical Systems Fig. 1 Blocking procedure to reduce a two-dimensional 4 × 4 lattice to a single tensor using alternating contractions in the horizontal and vertical directions. Fig. 1 Fig. 2 Illustration of the contraction T ⋆1 T = M along the 1-direction in a three-dimensional system, as in (2) . The square nodes represent the fusion of the original tensor indices into the combined fat indices (3) of M . Fig. 2 Fig. 3 Two tensors T are contracted over their shared vertical link, producing a tensor M with fat horizontal modes (green box). The fat modes of M are projected onto subspaces with the projectors P U = UU T and P V = VV T , respectively, to form the lower rank approximation M ˆ of (20) (blue box). As part of the construction one recognizes the core tensor S of (19) (red box). Note that the matrices U , U T , V and V T , described by diamonds in the figure, are applied from the inside to the outside, in correspondence with (19) and (20) . Fig. 3 Fig. 4 Two approximations M ˆ , constructed in Fig. 3 , are contracted in the horizontal direction. This illustrates how the projections performed in the first contraction are concatenated when making this second contraction, leading to a product P U P V . Here a new building block G = U T V arises, which we call a merger between two core tensors S . Note that, for consistency, the matrices operate in chronological order (from the inside to the outside with respect to M of Fig. 3 ) and not from left to right. The half-mergers on the left and right will connect to their counter parts in further contractions. Fig. 4 Fig. 5 The construction in Fig. 4 drastically simplifies when choosing U = V , as the merger G = 1 D in this case. The frames are solely needed to construct the core tensor S . This property will spread through the entire blocking procedure (including the final trace), such that the calculation can be performed using the core tensor only. Fig. 5 Fig. 6 Comparison of (ϵ − ϵ hooi )/ϵ hooi for approximations of 100 independent random tensors of dimension 10 × 10 × 100 × 100 truncated to dimension 10 × 10 × 10 × 10 using HOSVD, the Xie method, SuperQ and ISQ. The tensor elements are chosen randomly from a uniform distribution over [0,1] (left plot) and from a Gaussian distribution N (0;1) (right plot). The horizontal axis represents different random tensors. Fig. 6 Fig. 7 Comparison of (ϵ − ϵ hooi )/ϵ hooi for random tensors of dimension 30 × 30 × 30 × 30 truncated to dimension 10 × 10 × 10 × 10, using the same approximation methods and the same probability distributions for the tensor elements as in Fig. 6 . Fig. 7 Improved local truncation schemes for the higher-order tensor renormalization group method Jacques Bloch a jacques.bloch@ur.de Robert Lohmayer a b robert.lohmayer@ur.de Maximilian Meister a Michael Nunhofer a a Institute for Theoretical Physics, University of Regensburg, 93040 Regensburg, Germany Institute for Theoretical Physics University of Regensburg Regensburg 93040 Germany Institute for Theoretical Physics, University of Regensburg, 93040 Regensburg, Germany b Leibniz Institute for Immunotherapy (LIT), 93053 Regensburg, Germany Leibniz Institute for Immunotherapy (LIT) Regensburg 93053 Germany Leibniz Institute for Immunotherapy (LIT), 93053 Regensburg, Germany Editor: Hubert Saleur Abstract The higher-order tensor renormalization group is a tensor-network method providing estimates for the partition function and thermodynamical observables of classical and quantum systems in thermal equilibrium. At every step of the iterative blocking procedure, the coarse-grid tensor is truncated to keep the tensor dimension under control. For a consistent tensor blocking procedure, it is crucial that the forward and backward tensor modes are projected on the same lower dimensional subspaces. In this paper we present two methods, the SuperQ and the iterative SuperQ method, to construct tensor truncations that reduce or even minimize the local approximation errors, while satisfying this constraint. Data availability Data will be made available on request. 1 Introduction Physical systems in thermal equilibrium are described by their partition function, whose complexity grows exponentially in the volume. The standard method to simulate such statistical systems is the Markov chain Monte Carlo method (MC), which efficiently samples the relevant states of the system to produce reliable estimates of observables. A fundamental prerequisite for the MC method is the positivity of the sampling weights. Models which do not satisfy this condition cause the infamous sign problem and require alternative simulation methods. Quantum systems with complex actions are typical examples of systems with a sign problem. An important topical application in high energy physics is the simulation of quantum chromodynamics (QCD) at nonzero quark chemical potential, which allows for the investigation of the QCD phase diagram as a function of temperature and baryon density. There exist numerous methods to circumvent the sign problem, and some even solve it for particular systems [1–3] . Very mild sign problems can be circumvented by reweighting, which uses the Monte Carlo method on an auxiliary ensemble with positive weights, and reweights the observables to the target ensemble. The main issue with this method is that the statistical error increases exponentially with the volume such that it is hardly usable in any realistic situation, except for the validation of other methods in regions where the sign problem is small. Other methods which have shown their merit on some models, but are known to have fundamental problems for other ones, are the complex Langevin method, the thimbles, the density of states method and the method of dual variables, where the simulations are usually performed with the worm algorithm. Common to those methods is the stochastic sampling of the partition function. An alternative approach that has recently drawn a lot of interest is that of tensor networks, see [4] for a review. In these methods the partition function is first rewritten as a full contraction of a tensor network covering the entire lattice. The exact computation of the partition function and observables in this formulation would have an exponential complexity. The tensor renormalization group (TRG) [5] and higher order tensor renormalization group (HOTRG) [6] methods avoid this exponential cost by blocking the lattice iteratively and truncating the inflated dimensions of the coarse grid tensor at each blocking step using truncated higher order singular value decompositions (HOSVD) [7] , which are based on the matrix singular value decomposition (SVD). We consider the partition function of a d -dimensional classical or quantum system in thermal equilibrium, written as a fully contracted tensor network [8] ,(1) Z = tTr ∏ x = 1 V T i x , − 1 i x , 1 … i x , − d i x , d ( x ) , with a tensor T ( x ) at each site x = 1 , … , V . In general the local tensor is the same on all sites, i.e., T ( x ) = T for all x . For each lattice direction ν , the tensor has one mode for the forward and one mode for the backward orientation, corresponding to the indices i x , ν and i x , − ν ≡ i x − ν ˆ , ν , respectively, where ν ˆ is a unit step in the ν direction. We will often refer to these modes as backward and forward modes of the physical tensor. The trace in the partition function stands for a full contraction over all tensor indices, where two adjacent tensors share exactly one index. Thermodynamical observables, which are defined as derivatives of the partition function with respect to one of its parameters, can be computed using either a finite-difference approximation or an impurity tensor formulation involving the analytical derivative of T [9] . In the following we will restrict our discussion to HOTRG, because it can be applied to any number of dimensions, whereas TRG is limited to two-dimensional systems. The HOTRG method uses an iterative blocking procedure that reduces the size of the lattice by a factor of two during each blocking step by contracting pairs of adjacent tensors. The procedure is illustrated for a two-dimensional 4 × 4 lattice in Fig. 1 . Its extension to higher dimensions is obvious, and below we will further discuss the HOTRG method for the three-dimensional case. When contracting two adjacent tensors T over their shared link, a tensor M of higher order is produced. Such a contraction in the 1-direction is illustrated for the three-dimensional case in Fig. 2 and can be written as(2) M j X , − 1 j X , 1 j X , − 2 j X , 2 j X , − 3 j X , 3 = ∑ i x , 1 T i x , − 1 i x , 1 i x , − 2 i x , 2 i x , − 3 i x , 3 T i y , − 1 i y , 1 i y , − 2 i y , 2 i y , − 3 y x , 3 where y = x + 1 ˆ and therefore i y , − 1 = i x , 1 , by definition, X = ( x , y ) labels sites on the coarse grid and(3) j X , − 1 = i x , − 1 , j X , 1 = i y , 1 j X , − 2 = ( i x , − 2 , i y , − 2 ) , j X , 2 = ( i x , 2 , i y , 2 ) j X , − 3 = ( i x , − 3 , i y , − 3 ) , j X , 3 = ( i x , 3 , i y , 3 ) } fat indices . For any direction perpendicular to the direction of contraction, the tensor M has modes originating from both contracted tensors. To keep the order of the tensor unchanged, we gather every such pair of modes in a new fat mode corresponding to its direct product space. Assuming that the modes of the local tensor have dimension D , then the fat modes will have dimension D 2 . In HOTRG these fat modes are truncated back to dimension D using a modified version of the HOSVD approximation method, such that the dimension of the coarse grid tensor remains the same as that of the original local tensor throughout the entire blocking procedure. In general, step k + 1 of the HOTRG procedure can be summarized as(4) T [ k ] ⋆ ν T [ k ] = : M ⟶ truncate T [ k + 1 ] , where the ⋆ ν -operation symbolically represents a forward-backward contraction in direction ν . The precise construction of T [ k + 1 ] will be discussed in Sec. 3 . In the standard approximation procedure using HOSVD [7] , referred to as HOSVD approximation in the following, the dimension of each tensor mode gets reduced by projecting it on a lower dimensional subspace, which is generically different for each mode. This HOSVD approximation is modified when used as part of the iterative blocking procedure in the standard HOTRG algorithm, as it is essential for the accuracy and effectiveness of the method that the backward and forward modes for every direction get projected on the same subspace. Each of these subspaces will be characterized by a frame, which is a set of orthonormal basis vectors spanning the subspace. Constructing appropriate frames will be the major subject of this paper. The standard HOTRG procedure for the construction of frames [6] is not optimal, in particular when the local tensor is not symmetric in its backward and forward modes. In this paper we present two improved methods for the construction of common subspaces for pairs of backward and forward modes: the SuperQ and the iterative SuperQ method (ISQ), which is an iterative improvement of the former in search of the optimal subspaces. Note that the discussion in this paper solely focuses on the optimization of the rank reduction of the local tensors at every blocking step, but does not take into account global effects on the full contraction of the tensor network. Here is a brief outline of the paper. In Sec. 2 we review the standard HOSVD method to construct a reduced rank approximation for an arbitrary tensor. In Sec. 3 we explain why the HOTRG method uses a modification of this rank reduction procedure such that the backward and forward modes are projected on the same subspace. We then propose two methods to improve the standard HOTRG truncation: In Sec. 4 we present the SuperQ method, and in Sec. 5 we derive the more sophisticated ISQ method. Finally, we summarize and conclude in Sec. 7 . 2 Rank reduction and HOSVD approximation Below we first review the general idea of rank reduction for an arbitrary tensor, before describing the HOSVD procedure [7] which can be used to generate a quasi-optimal rank-reduced approximation in an efficient way. For a real tensor M of order n with dimension N 1 × ⋯ × N n , the left matrix-tensor multiplication A ⊙ r M is defined as a contraction of the second index of the matrix A with the r -th index of the tensor M ,(5) ( A ⊙ r M ) i 1 ⋯ i r − 1 j r i r + 1 ⋯ i n = ∑ i r A j r i r M i 1 ⋯ i n . A lower-rank approximation of M can be constructed as(6) M ˆ = P ( 1 ) ⊙ 1 P ( 2 ) ⊙ 2 ⋯ P ( n ) ⊙ n M , using N r × N r projectors P ( r ) of rank K r ≤ N r , r = 1 , … , n . In this approximation, the r -th tensor mode of dimension N r is projected onto a subspace of dimension K r , embedded in the original space. Typically the Frobenius norm ‖ M − M ˆ ‖ is used as a measure for the quality of the low-rank approximation and the aim is to determine optimal projectors P ( r ) with fixed ranks K r . The projectors can be represented as(7) P ( r ) = U ( r ) U ( r ) T with semi-orthogonal N r × K r matrices U ( r ) , which we will call frames in the following. Semi-orthogonal means that the columns of the frames are orthonormal, but not their rows (unless K r = N r ). The columns of each frame U ( r ) provide an orthonormal basis of the corresponding K r -dimensional subspace,(8) U ( r ) T U ( r ) = 1 K r . The approximation M ˆ can then be rewritten as(9) M ˆ = U ( 1 ) ⊙ 1 U ( 2 ) ⊙ 2 ⋯ U ( n ) ⊙ n S , with the K 1 × … × K n dimensional core tensor (10) S = U ( 1 ) T ⊙ 1 U ( 2 ) T ⊙ 2 ⋯ U ( n ) T ⊙ n M . The core tensor represents M ˆ in the bases of the subspaces, which are spanned by the columns of the frames U ( r ) . Note that M ˆ has the same dimension as M , but is generically of lower rank as the r -th mode is projected from a space of dimension N r on a subspace of dimension K r for r = 1 , … , n . In practice, M ˆ is usually not constructed explicitly, as many operations involving M ˆ can be performed at much lower cost using only the core tensor S and the frames U ( r ) , which contain the same information as M ˆ but condensed in lower-dimensional objects. A typical example of such an operation is the contraction of two tensors, as will be discussed in the next section. The squared Frobenius norm of M is given by(11) ‖ M ‖ 2 = 〈 M , M 〉 = ∑ { i } M i 1 ⋯ i n 2 , where for brevity we introduce the notation { i } = i 1 , … , i n for the summation indices, and the inner product between two real tensors of equal dimension is defined as(12) 〈 A , B 〉 = ∑ { i } A i 1 ⋯ i n B i 1 ⋯ i n . Since the reduced-rank tensor M ˆ is a projection of M , we have(13) 〈 M , M ˆ 〉 = ‖ M ˆ ‖ 2 = ‖ S ‖ 2 . Therefore, the squared approximation error is given by(14) ‖ M − M ˆ ‖ 2 = ‖ M ‖ 2 + ‖ M ˆ ‖ 2 − 2 〈 M , M ˆ 〉 = ‖ M ‖ 2 − ‖ M ˆ ‖ 2 = ‖ M ‖ 2 − ‖ S ‖ 2 , which does not require the explicit computation of M ˆ . In the HOSVD approximation procedure [7] the semi-orthogonal frames used to approximate M are constructed using properties of matrix SVDs. We first introduce the r -unfolding M ( r ) , which is a matrix containing the same entries as the tensor M , but reordered such that its rows correspond to the r -th mode of M and its columns correspond to a combination of all other tensor modes. The entries of the r -unfolding of M are given by(15) M i r , ( { i } ∖ i r ) ( r ) = M i 1 ⋯ i n , where the column index ( { i } ∖ i r ) = ( i 1 , … , i r − 1 , i r + 1 , … , i n ) is a linear index of coordinates in a space of dimension ∏ s ≠ r N s . The multi-rank of a tensor is defined by the n -tuple of the ranks of the individual unfoldings M ( r ) , r = 1 , … , n . Therefore M has at most multi-rank ( N 1 , … , N n ) and the approximation M ˆ of (6) at most multi-rank ( K 1 , … , K n ) . Note that the squared Frobenius norm (11) of a tensor is identical to that of any of its unfoldings, as it is just a sum over all squared components,(16) ‖ M ‖ 2 = ‖ M ( r ) ‖ 2 = Tr [ M ( r ) M ( r ) T ] for any r = 1 , … , n . To construct the HOSVD approximation M ˆ , we first consider the singular value decomposition for each r -unfolding M ( r ) of M (assuming real tensors for simplicity),(17) M ( r ) = L ( r ) Σ ( r ) R ( r ) T , where the columns of the N r × N r orthogonal matrix L ( r ) are the left singular vectors of M ( r ) . The columns of the orthogonal matrix R ( r ) contain the corresponding right singular vectors of M ( r ) . The diagonal entries of Σ ( r ) are the singular values of M ( r ) , which are always real and non-negative, while all other entries are zero. It is well-known in linear algebra that retaining the K r largest singular values in Σ ( r ) , while setting all others to zero, yields the best-possible matrix approximation A ( r ) of rank K r to M ( r ) (best-possible referring to a minimization of the Frobenius norm ‖ M ( r ) − A ( r ) ‖ ). The relative truncation error is given by(18) ϵ ( r ) = ‖ M ( r ) − A ( r ) ‖ ‖ M ( r ) ‖ = ∑ i = K r + 1 N r λ i ( r ) ∑ i = 1 N r λ i ( r ) , where λ i ( r ) are the eigenvalues of the Gramian M ( r ) M ( r ) T , i.e., the squared singular values of the unfolding M ( r ) , ordered such that λ 1 ( r ) ≥ … ≥ λ N r ( r ) . This matrix property is used in HOSVD by separately performing the matrix SVDs of all individual unfoldings M ( r ) of M for r = 1 , … , n and constructing the N r × K r frames U ( r ) with the singular vectors of L ( r ) corresponding to the K r largest singular values of the unfoldings M ( r ) . These frames are then used to construct the core tensor (10) and the matrix approximation (9) of HOSVD. Unlike for the matrix case, the HOSVD tensor approximation is in general not the best-possible approximation of a given multi-rank ( K 1 , … , K n ) , even though it is usually quite close to it [7] . In a variant of the HOSVD approximation, called interlaced HOSVD approximation, the rank reduction procedure is carried out in the following way: Starting with S = M , the frames are computed on successive unfoldings of the core tensor, which gets updated every time a new truncation frame is constructed until the core tensor is of dimension K 1 × … × K n . For the interlaced HOSVD approximation, the result depends on the order of the updates, but is usually close to that of the ordinary HOSVD approximation. The best-possible approximation of multi-rank ( K 1 , … , K n ) , which minimizes the Frobenius norm ‖ M − M ˆ ‖ , can be constructed numerically using the Higher Order Orthogonal Iteration (HOOI) [10] . Nevertheless, the HOSVD approximation is especially appealing because of its relative simplicity to produce an almost optimal approximation. 3 Backward-forward symmetric truncation in HOTRG We now discuss how the HOSVD formalism is used in HOTRG to avoid the exponential blow up of the tensor dimension during the blocking procedure, and why the standard HOSVD truncation is modified to avoid drawbacks related to accuracy and efficiency. To make our point we will use the two-dimensional case as it can be easiest illustrated and contains all the ingredients necessary for the discussion. Extending it to higher dimensions is straightforward. We consider the contraction M = T ⋆ 2 T of two local tensors T along the 2-direction. According to the discussion in the introduction, M will have thin backward and forward modes of dimension D in the contracted 2-direction, and fat backward and forward modes of dimension D 2 in the perpendicular 1-direction, which we want to reduce to lower rank by projecting on a D -dimensional subspace using (6) . This procedure of contraction and truncation, which we detail below, is illustrated in Fig. 3 . To reduce the dimension of the fat modes back from D 2 to D , while minimizing the loss of information, we apply the HOSVD approximation procedure, explained in Sec. 2 , where we only truncate the fat modes. The SVDs are computed for the unfoldings M ( 1 ) and M ( 2 ) for the backward and forward modes in the 1-direction, respectively, and the frames U and V of dimension D 2 × D are constructed with the singular vectors corresponding to their D largest singular values. For the modes in the contracted 2-direction no truncation is required. With these frames we construct a core tensor S of dimension D × D × D × D , according to (10) ,(19) S = U T ⊙ 1 V T ⊙ 2 M . The corresponding approximation M ˆ , defined in (9) , with the same dimension as M , but typically much lower rank, is given by(20) M ˆ = U ⊙ 1 V ⊙ 2 S = P U ⊙ 1 P V ⊙ 2 M , with D 2 × D 2 projectors P U = U U T and P V = V V T , with U T U = V T V = 1 D . As mentioned in Sec. 2 , operations involving M ˆ can typically be performed at much lower cost by using only the core tensor S and the frames U and V . Assume that in the next blocking step two M ˆ tensors are contracted in the 1-direction, as is illustrated in Fig. 4 . When using the standard HOSVD approximation (20) the backward and forward modes in M ˆ will have been projected on different subspaces using the projectors P U and P V , respectively. In a contraction M ˆ ⋆ 1 M ˆ the two projectors will be multiplied, as can be seen in the center of the figure. The decomposition of the approximation M ˆ in (20) can be used to reduce the computational effort, as the original contraction can be replaced by contractions of two core tensors S with a D × D dimensional merger G ≡ U T V in between, as can be seen in the figure.1 1 In fact this produces an amputated version of M ˆ ⋆ 1 M ˆ , which together with the mergers is all we need in the full contraction of the tensor network, see also Fig. 4 . The entries of G are scalar products of the basis vectors in U and V . At this point it is important to discuss a crucial modification introduced by the HOTRG method to the HOSVD truncation procedure presented above, which is rarely discussed in the literature. Although the HOSVD approximation M ˆ gives a close-to-best lower-rank approximation to M , it is in general not such a good and useful truncation when viewed as part of the iterative blocking procedure. Indeed, the product of projectors corresponds to a projection of a projection, which will unavoidably loose additional information if the projectors P U and P V are different. In this case, the contraction M ˆ ⋆ 1 M ˆ will no longer necessarily be a good approximation of M ⋆ 1 M , even if M ˆ itself was close to the best-possible approximation of M . We now observe that, due to the idempotence of projectors, there would be no additional loss if P U = P V , i.e., if the backward and forward modes of M in the 1-direction were projected on the same subspace. Note that in this case the merger G is an orthogonal matrix. Moreover, we can also get a serious gain in algorithmic simplicity, on top of this accuracy improvement, if we choose the same basis for both modes in the common subspace, i.e., we choose frames satisfying U = V , for which G = 1 D . When looking back at Fig. 4 we see that, in this case, the central merger just drops out, and the contraction can be replaced by a contraction of two core tensors, as is illustrated in Fig. 5 (the frames on the left and right of Fig. 4 will connect to their counter parts in further contractions, to form another merger G = 1 D , which will also drop out). For this reason, in HOTRG the backward and forward modes of each direction are truncated using a common frame U ( ν ) , ν = 1 , … , d , even when the HOSVD frames are different, which is typically the case for systems at nonzero chemical potential. With this backward-forward symmetric truncation, the core tensor S can be used as new coarse grid tensor after each blocking step, where at the ( k + 1 ) -th blocking step two tensors of step k are contracted to form a new coarse grid tensor,(21) T [ k ] ⋆ ν T [ k ] = : M ⟶ BF S = : T [ k + 1 ] . The abbreviation BF on the arrow means that we apply a backward-forward symmetric truncation to construct the core tensor, which then becomes the new local tensor on the coarse grid. The frames are only needed to construct the core tensor with (19) , and can then be discarded. The same reasoning also holds for a contraction in the other direction, where the directions of thin and fat modes are interchanged. Moreover, the procedure naturally generalizes to d dimensions, where we have d − 1 directions ν with backward and forward fat modes: If the backward and forward frames are chosen to be identical for each direction, the core tensor (10) can be used as the new coarse grid tensor in the HOTRG blocking procedure. This strategy of choosing the same frame to truncate the backward and forward mode for each individual direction in HOTRG, makes it fundamentally different from the HOSVD approximation, as it can no longer directly rely on the optimal low-rank properties of matrix SVD. Geometrically, truncating the backward and forward modes with the same frame means that these modes get projected on the same subspace and are described in the same basis. This explains why the full contraction of the tensor network into a scalar can be rewritten in terms of the core tensors only. Note that if one would use the standard HOSVD approximation procedure and work with different backward and forward frames, we would have to use both the core tensors and the mergers G defined above when performing the iterative contractions, to ensure that the different subspaces are matched onto one another, see Fig. 4 . Although this is no conceptual problem, it would complicate the algorithm, require additional computational work, and most of all the product of projectors would deteriorate the results further. The construction of the shared backward-forward frames in HOTRG has not been given a lot of attention in the literature until now. There is a brief discussion of this issue in the original HOTRG paper [6] , where either the backward or forward frame is chosen and applied to both modes, depending on which one gives the smallest SVD truncation error. The error introduced by this choice on the other mode is however not taken into account. We observed that for tensors lacking a backward-forward symmetry, this choice of frame is not optimal and can be improved upon. Below we present two new methods to improve the construction of shared frames for the backward and forward modes. The first one, called SuperQ method and presented in Sec. 4 , minimizes a combined error on the backward and forward unfoldings for each individual direction. The second method, which we call iterative SuperQ (ISQ) method is presented in Sec. 5 . This iterative method aims at determining the best-possible approximation to M for a given multi-rank, satisfying the requirement that the backward and forward frames for each direction are identical. The ISQ method leans on ideas developed for the higher order orthogonal iteration (HOOI) method [10] , which constructs the best-possible approximation of a given multi-rank with independent frames for all modes. We will see that the SuperQ solution can be used as a natural starting point for the ISQ procedure. Note that the SuperQ and ISQ methods are specifically conceived for tensors which are part of a physical tensor network on a space-time lattice and have modes corresponding to backward and forward orientations. 4 The SuperQ method To discuss the construction of truncations satisfying the requirement that the frames for the backward and forward modes are identical, we consider a tensor M with d pairs of backward and forward modes. The tensor is thus of order 2d with dimension N 1 × N 1 × ⋯ × N d × N d , which will be truncated to dimension K 1 × K 1 × ⋯ × K d × K d using d semi-orthogonal frames U ( ν ) of dimension N ν × K ν . With these frames the core tensor is constructed using(22) S = U ( 1 ) T ⊙ 1 U ( 1 ) T ⊙ 2 ⋯ U ( d ) T ⊙ 2 d − 1 U ( d ) T ⊙ 2 d M . Consider the positive semi-definite Gram matrices(23) Q b ( ν ) = M b ( ν ) M b ( ν ) T , Q f ( ν ) = M f ( ν ) M f ( ν ) T , where M b ( ν ) and M f ( ν ) are the unfoldings of M with respect to the backward and forward modes for direction ν , i.e.,(24) M b ( ν ) ≡ M ( 2 ν − 1 ) , M f ( ν ) ≡ M ( 2 ν ) , and M ( r ) is the r -unfolding of the tensor M defined in (15) . We denote the N ν × K ν frames constructed with the eigenvectors corresponding to the K ν largest eigenvalues of Q b ( ν ) and Q f ( ν ) as U b ( ν ) and U f ( ν ) , respectively. As the backward and forward Gramians Q b ( ν ) and Q f ( ν ) are in general not identical, the corresponding subspaces spanned by the vectors of the frames U b ( ν ) and U f ( ν ) will be different too.2 2 This can even be the case if the eigenvalues of both Gramians coincide, as we have observed for the O ( 2 ) model with chemical potential. In the standard HOTRG procedure [6] it is suggested to choose either U b ( ν ) or U f ( ν ) for the unique U ( ν ) , depending which of both gives the smallest SVD truncation error (18) . Even though this choice of frame optimizes the truncation error for one mode, it does not take into account its effect on the mode corresponding to the opposite orientation. Therefore, it is clear that, generically, better choices of frames should exist, and our aim is to construct frames U ( ν ) that reduce the combined truncation error when applied simultaneously to the backward and forward modes for the ν direction. Let us now consider a single truncation frame U ( ν ) which we use to reduce the rank of the unfoldings M b ( ν ) and M f ( ν ) . Using (14) and (16) , the relative truncation errors on the backward and forward unfoldings are(25) ϵ b ( ν ) = ‖ M b ( ν ) − A b ( ν ) ‖ ‖ M b ( ν ) ‖ = 1 − Tr ( U ( ν ) T Q b ( ν ) U ( ν ) ) Tr Q b ( ν ) , (26) ϵ f ( ν ) = ‖ M f ( ν ) − A f ( ν ) ‖ ‖ M f ( ν ) ‖ = 1 − Tr ( U ( ν ) T Q f ( ν ) U ( ν ) ) Tr Q f ( ν ) , where A b ( ν ) = U ( ν ) U ( ν ) T M b ( ν ) and A f ( ν ) = U ( ν ) U ( ν ) T M f ( ν ) are the rank-K ν approximations to the unfoldings M b ( ν ) and M f ( ν ) , respectively, obtained with the same frame U ( ν ) . To improve upon using either U b ( ν ) or U f ( ν ) , we determine the common U ( ν ) by minimizing the combination of both errors in(27) ϵ S ( ν ) 2 = 1 2 ( ϵ b ( ν ) 2 + ϵ f ( ν ) 2 ) = 1 − 1 2 ‖ M ‖ 2 Tr [ U ( ν ) T ( Q b ( ν ) + Q f ( ν ) ) U ( ν ) ] , where we also used Tr Q b ( ν ) = Tr Q f ( ν ) = ‖ M ‖ 2 . We define the SuperQ matrix for direction ν as(28) Q S ( ν ) = Q b ( ν ) + Q f ( ν ) , which is symmetric and positive semi-definite as it is a sum of two symmetric positive semi-definite matrices. Therefore, if we diagonalize the SuperQ matrix and truncate the eigenvector matrix, retaining the eigenvectors corresponding to the K ν largest eigenvalues, then this semi-orthogonal frame U ( ν ) minimizes the truncation error (27) on Q S ( ν ) . This SuperQ procedure is repeated on all d directions to determine all frames U ( ν ) , which can then be used to approximate M and to construct the corresponding core tensor S , see (22) . The SuperQ method is computationally efficient since it only requires a single eigenvalue decomposition for each pair of backward and forward fat modes, while the standard HOTRG procedure [6] performs separate decompositions on these modes. When applying the SuperQ method to HOTRG, where M is a contraction T ⋆ ν T along one of the directions, only 2 d − 2 modes will actually be truncated, as the backward and forward modes for the contracted direction need not be truncated. In analogy to the interlaced HOSVD approximation, see Sec. 2 , we can also define an interlaced version of the SuperQ method where we determine the frames U ( ν ) by applying the method to an intermediate core tensor, which gets updated direction-by-direction (starting from S = M ) by truncating the respective backward and forward mode each time a frame has been computed. This interlaced SuperQ method is also of interest in the light of the iterative procedure derived in the next section. 5 Optimized frames with iterative SuperQ Although the HOSVD method, see Sec. 2 , typically yields a good tensor approximation M ˆ [7] , the best-possible one, which minimizes ‖ M − M ˆ ‖ 2 , can be constructed with an iterative procedure called higher order orthogonal iteration (HOOI) method [10] . According to the discussion of the backward-forward symmetric truncation in Sec. 3 , it is clear that HOOI is itself not applicable in a tensor network approach to statistical physics, because the backward and forward modes should be truncated with the same semi-orthogonal frame for each direction, while HOOI very generically generates different frames for all modes. Below we present the iterative SuperQ (ISQ) method, which is inspired by the original HOOI procedure but imposes the requirement that the same frame has to be used to truncate the backward and forward modes of each direction. As in Sec. 4 , we consider a tensor M with d pairs of backward and forward modes, i.e., the tensor is of order 2d with dimensions N 1 × N 1 ⋯ × N d × N d , which will be truncated to dimensions K 1 × K 1 × ⋯ × K d × K d , see (22) . Our aim is to minimize the squared Frobenius norm (14) (29) ‖ M − M ˆ ‖ 2 = ‖ M ‖ 2 − ‖ S ‖ 2 , over all semi-orthogonal N ν × K ν frames U ( ν ) , ν = 1 … d , for fixed K ν , with the additional condition that the backward and forward modes for each direction ν are truncated with the same frame U ( ν ) . The semi-orthogonality of the frames is imposed explicitly by orthonormality conditions for the column vectors of U ( ν ) ,(30) ∑ i = 1 N ν U i a ( ν ) U i b ( ν ) = δ a b , for 1 ≤ a , b ≤ K ν and 1 ≤ ν ≤ d , in the constrained minimization of (29) . This leads to the cost function(31) g = ‖ S ‖ 2 + C = ‖ U ( 1 ) T ⊙ 1 U ( 1 ) T ⊙ 2 ⋯ U ( d ) T ⊙ 2 d − 1 U ( d ) T ⊙ 2 d M ‖ 2 + ∑ ν = 1 d Tr [ Λ ( ν ) ( 1 − U ( ν ) T U ( ν ) ) ] , with matrices Λ ( ν ) containing the Lagrange multipliers. The orthonormalization conditions are symmetric under the exchange a ↔ b , and so Λ ( ν ) will be symmetric too. If we diagonalize Λ diag ( ν ) = O ( ν ) T Λ ( ν ) O ( ν ) , redefine U ( ν ) O ( ν ) → U ( ν ) and Λ diag ( ν ) → Λ ( ν ) , and use the orthogonality of O ( ν ) , then Eq. (31) remains unaltered albeit now with diagonal Λ ( ν ) = diag ( λ 1 ( ν ) , … , λ K ν ( ν ) ) , and this without loss of generality. Written out in components this is(32) g = ∑ { a , b } S a 1 b 1 ⋯ a d b d 2 + ∑ ν = 1 d ∑ c = 1 K ν λ c ( ν ) ( 1 − ∑ k = 1 N ν U k c ( ν ) 2 ) = ∑ { a , b } ( ∑ { i , j } [ ∏ μ = 1 d U i μ a μ ( μ ) U j μ b μ ( μ ) ] M i 1 j 1 ⋯ i d j d ) 2 + ∑ ν = 1 d ∑ c = 1 K ν λ c ( ν ) ( 1 − ∑ k = 1 N ν U k c ( ν ) 2 ) . For a constrained maximum of ‖ S ‖ 2 , the partial derivative of g with respect to the ( k , c ) -entry of the ν -th orthogonal frame has to satisfy(33) 0 = ∂ g ∂ U k c ( ν ) = ∑ { a , b } 2 S a 1 b 1 ⋯ a d b d ∂ ∂ U k c ( ν ) ( ∑ { i , j } [ ∏ μ = 1 d U i μ a μ ( μ ) U j μ b μ ( μ ) ] M i 1 j 1 ⋯ i d j d ) − 2 λ c ( ν ) U k c ( ν ) , for 1 ≤ k ≤ N ν , 1 ≤ c ≤ K ν and 1 ≤ ν ≤ d . Note that the same frame U ( ν ) appears twice in S , as it is used to truncate the modes in the backward and forward ν direction. We therefore obtain(34) ∑ { a , b } S a 1 b 1 ⋯ a d b d ( ∑ { i , j } δ k , i ν δ c , a ν [ ∏ μ ≠ ν U i μ a μ ( μ ) U j μ b μ ( μ ) ] U j ν b ν ( ν ) M i 1 j 1 ⋯ i n j n + ∑ { i , j } δ k , j ν δ c , b ν [ ∏ μ ≠ ν U i μ a μ ( μ ) U j μ b μ ( μ ) ] U i ν a ν ( ν ) M i 1 j 1 ⋯ i n j n ) = λ c ( ν ) U k c ( ν ) . After eliminating the Kronecker deltas we get(35) ∑ { a , b } ∖ a ν S a 1 b 1 ⋯ c b ν ⋯ a d b d ∑ { i , j } ∖ i ν [ ∏ μ ≠ ν U i μ a μ ( μ ) U j μ b μ ( μ ) ] U j ν b ν ( ν ) M i 1 j 1 ⋯ k j ν ⋯ i d j d + ∑ { a , b } ∖ b ν S a 1 b 1 ⋯ a ν c ⋯ a d b d ∑ { i , j } ∖ j ν [ ∏ μ ≠ ν U i μ a μ ( μ ) U j μ b μ ( μ ) ] U i ν a ν ( ν ) M i 1 j 1 ⋯ i ν k ⋯ i d j d = λ c ( ν ) U k c ( ν ) . Let us define the unfolding matrices B ( ν ) and F ( ν ) with dimensions N ν × ( K 1 2 ⋯ K ν − 1 2 K ν K ν + 1 2 ⋯ K d 2 ) , where all directions of M are truncated, except for the backward-ν mode for B ( ν ) , and the forward-ν mode for F ( ν ) , and the unfolding is performed with respect to the untruncated mode,(36) B i ν , ( { a , b } ∖ a ν ) ( ν ) = ∑ { i , j } ∖ i ν [ ∏ μ ≠ ν U i μ a μ ( μ ) U j μ b μ ( μ ) ] U j ν b ν ( ν ) M i 1 j 1 ⋯ ⋯ i d j d (37) F j ν , ( { a , b } ∖ b ν ) ( ν ) = ∑ { i , j } ∖ j ν [ ∏ μ ≠ ν U i μ a μ ( μ ) U j μ b μ ( μ ) ] U i ν a ν ( ν ) M i 1 j 1 ⋯ i d j d , where we used the notation introduced in (15) for the matrix indices. The core tensor can also be written in terms of B ( ν ) and F ( ν ) by truncating the remaining untruncated index:(38) S a 1 b 1 ⋯ a d b d = ∑ i ν B i ν , ( { a , b } ∖ a ν ) ( ν ) U i ν a ν ( ν ) = ∑ j ν F j ν , ( { a , b } ∖ b ν ) ( ν ) U j ν b ν ( ν ) . After substituting B ( ν ) and F ( ν ) in (35) we obtain(39) ∑ { a , b } ∖ a ν ∑ i ν B k , ( { a , b } ∖ a ν ) ( ν ) B i ν , ( { a , b } ∖ a ν ) ( ν ) U i ν c ( ν ) + ∑ { a , b } ∖ b ν ∑ j ν F k , ( { a , b } ∖ b ν ) ( ν ) F j ν , ( { a , b } ∖ b ν ) ( ν ) U j ν c ( ν ) = λ c ( ν ) U k c ( ν ) , or(40) ∑ i ν [ B ( ν ) B ( ν ) T ] k i ν U i ν c ( ν ) + ∑ j ν [ F ( ν ) F ( ν ) T ] k j ν U j ν c ( ν ) = λ c ( ν ) U k c ( ν ) . If we introduce the positive semi-definite matrices(41) Q ( ν ) ( U ( 1 ) , … , U ( d ) ) = B ( ν ) B ( ν ) T + F ( ν ) F ( ν ) T , ν = 1 , … , d , we can identify (40) as a coupled nonlinear eigenvalue problem (which is nonlinear in the eigenvectors)(42) ν = 1 … d , Q ( ν ) ( U ( 1 ) , … , U ( d ) ) U ( ν ) = U ( ν ) Λ ( ν ) , ν = 1 , … , d , for the semi-orthogonal frames U ( ν ) of dimension N ν × K ν and the K ν -dimensional diagonal matrices Λ ( ν ) . Note that all frames U ( μ ) , μ = 1 … d , appear in Q ( ν ) (as projections P ( μ ) = U ( μ ) U ( μ ) T ). This is even true for the direction ν itself, as its projector is applied to the forward mode in B ( ν ) B ( ν ) T and to the backward mode in F ( ν ) F ( ν ) T . Therefore the ν -th equation is cubic in U ( ν ) and quartic in all other U ( μ ) , μ ≠ ν . It is crucial to keep in mind that the self-consistent solutions to (42) , which we are looking for, are required to be semi-orthogonal matrices with K ν ≤ N ν columns, in order to satisfy the constraints (30) . It is useful to note that if we replace the matrices Q ( ν ) in (42) by fixed matrices Q ˆ ( ν ) , the d matrix equations decouple and each one of them is a linear eigenvalue equation for the frame U ( ν ) . The solutions of these linearized equations are however in general no solution of the original nonlinear equations (42) . On the other hand, if (42) is satisfied, then U ( ν ) is a solution of the linear eigenvalue problem for the specific matrix Q ˆ ( ν ) = Q ( ν ) ( U ( 1 ) , … , U ( d ) ) . Therefore, we propose to solve the coupled system of equations using an iterative procedure, where at each iteration step, Q ˆ ( ν ) is computed with (41) using the current frames U ( μ ) , μ = 1 , … , d , and the eigenvalue problem(43) Q ˆ ( ν ) ( U ( 1 ) , … , U ( d ) ) u ( ν ) = λ ( ν ) u ( ν ) , is solved for the eigenvalues λ ( ν ) and eigenvectors u ( ν ) of the positive semi-definite matrix Q ˆ ( ν ) . We then take the normalized eigenvectors corresponding to the K ν largest eigenvalues to form a new frame U ( ν ) . Note that the eigenvectors of the symmetric matrix Q ˆ ( ν ) are orthogonal and therefore the constraint (30) is automatically satisfied for U ( ν ) . This strategy can be motivated in the following way. We are looking for the solution of (42) which maximizes ‖ S ‖ 2 . Using the expression (38) for the core tensor S in terms of the unfoldings B ( ν ) and F ( ν ) , the squared norm in the cost function (31) can be written as(44) ‖ S ‖ 2 = 1 2 ( Tr [ U ( ν ) T B ( ν ) B ( ν ) T U ( ν ) ] + Tr [ U ( ν ) T F ( ν ) F ( ν ) T U ( ν ) ] ) = 1 2 Tr [ U ( ν ) T Q ( ν ) U ( ν ) ] , which explicitly contains the matrix Q ( ν ) ≡ Q ( ν ) ( U ( 1 ) , … , U ( d ) ) defined in (41) (note that this equation yields the same ‖ S ‖ 2 for each ν = 1 , … , d ). It is straightforward to show that, at each iteration step, the solution obtained from the linearization (43) corresponds to the constrained optimization of the approximation(45) ‖ S ‖ 2 ≈ 1 2 Tr [ U ( ν ) T Q ˆ ( ν ) U ( ν ) ] to (44) , where Q ˆ ( ν ) is fixed and computed with the most recent frames U ( μ ) , μ = 1 , … , d . As this approximation is quadratic in the new frame U ( ν ) , its maximum will be given by the eigenvectors corresponding to the K ν largest eigenvalues of the symmetric, positive semi-definite matrix Q ˆ ( ν ) . The iterative procedure can be interpreted as an iterative SuperQ method, where at each step all modes of M are truncated using the last known frames, except for a backward mode in B and the corresponding forward mode in F . During the iterative procedure we cycle through the d dimensions ν and determine a new frame U ( ν ) at each step using (43) . Then, we repeat these d iteration steps until all frames have converged. In practice we observed that the first iteration for each direction is the most important one, and further iterations of the same direction only give small corrections. An alternative procedure would be to iterate (43) for a single U ( ν ) (keeping all other U ( μ ) fixed) until convergence has been reached (reevaluating Q ˆ ( ν ) with the most recent U ( ν ) at every step), and then go on to the next frame. Once all frames have been iterated, this whole procedure is repeated until all frames converge together. Note that the inner iterations are computationally cheap, as all frames but one are kept fixed and all matrix-tensor multiplications in Q ˆ and ‖ S ‖ 2 involving these fixed frames have to be computed only once. However, this procedure does not seem to give an overall faster convergence. A natural choice for the starting frames U ( ν ) , ν = 1 … d , in the iterative procedure are the frames obtained from the interlaced SuperQ method, see Sec. 4 . Note that the iterative procedure is not guaranteed to converge, and even when it does, the solution is not necessarily the global maximum. This can be improved upon by tuning the starting frames or by applying an under-relaxation procedure to the intermediate Q matrix. In this procedure, we replace Q ˆ ( ν ) in (43) by(46) Q ‾ ( ν ) = ω Q ˆ ( ν ) + ( 1 − ω ) Q ˆ prev ( ν ) , where Q ˆ prev ( ν ) was used to obtain the previous U ( ν ) in the iterative procedure. The relaxation procedure can be used to optimize ‖ S ‖ 2 by tuning the local parameter 0 ≤ ω ≤ 1 . We observed that a coarse tuning of ω is sufficient to improve the overall convergence of the iterative procedure. When applying the ISQ method to HOTRG, where M is a contraction T ⋆ ν T along one of the directions, only 2 d − 2 of 2d modes will actually be truncated, as the backward and forward modes for the contracted direction are left unchanged. For the two-dimensional case, where only one frame has to be determined after each contraction, an alternative method to optimize the truncation using a linearization was proposed in the projective truncation of Ref. [11] . 6 Examples In the following, we illustrate the effect of the SuperQ and ISQ methods. For various random tensors A , we compute core tensors S using the standard HOSVD approximation [7] and the best possible approximation of a given multi-rank with the HOOI method [10] . These results are compared with the following backward-forward symmetric approximations: the method proposed by Xie et al. (used in standard HOTRG [6] ), the SuperQ approximation of Sec. 4 , and the ISQ approximation of Sec. 5 . The relative error for each of these approximations is given by(47) ϵ = 1 − ‖ S ‖ 2 ‖ A ‖ 2 . The comparison of the different methods will be illustrated by plotting ( ϵ − ϵ hooi ) / ϵ hooi in the figures below. For the ISQ method the iterations are initialized with the frames obtained using the interlaced SuperQ method. The ISQ method is iterated until the relative error ϵ has a relative change of less than 10 − 6 between two major iterations, where a major iteration corresponds to one update of all frames. For each iteration we tune the relaxation parameter ω to reduce the error. The number of major iterations varies between 20 and 100 with an average of around 50. In a first example we consider random tensors A of order 4 with dimension 10 × 10 × 100 × 100 , whose rank is reduced by truncating the last two indices to dimension 10. The results shown in Fig. 6 were computed for initial tensors filled with uniformly distributed elements in [ 0 , 1 ] (left panel) and normally distributed elements with mean μ = 0 and standard deviation σ = 1 (right panel). In another example, shown in Fig. 7 , all modes of random 30 × 30 × 30 × 30 tensors are reduced to dimension 10. Again the random tensors are filled with elements from a uniform distribution (left) and a normal distribution (right). In all examples, the hierarchy between the approximations is the same. In decreasing order of accuracy we find: HOOI, ISQ, HOSVD, SuperQ, and finally the Xie-method. We notice that, as we suggested in the derivation of Sec. 4 , the SuperQ method performs better than the Xie-method. Both of them are superseded by HOSVD, which is logical as the latter does not have to satisfy the additional backward-forward symmetry constraint. However, a somewhat unexpected result is that in all examples, the ISQ method performs better than the standard HOSVD approximation, even though the former does satisfy the additional backward-forward symmetry constraint. In all cases HOOI performs best, as it is the best possible approximation of the given multi-rank. 7 Conclusions In this paper we consider the reduction of the local truncation error in a single blocking step of the HOTRG procedure. We have discussed in detail the constraints imposed on the semi-orthogonal truncation frames in the HOTRG algorithm, where the backward and forward modes for each direction have to be projected on the same lower-dimensional subspace at each blocking step. We first introduced the SuperQ method, which minimizes a combined error on the backward and forward unfoldings for each individual direction. The method is computationally more efficient and generically yields a reduced local truncation error when compared to the original HOTRG truncation. As a further improvement, we presented the iterative SuperQ method, where we formulate a constrained minimization problem, which leads to equations that have to be satisfied by the semi-orthogonal truncation frames in order to minimize the error on the lower-rank tensor approximation, while satisfying the backward-forward symmetry constraints. The method is inspired by the HOOI method, with the additional requirement that the same frames are used on the backward and forward modes of each direction. The equations form a coupled nonlinear eigenvalue problem, which we propose to solve using an iterative procedure, where decoupled linear eigenvalue problems are solved at each iteration step. Computing the optimal backward-forward symmetric truncation frames with the ISQ method is more expensive than the truncation applied in the original HOTRG method, as each iteration step requires new eigenvalue decompositions. Therefore, in practice, we generally use the SuperQ truncation in the HOTRG blocking procedure, since it provides the best trade-off between computational cost and truncation accuracy. Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. References [1] G. 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Evenbly Algorithms for tensor network renormalization Phys. Rev. B 95 2017 045117 10.1103/PhysRevB.95.045117 arXiv:1509.07484 G. Evenbly, Algorithms for tensor network renormalization, Phys. Rev. B95 (2017) 045117. arXiv:1509.07484, doi:10.1103/PhysRevB.95.045117.
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+]>NUPHB 116107 116107 S0550-3213(23)00036-6 10.1016/j.nuclphysb.2023.116107 The Author(s) Quantum Field Theory and Statistical Systems Fig. 1 Blocking procedure to reduce a two-dimensional 4 × 4 lattice to a single tensor using alternating contractions in the horizontal and vertical directions. Fig. 1 Fig. 2 Illustration of the contraction T ⋆1 T = M along the 1-direction in a three-dimensional system, as in (2) . The square nodes represent the fusion of the original tensor indices into the combined fat indices (3) of M . Fig. 2 Fig. 3 Two tensors T are contracted over their shared vertical link, producing a tensor M with fat horizontal modes (green box). The fat modes of M are projected onto subspaces with the projectors P U = UU T and P V = VV T , respectively, to form the lower rank approximation M ˆ of (20) (blue box). As part of the construction one recognizes the core tensor S of (19) (red box). Note that the matrices U , U T , V and V T , described by diamonds in the figure, are applied from the inside to the outside, in correspondence with (19) and (20) . Fig. 3 Fig. 4 Two approximations M ˆ , constructed in Fig. 3 , are contracted in the horizontal direction. This illustrates how the projections performed in the first contraction are concatenated when making this second contraction, leading to a product P U P V . Here a new building block G = U T V arises, which we call a merger between two core tensors S . Note that, for consistency, the matrices operate in chronological order (from the inside to the outside with respect to M of Fig. 3 ) and not from left to right. The half-mergers on the left and right will connect to their counter parts in further contractions. Fig. 4 Fig. 5 The construction in Fig. 4 drastically simplifies when choosing U = V , as the merger G = 1 D in this case. The frames are solely needed to construct the core tensor S . This property will spread through the entire blocking procedure (including the final trace), such that the calculation can be performed using the core tensor only. Fig. 5 Fig. 6 Comparison of (ϵ − ϵ hooi )/ϵ hooi for approximations of 100 independent random tensors of dimension 10 × 10 × 100 × 100 truncated to dimension 10 × 10 × 10 × 10 using HOSVD, the Xie method, SuperQ and ISQ. The tensor elements are chosen randomly from a uniform distribution over [0,1] (left plot) and from a Gaussian distribution N (0;1) (right plot). The horizontal axis represents different random tensors. Fig. 6 Fig. 7 Comparison of (ϵ − ϵ hooi )/ϵ hooi for random tensors of dimension 30 × 30 × 30 × 30 truncated to dimension 10 × 10 × 10 × 10, using the same approximation methods and the same probability distributions for the tensor elements as in Fig. 6 . Fig. 7 Improved local truncation schemes for the higher-order tensor renormalization group method Jacques Bloch a jacques.bloch@ur.de Robert Lohmayer a b robert.lohmayer@ur.de Maximilian Meister a Michael Nunhofer a a Institute for Theoretical Physics, University of Regensburg, 93040 Regensburg, Germany Institute for Theoretical Physics University of Regensburg Regensburg 93040 Germany Institute for Theoretical Physics, University of Regensburg, 93040 Regensburg, Germany b Leibniz Institute for Immunotherapy (LIT), 93053 Regensburg, Germany Leibniz Institute for Immunotherapy (LIT) Regensburg 93053 Germany Leibniz Institute for Immunotherapy (LIT), 93053 Regensburg, Germany Editor: Hubert Saleur Abstract The higher-order tensor renormalization group is a tensor-network method providing estimates for the partition function and thermodynamical observables of classical and quantum systems in thermal equilibrium. At every step of the iterative blocking procedure, the coarse-grid tensor is truncated to keep the tensor dimension under control. For a consistent tensor blocking procedure, it is crucial that the forward and backward tensor modes are projected on the same lower dimensional subspaces. In this paper we present two methods, the SuperQ and the iterative SuperQ method, to construct tensor truncations that reduce or even minimize the local approximation errors, while satisfying this constraint. Data availability Data will be made available on request. 1 Introduction Physical systems in thermal equilibrium are described by their partition function, whose complexity grows exponentially in the volume. The standard method to simulate such statistical systems is the Markov chain Monte Carlo method (MC), which efficiently samples the relevant states of the system to produce reliable estimates of observables. A fundamental prerequisite for the MC method is the positivity of the sampling weights. Models which do not satisfy this condition cause the infamous sign problem and require alternative simulation methods. Quantum systems with complex actions are typical examples of systems with a sign problem. An important topical application in high energy physics is the simulation of quantum chromodynamics (QCD) at nonzero quark chemical potential, which allows for the investigation of the QCD phase diagram as a function of temperature and baryon density. There exist numerous methods to circumvent the sign problem, and some even solve it for particular systems [1–3] . Very mild sign problems can be circumvented by reweighting, which uses the Monte Carlo method on an auxiliary ensemble with positive weights, and reweights the observables to the target ensemble. The main issue with this method is that the statistical error increases exponentially with the volume such that it is hardly usable in any realistic situation, except for the validation of other methods in regions where the sign problem is small. Other methods which have shown their merit on some models, but are known to have fundamental problems for other ones, are the complex Langevin method, the thimbles, the density of states method and the method of dual variables, where the simulations are usually performed with the worm algorithm. Common to those methods is the stochastic sampling of the partition function. An alternative approach that has recently drawn a lot of interest is that of tensor networks, see [4] for a review. In these methods the partition function is first rewritten as a full contraction of a tensor network covering the entire lattice. The exact computation of the partition function and observables in this formulation would have an exponential complexity. The tensor renormalization group (TRG) [5] and higher order tensor renormalization group (HOTRG) [6] methods avoid this exponential cost by blocking the lattice iteratively and truncating the inflated dimensions of the coarse grid tensor at each blocking step using truncated higher order singular value decompositions (HOSVD) [7] , which are based on the matrix singular value decomposition (SVD). We consider the partition function of a d -dimensional classical or quantum system in thermal equilibrium, written as a fully contracted tensor network [8] ,(1) Z = tTr ∏ x = 1 V T i x , − 1 i x , 1 … i x , − d i x , d ( x ) , with a tensor T ( x ) at each site x = 1 , … , V . In general the local tensor is the same on all sites, i.e., T ( x ) = T for all x . For each lattice direction ν , the tensor has one mode for the forward and one mode for the backward orientation, corresponding to the indices i x , ν and i x , − ν ≡ i x − ν ˆ , ν , respectively, where ν ˆ is a unit step in the ν direction. We will often refer to these modes as backward and forward modes of the physical tensor. The trace in the partition function stands for a full contraction over all tensor indices, where two adjacent tensors share exactly one index. Thermodynamical observables, which are defined as derivatives of the partition function with respect to one of its parameters, can be computed using either a finite-difference approximation or an impurity tensor formulation involving the analytical derivative of T [9] . In the following we will restrict our discussion to HOTRG, because it can be applied to any number of dimensions, whereas TRG is limited to two-dimensional systems. The HOTRG method uses an iterative blocking procedure that reduces the size of the lattice by a factor of two during each blocking step by contracting pairs of adjacent tensors. The procedure is illustrated for a two-dimensional 4 × 4 lattice in Fig. 1 . Its extension to higher dimensions is obvious, and below we will further discuss the HOTRG method for the three-dimensional case. When contracting two adjacent tensors T over their shared link, a tensor M of higher order is produced. Such a contraction in the 1-direction is illustrated for the three-dimensional case in Fig. 2 and can be written as(2) M j X , − 1 j X , 1 j X , − 2 j X , 2 j X , − 3 j X , 3 = ∑ i x , 1 T i x , − 1 i x , 1 i x , − 2 i x , 2 i x , − 3 i x , 3 T i y , − 1 i y , 1 i y , − 2 i y , 2 i y , − 3 y x , 3 where y = x + 1 ˆ and therefore i y , − 1 = i x , 1 , by definition, X = ( x , y ) labels sites on the coarse grid and(3) j X , − 1 = i x , − 1 , j X , 1 = i y , 1 j X , − 2 = ( i x , − 2 , i y , − 2 ) , j X , 2 = ( i x , 2 , i y , 2 ) j X , − 3 = ( i x , − 3 , i y , − 3 ) , j X , 3 = ( i x , 3 , i y , 3 ) } fat indices . For any direction perpendicular to the direction of contraction, the tensor M has modes originating from both contracted tensors. To keep the order of the tensor unchanged, we gather every such pair of modes in a new fat mode corresponding to its direct product space. Assuming that the modes of the local tensor have dimension D , then the fat modes will have dimension D 2 . In HOTRG these fat modes are truncated back to dimension D using a modified version of the HOSVD approximation method, such that the dimension of the coarse grid tensor remains the same as that of the original local tensor throughout the entire blocking procedure. In general, step k + 1 of the HOTRG procedure can be summarized as(4) T [ k ] ⋆ ν T [ k ] = : M ⟶ truncate T [ k + 1 ] , where the ⋆ ν -operation symbolically represents a forward-backward contraction in direction ν . The precise construction of T [ k + 1 ] will be discussed in Sec. 3 . In the standard approximation procedure using HOSVD [7] , referred to as HOSVD approximation in the following, the dimension of each tensor mode gets reduced by projecting it on a lower dimensional subspace, which is generically different for each mode. This HOSVD approximation is modified when used as part of the iterative blocking procedure in the standard HOTRG algorithm, as it is essential for the accuracy and effectiveness of the method that the backward and forward modes for every direction get projected on the same subspace. Each of these subspaces will be characterized by a frame, which is a set of orthonormal basis vectors spanning the subspace. Constructing appropriate frames will be the major subject of this paper. The standard HOTRG procedure for the construction of frames [6] is not optimal, in particular when the local tensor is not symmetric in its backward and forward modes. In this paper we present two improved methods for the construction of common subspaces for pairs of backward and forward modes: the SuperQ and the iterative SuperQ method (ISQ), which is an iterative improvement of the former in search of the optimal subspaces. Note that the discussion in this paper solely focuses on the optimization of the rank reduction of the local tensors at every blocking step, but does not take into account global effects on the full contraction of the tensor network. Here is a brief outline of the paper. In Sec. 2 we review the standard HOSVD method to construct a reduced rank approximation for an arbitrary tensor. In Sec. 3 we explain why the HOTRG method uses a modification of this rank reduction procedure such that the backward and forward modes are projected on the same subspace. We then propose two methods to improve the standard HOTRG truncation: In Sec. 4 we present the SuperQ method, and in Sec. 5 we derive the more sophisticated ISQ method. Finally, we summarize and conclude in Sec. 7 . 2 Rank reduction and HOSVD approximation Below we first review the general idea of rank reduction for an arbitrary tensor, before describing the HOSVD procedure [7] which can be used to generate a quasi-optimal rank-reduced approximation in an efficient way. For a real tensor M of order n with dimension N 1 × ⋯ × N n , the left matrix-tensor multiplication A ⊙ r M is defined as a contraction of the second index of the matrix A with the r -th index of the tensor M ,(5) ( A ⊙ r M ) i 1 ⋯ i r − 1 j r i r + 1 ⋯ i n = ∑ i r A j r i r M i 1 ⋯ i n . A lower-rank approximation of M can be constructed as(6) M ˆ = P ( 1 ) ⊙ 1 P ( 2 ) ⊙ 2 ⋯ P ( n ) ⊙ n M , using N r × N r projectors P ( r ) of rank K r ≤ N r , r = 1 , … , n . In this approximation, the r -th tensor mode of dimension N r is projected onto a subspace of dimension K r , embedded in the original space. Typically the Frobenius norm ‖ M − M ˆ ‖ is used as a measure for the quality of the low-rank approximation and the aim is to determine optimal projectors P ( r ) with fixed ranks K r . The projectors can be represented as(7) P ( r ) = U ( r ) U ( r ) T with semi-orthogonal N r × K r matrices U ( r ) , which we will call frames in the following. Semi-orthogonal means that the columns of the frames are orthonormal, but not their rows (unless K r = N r ). The columns of each frame U ( r ) provide an orthonormal basis of the corresponding K r -dimensional subspace,(8) U ( r ) T U ( r ) = 1 K r . The approximation M ˆ can then be rewritten as(9) M ˆ = U ( 1 ) ⊙ 1 U ( 2 ) ⊙ 2 ⋯ U ( n ) ⊙ n S , with the K 1 × … × K n dimensional core tensor (10) S = U ( 1 ) T ⊙ 1 U ( 2 ) T ⊙ 2 ⋯ U ( n ) T ⊙ n M . The core tensor represents M ˆ in the bases of the subspaces, which are spanned by the columns of the frames U ( r ) . Note that M ˆ has the same dimension as M , but is generically of lower rank as the r -th mode is projected from a space of dimension N r on a subspace of dimension K r for r = 1 , … , n . In practice, M ˆ is usually not constructed explicitly, as many operations involving M ˆ can be performed at much lower cost using only the core tensor S and the frames U ( r ) , which contain the same information as M ˆ but condensed in lower-dimensional objects. A typical example of such an operation is the contraction of two tensors, as will be discussed in the next section. The squared Frobenius norm of M is given by(11) ‖ M ‖ 2 = 〈 M , M 〉 = ∑ { i } M i 1 ⋯ i n 2 , where for brevity we introduce the notation { i } = i 1 , … , i n for the summation indices, and the inner product between two real tensors of equal dimension is defined as(12) 〈 A , B 〉 = ∑ { i } A i 1 ⋯ i n B i 1 ⋯ i n . Since the reduced-rank tensor M ˆ is a projection of M , we have(13) 〈 M , M ˆ 〉 = ‖ M ˆ ‖ 2 = ‖ S ‖ 2 . Therefore, the squared approximation error is given by(14) ‖ M − M ˆ ‖ 2 = ‖ M ‖ 2 + ‖ M ˆ ‖ 2 − 2 〈 M , M ˆ 〉 = ‖ M ‖ 2 − ‖ M ˆ ‖ 2 = ‖ M ‖ 2 − ‖ S ‖ 2 , which does not require the explicit computation of M ˆ . In the HOSVD approximation procedure [7] the semi-orthogonal frames used to approximate M are constructed using properties of matrix SVDs. We first introduce the r -unfolding M ( r ) , which is a matrix containing the same entries as the tensor M , but reordered such that its rows correspond to the r -th mode of M and its columns correspond to a combination of all other tensor modes. The entries of the r -unfolding of M are given by(15) M i r , ( { i } ∖ i r ) ( r ) = M i 1 ⋯ i n , where the column index ( { i } ∖ i r ) = ( i 1 , … , i r − 1 , i r + 1 , … , i n ) is a linear index of coordinates in a space of dimension ∏ s ≠ r N s . The multi-rank of a tensor is defined by the n -tuple of the ranks of the individual unfoldings M ( r ) , r = 1 , … , n . Therefore M has at most multi-rank ( N 1 , … , N n ) and the approximation M ˆ of (6) at most multi-rank ( K 1 , … , K n ) . Note that the squared Frobenius norm (11) of a tensor is identical to that of any of its unfoldings, as it is just a sum over all squared components,(16) ‖ M ‖ 2 = ‖ M ( r ) ‖ 2 = Tr [ M ( r ) M ( r ) T ] for any r = 1 , … , n . To construct the HOSVD approximation M ˆ , we first consider the singular value decomposition for each r -unfolding M ( r ) of M (assuming real tensors for simplicity),(17) M ( r ) = L ( r ) Σ ( r ) R ( r ) T , where the columns of the N r × N r orthogonal matrix L ( r ) are the left singular vectors of M ( r ) . The columns of the orthogonal matrix R ( r ) contain the corresponding right singular vectors of M ( r ) . The diagonal entries of Σ ( r ) are the singular values of M ( r ) , which are always real and non-negative, while all other entries are zero. It is well-known in linear algebra that retaining the K r largest singular values in Σ ( r ) , while setting all others to zero, yields the best-possible matrix approximation A ( r ) of rank K r to M ( r ) (best-possible referring to a minimization of the Frobenius norm ‖ M ( r ) − A ( r ) ‖ ). The relative truncation error is given by(18) ϵ ( r ) = ‖ M ( r ) − A ( r ) ‖ ‖ M ( r ) ‖ = ∑ i = K r + 1 N r λ i ( r ) ∑ i = 1 N r λ i ( r ) , where λ i ( r ) are the eigenvalues of the Gramian M ( r ) M ( r ) T , i.e., the squared singular values of the unfolding M ( r ) , ordered such that λ 1 ( r ) ≥ … ≥ λ N r ( r ) . This matrix property is used in HOSVD by separately performing the matrix SVDs of all individual unfoldings M ( r ) of M for r = 1 , … , n and constructing the N r × K r frames U ( r ) with the singular vectors of L ( r ) corresponding to the K r largest singular values of the unfoldings M ( r ) . These frames are then used to construct the core tensor (10) and the matrix approximation (9) of HOSVD. Unlike for the matrix case, the HOSVD tensor approximation is in general not the best-possible approximation of a given multi-rank ( K 1 , … , K n ) , even though it is usually quite close to it [7] . In a variant of the HOSVD approximation, called interlaced HOSVD approximation, the rank reduction procedure is carried out in the following way: Starting with S = M , the frames are computed on successive unfoldings of the core tensor, which gets updated every time a new truncation frame is constructed until the core tensor is of dimension K 1 × … × K n . For the interlaced HOSVD approximation, the result depends on the order of the updates, but is usually close to that of the ordinary HOSVD approximation. The best-possible approximation of multi-rank ( K 1 , … , K n ) , which minimizes the Frobenius norm ‖ M − M ˆ ‖ , can be constructed numerically using the Higher Order Orthogonal Iteration (HOOI) [10] . Nevertheless, the HOSVD approximation is especially appealing because of its relative simplicity to produce an almost optimal approximation. 3 Backward-forward symmetric truncation in HOTRG We now discuss how the HOSVD formalism is used in HOTRG to avoid the exponential blow up of the tensor dimension during the blocking procedure, and why the standard HOSVD truncation is modified to avoid drawbacks related to accuracy and efficiency. To make our point we will use the two-dimensional case as it can be easiest illustrated and contains all the ingredients necessary for the discussion. Extending it to higher dimensions is straightforward. We consider the contraction M = T ⋆ 2 T of two local tensors T along the 2-direction. According to the discussion in the introduction, M will have thin backward and forward modes of dimension D in the contracted 2-direction, and fat backward and forward modes of dimension D 2 in the perpendicular 1-direction, which we want to reduce to lower rank by projecting on a D -dimensional subspace using (6) . This procedure of contraction and truncation, which we detail below, is illustrated in Fig. 3 . To reduce the dimension of the fat modes back from D 2 to D , while minimizing the loss of information, we apply the HOSVD approximation procedure, explained in Sec. 2 , where we only truncate the fat modes. The SVDs are computed for the unfoldings M ( 1 ) and M ( 2 ) for the backward and forward modes in the 1-direction, respectively, and the frames U and V of dimension D 2 × D are constructed with the singular vectors corresponding to their D largest singular values. For the modes in the contracted 2-direction no truncation is required. With these frames we construct a core tensor S of dimension D × D × D × D , according to (10) ,(19) S = U T ⊙ 1 V T ⊙ 2 M . The corresponding approximation M ˆ , defined in (9) , with the same dimension as M , but typically much lower rank, is given by(20) M ˆ = U ⊙ 1 V ⊙ 2 S = P U ⊙ 1 P V ⊙ 2 M , with D 2 × D 2 projectors P U = U U T and P V = V V T , with U T U = V T V = 1 D . As mentioned in Sec. 2 , operations involving M ˆ can typically be performed at much lower cost by using only the core tensor S and the frames U and V . Assume that in the next blocking step two M ˆ tensors are contracted in the 1-direction, as is illustrated in Fig. 4 . When using the standard HOSVD approximation (20) the backward and forward modes in M ˆ will have been projected on different subspaces using the projectors P U and P V , respectively. In a contraction M ˆ ⋆ 1 M ˆ the two projectors will be multiplied, as can be seen in the center of the figure. The decomposition of the approximation M ˆ in (20) can be used to reduce the computational effort, as the original contraction can be replaced by contractions of two core tensors S with a D × D dimensional merger G ≡ U T V in between, as can be seen in the figure.1 1 In fact this produces an amputated version of M ˆ ⋆ 1 M ˆ , which together with the mergers is all we need in the full contraction of the tensor network, see also Fig. 4 . The entries of G are scalar products of the basis vectors in U and V . At this point it is important to discuss a crucial modification introduced by the HOTRG method to the HOSVD truncation procedure presented above, which is rarely discussed in the literature. Although the HOSVD approximation M ˆ gives a close-to-best lower-rank approximation to M , it is in general not such a good and useful truncation when viewed as part of the iterative blocking procedure. Indeed, the product of projectors corresponds to a projection of a projection, which will unavoidably loose additional information if the projectors P U and P V are different. In this case, the contraction M ˆ ⋆ 1 M ˆ will no longer necessarily be a good approximation of M ⋆ 1 M , even if M ˆ itself was close to the best-possible approximation of M . We now observe that, due to the idempotence of projectors, there would be no additional loss if P U = P V , i.e., if the backward and forward modes of M in the 1-direction were projected on the same subspace. Note that in this case the merger G is an orthogonal matrix. Moreover, we can also get a serious gain in algorithmic simplicity, on top of this accuracy improvement, if we choose the same basis for both modes in the common subspace, i.e., we choose frames satisfying U = V , for which G = 1 D . When looking back at Fig. 4 we see that, in this case, the central merger just drops out, and the contraction can be replaced by a contraction of two core tensors, as is illustrated in Fig. 5 (the frames on the left and right of Fig. 4 will connect to their counter parts in further contractions, to form another merger G = 1 D , which will also drop out). For this reason, in HOTRG the backward and forward modes of each direction are truncated using a common frame U ( ν ) , ν = 1 , … , d , even when the HOSVD frames are different, which is typically the case for systems at nonzero chemical potential. With this backward-forward symmetric truncation, the core tensor S can be used as new coarse grid tensor after each blocking step, where at the ( k + 1 ) -th blocking step two tensors of step k are contracted to form a new coarse grid tensor,(21) T [ k ] ⋆ ν T [ k ] = : M ⟶ BF S = : T [ k + 1 ] . The abbreviation BF on the arrow means that we apply a backward-forward symmetric truncation to construct the core tensor, which then becomes the new local tensor on the coarse grid. The frames are only needed to construct the core tensor with (19) , and can then be discarded. The same reasoning also holds for a contraction in the other direction, where the directions of thin and fat modes are interchanged. Moreover, the procedure naturally generalizes to d dimensions, where we have d − 1 directions ν with backward and forward fat modes: If the backward and forward frames are chosen to be identical for each direction, the core tensor (10) can be used as the new coarse grid tensor in the HOTRG blocking procedure. This strategy of choosing the same frame to truncate the backward and forward mode for each individual direction in HOTRG, makes it fundamentally different from the HOSVD approximation, as it can no longer directly rely on the optimal low-rank properties of matrix SVD. Geometrically, truncating the backward and forward modes with the same frame means that these modes get projected on the same subspace and are described in the same basis. This explains why the full contraction of the tensor network into a scalar can be rewritten in terms of the core tensors only. Note that if one would use the standard HOSVD approximation procedure and work with different backward and forward frames, we would have to use both the core tensors and the mergers G defined above when performing the iterative contractions, to ensure that the different subspaces are matched onto one another, see Fig. 4 . Although this is no conceptual problem, it would complicate the algorithm, require additional computational work, and most of all the product of projectors would deteriorate the results further. The construction of the shared backward-forward frames in HOTRG has not been given a lot of attention in the literature until now. There is a brief discussion of this issue in the original HOTRG paper [6] , where either the backward or forward frame is chosen and applied to both modes, depending on which one gives the smallest SVD truncation error. The error introduced by this choice on the other mode is however not taken into account. We observed that for tensors lacking a backward-forward symmetry, this choice of frame is not optimal and can be improved upon. Below we present two new methods to improve the construction of shared frames for the backward and forward modes. The first one, called SuperQ method and presented in Sec. 4 , minimizes a combined error on the backward and forward unfoldings for each individual direction. The second method, which we call iterative SuperQ (ISQ) method is presented in Sec. 5 . This iterative method aims at determining the best-possible approximation to M for a given multi-rank, satisfying the requirement that the backward and forward frames for each direction are identical. The ISQ method leans on ideas developed for the higher order orthogonal iteration (HOOI) method [10] , which constructs the best-possible approximation of a given multi-rank with independent frames for all modes. We will see that the SuperQ solution can be used as a natural starting point for the ISQ procedure. Note that the SuperQ and ISQ methods are specifically conceived for tensors which are part of a physical tensor network on a space-time lattice and have modes corresponding to backward and forward orientations. 4 The SuperQ method To discuss the construction of truncations satisfying the requirement that the frames for the backward and forward modes are identical, we consider a tensor M with d pairs of backward and forward modes. The tensor is thus of order 2d with dimension N 1 × N 1 × ⋯ × N d × N d , which will be truncated to dimension K 1 × K 1 × ⋯ × K d × K d using d semi-orthogonal frames U ( ν ) of dimension N ν × K ν . With these frames the core tensor is constructed using(22) S = U ( 1 ) T ⊙ 1 U ( 1 ) T ⊙ 2 ⋯ U ( d ) T ⊙ 2 d − 1 U ( d ) T ⊙ 2 d M . Consider the positive semi-definite Gram matrices(23) Q b ( ν ) = M b ( ν ) M b ( ν ) T , Q f ( ν ) = M f ( ν ) M f ( ν ) T , where M b ( ν ) and M f ( ν ) are the unfoldings of M with respect to the backward and forward modes for direction ν , i.e.,(24) M b ( ν ) ≡ M ( 2 ν − 1 ) , M f ( ν ) ≡ M ( 2 ν ) , and M ( r ) is the r -unfolding of the tensor M defined in (15) . We denote the N ν × K ν frames constructed with the eigenvectors corresponding to the K ν largest eigenvalues of Q b ( ν ) and Q f ( ν ) as U b ( ν ) and U f ( ν ) , respectively. As the backward and forward Gramians Q b ( ν ) and Q f ( ν ) are in general not identical, the corresponding subspaces spanned by the vectors of the frames U b ( ν ) and U f ( ν ) will be different too.2 2 This can even be the case if the eigenvalues of both Gramians coincide, as we have observed for the O ( 2 ) model with chemical potential. In the standard HOTRG procedure [6] it is suggested to choose either U b ( ν ) or U f ( ν ) for the unique U ( ν ) , depending which of both gives the smallest SVD truncation error (18) . Even though this choice of frame optimizes the truncation error for one mode, it does not take into account its effect on the mode corresponding to the opposite orientation. Therefore, it is clear that, generically, better choices of frames should exist, and our aim is to construct frames U ( ν ) that reduce the combined truncation error when applied simultaneously to the backward and forward modes for the ν direction. Let us now consider a single truncation frame U ( ν ) which we use to reduce the rank of the unfoldings M b ( ν ) and M f ( ν ) . Using (14) and (16) , the relative truncation errors on the backward and forward unfoldings are(25) ϵ b ( ν ) = ‖ M b ( ν ) − A b ( ν ) ‖ ‖ M b ( ν ) ‖ = 1 − Tr ( U ( ν ) T Q b ( ν ) U ( ν ) ) Tr Q b ( ν ) , (26) ϵ f ( ν ) = ‖ M f ( ν ) − A f ( ν ) ‖ ‖ M f ( ν ) ‖ = 1 − Tr ( U ( ν ) T Q f ( ν ) U ( ν ) ) Tr Q f ( ν ) , where A b ( ν ) = U ( ν ) U ( ν ) T M b ( ν ) and A f ( ν ) = U ( ν ) U ( ν ) T M f ( ν ) are the rank-K ν approximations to the unfoldings M b ( ν ) and M f ( ν ) , respectively, obtained with the same frame U ( ν ) . To improve upon using either U b ( ν ) or U f ( ν ) , we determine the common U ( ν ) by minimizing the combination of both errors in(27) ϵ S ( ν ) 2 = 1 2 ( ϵ b ( ν ) 2 + ϵ f ( ν ) 2 ) = 1 − 1 2 ‖ M ‖ 2 Tr [ U ( ν ) T ( Q b ( ν ) + Q f ( ν ) ) U ( ν ) ] , where we also used Tr Q b ( ν ) = Tr Q f ( ν ) = ‖ M ‖ 2 . We define the SuperQ matrix for direction ν as(28) Q S ( ν ) = Q b ( ν ) + Q f ( ν ) , which is symmetric and positive semi-definite as it is a sum of two symmetric positive semi-definite matrices. Therefore, if we diagonalize the SuperQ matrix and truncate the eigenvector matrix, retaining the eigenvectors corresponding to the K ν largest eigenvalues, then this semi-orthogonal frame U ( ν ) minimizes the truncation error (27) on Q S ( ν ) . This SuperQ procedure is repeated on all d directions to determine all frames U ( ν ) , which can then be used to approximate M and to construct the corresponding core tensor S , see (22) . The SuperQ method is computationally efficient since it only requires a single eigenvalue decomposition for each pair of backward and forward fat modes, while the standard HOTRG procedure [6] performs separate decompositions on these modes. When applying the SuperQ method to HOTRG, where M is a contraction T ⋆ ν T along one of the directions, only 2 d − 2 modes will actually be truncated, as the backward and forward modes for the contracted direction need not be truncated. In analogy to the interlaced HOSVD approximation, see Sec. 2 , we can also define an interlaced version of the SuperQ method where we determine the frames U ( ν ) by applying the method to an intermediate core tensor, which gets updated direction-by-direction (starting from S = M ) by truncating the respective backward and forward mode each time a frame has been computed. This interlaced SuperQ method is also of interest in the light of the iterative procedure derived in the next section. 5 Optimized frames with iterative SuperQ Although the HOSVD method, see Sec. 2 , typically yields a good tensor approximation M ˆ [7] , the best-possible one, which minimizes ‖ M − M ˆ ‖ 2 , can be constructed with an iterative procedure called higher order orthogonal iteration (HOOI) method [10] . According to the discussion of the backward-forward symmetric truncation in Sec. 3 , it is clear that HOOI is itself not applicable in a tensor network approach to statistical physics, because the backward and forward modes should be truncated with the same semi-orthogonal frame for each direction, while HOOI very generically generates different frames for all modes. Below we present the iterative SuperQ (ISQ) method, which is inspired by the original HOOI procedure but imposes the requirement that the same frame has to be used to truncate the backward and forward modes of each direction. As in Sec. 4 , we consider a tensor M with d pairs of backward and forward modes, i.e., the tensor is of order 2d with dimensions N 1 × N 1 ⋯ × N d × N d , which will be truncated to dimensions K 1 × K 1 × ⋯ × K d × K d , see (22) . Our aim is to minimize the squared Frobenius norm (14) (29) ‖ M − M ˆ ‖ 2 = ‖ M ‖ 2 − ‖ S ‖ 2 , over all semi-orthogonal N ν × K ν frames U ( ν ) , ν = 1 … d , for fixed K ν , with the additional condition that the backward and forward modes for each direction ν are truncated with the same frame U ( ν ) . The semi-orthogonality of the frames is imposed explicitly by orthonormality conditions for the column vectors of U ( ν ) ,(30) ∑ i = 1 N ν U i a ( ν ) U i b ( ν ) = δ a b , for 1 ≤ a , b ≤ K ν and 1 ≤ ν ≤ d , in the constrained minimization of (29) . This leads to the cost function(31) g = ‖ S ‖ 2 + C = ‖ U ( 1 ) T ⊙ 1 U ( 1 ) T ⊙ 2 ⋯ U ( d ) T ⊙ 2 d − 1 U ( d ) T ⊙ 2 d M ‖ 2 + ∑ ν = 1 d Tr [ Λ ( ν ) ( 1 − U ( ν ) T U ( ν ) ) ] , with matrices Λ ( ν ) containing the Lagrange multipliers. The orthonormalization conditions are symmetric under the exchange a ↔ b , and so Λ ( ν ) will be symmetric too. If we diagonalize Λ diag ( ν ) = O ( ν ) T Λ ( ν ) O ( ν ) , redefine U ( ν ) O ( ν ) → U ( ν ) and Λ diag ( ν ) → Λ ( ν ) , and use the orthogonality of O ( ν ) , then Eq. (31) remains unaltered albeit now with diagonal Λ ( ν ) = diag ( λ 1 ( ν ) , … , λ K ν ( ν ) ) , and this without loss of generality. Written out in components this is(32) g = ∑ { a , b } S a 1 b 1 ⋯ a d b d 2 + ∑ ν = 1 d ∑ c = 1 K ν λ c ( ν ) ( 1 − ∑ k = 1 N ν U k c ( ν ) 2 ) = ∑ { a , b } ( ∑ { i , j } [ ∏ μ = 1 d U i μ a μ ( μ ) U j μ b μ ( μ ) ] M i 1 j 1 ⋯ i d j d ) 2 + ∑ ν = 1 d ∑ c = 1 K ν λ c ( ν ) ( 1 − ∑ k = 1 N ν U k c ( ν ) 2 ) . For a constrained maximum of ‖ S ‖ 2 , the partial derivative of g with respect to the ( k , c ) -entry of the ν -th orthogonal frame has to satisfy(33) 0 = ∂ g ∂ U k c ( ν ) = ∑ { a , b } 2 S a 1 b 1 ⋯ a d b d ∂ ∂ U k c ( ν ) ( ∑ { i , j } [ ∏ μ = 1 d U i μ a μ ( μ ) U j μ b μ ( μ ) ] M i 1 j 1 ⋯ i d j d ) − 2 λ c ( ν ) U k c ( ν ) , for 1 ≤ k ≤ N ν , 1 ≤ c ≤ K ν and 1 ≤ ν ≤ d . Note that the same frame U ( ν ) appears twice in S , as it is used to truncate the modes in the backward and forward ν direction. We therefore obtain(34) ∑ { a , b } S a 1 b 1 ⋯ a d b d ( ∑ { i , j } δ k , i ν δ c , a ν [ ∏ μ ≠ ν U i μ a μ ( μ ) U j μ b μ ( μ ) ] U j ν b ν ( ν ) M i 1 j 1 ⋯ i n j n + ∑ { i , j } δ k , j ν δ c , b ν [ ∏ μ ≠ ν U i μ a μ ( μ ) U j μ b μ ( μ ) ] U i ν a ν ( ν ) M i 1 j 1 ⋯ i n j n ) = λ c ( ν ) U k c ( ν ) . After eliminating the Kronecker deltas we get(35) ∑ { a , b } ∖ a ν S a 1 b 1 ⋯ c b ν ⋯ a d b d ∑ { i , j } ∖ i ν [ ∏ μ ≠ ν U i μ a μ ( μ ) U j μ b μ ( μ ) ] U j ν b ν ( ν ) M i 1 j 1 ⋯ k j ν ⋯ i d j d + ∑ { a , b } ∖ b ν S a 1 b 1 ⋯ a ν c ⋯ a d b d ∑ { i , j } ∖ j ν [ ∏ μ ≠ ν U i μ a μ ( μ ) U j μ b μ ( μ ) ] U i ν a ν ( ν ) M i 1 j 1 ⋯ i ν k ⋯ i d j d = λ c ( ν ) U k c ( ν ) . Let us define the unfolding matrices B ( ν ) and F ( ν ) with dimensions N ν × ( K 1 2 ⋯ K ν − 1 2 K ν K ν + 1 2 ⋯ K d 2 ) , where all directions of M are truncated, except for the backward-ν mode for B ( ν ) , and the forward-ν mode for F ( ν ) , and the unfolding is performed with respect to the untruncated mode,(36) B i ν , ( { a , b } ∖ a ν ) ( ν ) = ∑ { i , j } ∖ i ν [ ∏ μ ≠ ν U i μ a μ ( μ ) U j μ b μ ( μ ) ] U j ν b ν ( ν ) M i 1 j 1 ⋯ ⋯ i d j d (37) F j ν , ( { a , b } ∖ b ν ) ( ν ) = ∑ { i , j } ∖ j ν [ ∏ μ ≠ ν U i μ a μ ( μ ) U j μ b μ ( μ ) ] U i ν a ν ( ν ) M i 1 j 1 ⋯ i d j d , where we used the notation introduced in (15) for the matrix indices. The core tensor can also be written in terms of B ( ν ) and F ( ν ) by truncating the remaining untruncated index:(38) S a 1 b 1 ⋯ a d b d = ∑ i ν B i ν , ( { a , b } ∖ a ν ) ( ν ) U i ν a ν ( ν ) = ∑ j ν F j ν , ( { a , b } ∖ b ν ) ( ν ) U j ν b ν ( ν ) . After substituting B ( ν ) and F ( ν ) in (35) we obtain(39) ∑ { a , b } ∖ a ν ∑ i ν B k , ( { a , b } ∖ a ν ) ( ν ) B i ν , ( { a , b } ∖ a ν ) ( ν ) U i ν c ( ν ) + ∑ { a , b } ∖ b ν ∑ j ν F k , ( { a , b } ∖ b ν ) ( ν ) F j ν , ( { a , b } ∖ b ν ) ( ν ) U j ν c ( ν ) = λ c ( ν ) U k c ( ν ) , or(40) ∑ i ν [ B ( ν ) B ( ν ) T ] k i ν U i ν c ( ν ) + ∑ j ν [ F ( ν ) F ( ν ) T ] k j ν U j ν c ( ν ) = λ c ( ν ) U k c ( ν ) . If we introduce the positive semi-definite matrices(41) Q ( ν ) ( U ( 1 ) , … , U ( d ) ) = B ( ν ) B ( ν ) T + F ( ν ) F ( ν ) T , ν = 1 , … , d , we can identify (40) as a coupled nonlinear eigenvalue problem (which is nonlinear in the eigenvectors)(42) ν = 1 … d , Q ( ν ) ( U ( 1 ) , … , U ( d ) ) U ( ν ) = U ( ν ) Λ ( ν ) , ν = 1 , … , d , for the semi-orthogonal frames U ( ν ) of dimension N ν × K ν and the K ν -dimensional diagonal matrices Λ ( ν ) . Note that all frames U ( μ ) , μ = 1 … d , appear in Q ( ν ) (as projections P ( μ ) = U ( μ ) U ( μ ) T ). This is even true for the direction ν itself, as its projector is applied to the forward mode in B ( ν ) B ( ν ) T and to the backward mode in F ( ν ) F ( ν ) T . Therefore the ν -th equation is cubic in U ( ν ) and quartic in all other U ( μ ) , μ ≠ ν . It is crucial to keep in mind that the self-consistent solutions to (42) , which we are looking for, are required to be semi-orthogonal matrices with K ν ≤ N ν columns, in order to satisfy the constraints (30) . It is useful to note that if we replace the matrices Q ( ν ) in (42) by fixed matrices Q ˆ ( ν ) , the d matrix equations decouple and each one of them is a linear eigenvalue equation for the frame U ( ν ) . The solutions of these linearized equations are however in general no solution of the original nonlinear equations (42) . On the other hand, if (42) is satisfied, then U ( ν ) is a solution of the linear eigenvalue problem for the specific matrix Q ˆ ( ν ) = Q ( ν ) ( U ( 1 ) , … , U ( d ) ) . Therefore, we propose to solve the coupled system of equations using an iterative procedure, where at each iteration step, Q ˆ ( ν ) is computed with (41) using the current frames U ( μ ) , μ = 1 , … , d , and the eigenvalue problem(43) Q ˆ ( ν ) ( U ( 1 ) , … , U ( d ) ) u ( ν ) = λ ( ν ) u ( ν ) , is solved for the eigenvalues λ ( ν ) and eigenvectors u ( ν ) of the positive semi-definite matrix Q ˆ ( ν ) . We then take the normalized eigenvectors corresponding to the K ν largest eigenvalues to form a new frame U ( ν ) . Note that the eigenvectors of the symmetric matrix Q ˆ ( ν ) are orthogonal and therefore the constraint (30) is automatically satisfied for U ( ν ) . This strategy can be motivated in the following way. We are looking for the solution of (42) which maximizes ‖ S ‖ 2 . Using the expression (38) for the core tensor S in terms of the unfoldings B ( ν ) and F ( ν ) , the squared norm in the cost function (31) can be written as(44) ‖ S ‖ 2 = 1 2 ( Tr [ U ( ν ) T B ( ν ) B ( ν ) T U ( ν ) ] + Tr [ U ( ν ) T F ( ν ) F ( ν ) T U ( ν ) ] ) = 1 2 Tr [ U ( ν ) T Q ( ν ) U ( ν ) ] , which explicitly contains the matrix Q ( ν ) ≡ Q ( ν ) ( U ( 1 ) , … , U ( d ) ) defined in (41) (note that this equation yields the same ‖ S ‖ 2 for each ν = 1 , … , d ). It is straightforward to show that, at each iteration step, the solution obtained from the linearization (43) corresponds to the constrained optimization of the approximation(45) ‖ S ‖ 2 ≈ 1 2 Tr [ U ( ν ) T Q ˆ ( ν ) U ( ν ) ] to (44) , where Q ˆ ( ν ) is fixed and computed with the most recent frames U ( μ ) , μ = 1 , … , d . As this approximation is quadratic in the new frame U ( ν ) , its maximum will be given by the eigenvectors corresponding to the K ν largest eigenvalues of the symmetric, positive semi-definite matrix Q ˆ ( ν ) . The iterative procedure can be interpreted as an iterative SuperQ method, where at each step all modes of M are truncated using the last known frames, except for a backward mode in B and the corresponding forward mode in F . During the iterative procedure we cycle through the d dimensions ν and determine a new frame U ( ν ) at each step using (43) . Then, we repeat these d iteration steps until all frames have converged. In practice we observed that the first iteration for each direction is the most important one, and further iterations of the same direction only give small corrections. An alternative procedure would be to iterate (43) for a single U ( ν ) (keeping all other U ( μ ) fixed) until convergence has been reached (reevaluating Q ˆ ( ν ) with the most recent U ( ν ) at every step), and then go on to the next frame. Once all frames have been iterated, this whole procedure is repeated until all frames converge together. Note that the inner iterations are computationally cheap, as all frames but one are kept fixed and all matrix-tensor multiplications in Q ˆ and ‖ S ‖ 2 involving these fixed frames have to be computed only once. However, this procedure does not seem to give an overall faster convergence. A natural choice for the starting frames U ( ν ) , ν = 1 … d , in the iterative procedure are the frames obtained from the interlaced SuperQ method, see Sec. 4 . Note that the iterative procedure is not guaranteed to converge, and even when it does, the solution is not necessarily the global maximum. This can be improved upon by tuning the starting frames or by applying an under-relaxation procedure to the intermediate Q matrix. In this procedure, we replace Q ˆ ( ν ) in (43) by(46) Q ‾ ( ν ) = ω Q ˆ ( ν ) + ( 1 − ω ) Q ˆ prev ( ν ) , where Q ˆ prev ( ν ) was used to obtain the previous U ( ν ) in the iterative procedure. The relaxation procedure can be used to optimize ‖ S ‖ 2 by tuning the local parameter 0 ≤ ω ≤ 1 . We observed that a coarse tuning of ω is sufficient to improve the overall convergence of the iterative procedure. When applying the ISQ method to HOTRG, where M is a contraction T ⋆ ν T along one of the directions, only 2 d − 2 of 2d modes will actually be truncated, as the backward and forward modes for the contracted direction are left unchanged. For the two-dimensional case, where only one frame has to be determined after each contraction, an alternative method to optimize the truncation using a linearization was proposed in the projective truncation of Ref. [11] . 6 Examples In the following, we illustrate the effect of the SuperQ and ISQ methods. For various random tensors A , we compute core tensors S using the standard HOSVD approximation [7] and the best possible approximation of a given multi-rank with the HOOI method [10] . These results are compared with the following backward-forward symmetric approximations: the method proposed by Xie et al. (used in standard HOTRG [6] ), the SuperQ approximation of Sec. 4 , and the ISQ approximation of Sec. 5 . The relative error for each of these approximations is given by(47) ϵ = 1 − ‖ S ‖ 2 ‖ A ‖ 2 . The comparison of the different methods will be illustrated by plotting ( ϵ − ϵ hooi ) / ϵ hooi in the figures below. For the ISQ method the iterations are initialized with the frames obtained using the interlaced SuperQ method. The ISQ method is iterated until the relative error ϵ has a relative change of less than 10 − 6 between two major iterations, where a major iteration corresponds to one update of all frames. For each iteration we tune the relaxation parameter ω to reduce the error. The number of major iterations varies between 20 and 100 with an average of around 50. In a first example we consider random tensors A of order 4 with dimension 10 × 10 × 100 × 100 , whose rank is reduced by truncating the last two indices to dimension 10. The results shown in Fig. 6 were computed for initial tensors filled with uniformly distributed elements in [ 0 , 1 ] (left panel) and normally distributed elements with mean μ = 0 and standard deviation σ = 1 (right panel). In another example, shown in Fig. 7 , all modes of random 30 × 30 × 30 × 30 tensors are reduced to dimension 10. Again the random tensors are filled with elements from a uniform distribution (left) and a normal distribution (right). In all examples, the hierarchy between the approximations is the same. In decreasing order of accuracy we find: HOOI, ISQ, HOSVD, SuperQ, and finally the Xie-method. We notice that, as we suggested in the derivation of Sec. 4 , the SuperQ method performs better than the Xie-method. Both of them are superseded by HOSVD, which is logical as the latter does not have to satisfy the additional backward-forward symmetry constraint. However, a somewhat unexpected result is that in all examples, the ISQ method performs better than the standard HOSVD approximation, even though the former does satisfy the additional backward-forward symmetry constraint. In all cases HOOI performs best, as it is the best possible approximation of the given multi-rank. 7 Conclusions In this paper we consider the reduction of the local truncation error in a single blocking step of the HOTRG procedure. We have discussed in detail the constraints imposed on the semi-orthogonal truncation frames in the HOTRG algorithm, where the backward and forward modes for each direction have to be projected on the same lower-dimensional subspace at each blocking step. We first introduced the SuperQ method, which minimizes a combined error on the backward and forward unfoldings for each individual direction. The method is computationally more efficient and generically yields a reduced local truncation error when compared to the original HOTRG truncation. As a further improvement, we presented the iterative SuperQ method, where we formulate a constrained minimization problem, which leads to equations that have to be satisfied by the semi-orthogonal truncation frames in order to minimize the error on the lower-rank tensor approximation, while satisfying the backward-forward symmetry constraints. The method is inspired by the HOOI method, with the additional requirement that the same frames are used on the backward and forward modes of each direction. The equations form a coupled nonlinear eigenvalue problem, which we propose to solve using an iterative procedure, where decoupled linear eigenvalue problems are solved at each iteration step. Computing the optimal backward-forward symmetric truncation frames with the ISQ method is more expensive than the truncation applied in the original HOTRG method, as each iteration step requires new eigenvalue decompositions. Therefore, in practice, we generally use the SuperQ truncation in the HOTRG blocking procedure, since it provides the best trade-off between computational cost and truncation accuracy. Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. References [1] G. 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Langfeld, Approaches to the sign problem in lattice field theory, International Journal of Modern Physics A 31 (22) (2016) 1643007. arXiv:1603.09517, doi:10.1142/S0217751X16430077. [4] Y. Meurice R. Sakai J. Unmuth-Yockey Tensor lattice field theory for renormalization and quantum computing Rev. Mod. Phys. 94 2 2022 025005 10.1103/RevModPhys.94.025005 arXiv:2010.06539 Y. Meurice, R. Sakai, J. Unmuth-Yockey, Tensor lattice field theory for renormalization and quantum computing, Rev. Mod. Phys. 94 (2) (2022) 025005. arXiv:2010.06539, doi:10.1103/RevModPhys.94.025005. [5] M. Levin C.P. Nave Tensor renormalization group approach to two-dimensional classical lattice models Phys. Rev. Lett. 99 12 2007 120601 10.1103/PhysRevLett.99.120601 arXiv:cond-mat/0611687 M. Levin, C. P. Nave, Tensor renormalization group approach to two-dimensional classical lattice models, Phys. Rev. Lett. 99 (12) (2007) 120601. arXiv:cond-mat/0611687, doi:10.1103/PhysRevLett.99.120601. [6] Z.Y. Xie J. Chen M.P. 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Evenbly Algorithms for tensor network renormalization Phys. Rev. B 95 2017 045117 10.1103/PhysRevB.95.045117 arXiv:1509.07484 G. Evenbly, Algorithms for tensor network renormalization, Phys. Rev. B95 (2017) 045117. arXiv:1509.07484, doi:10.1103/PhysRevB.95.045117.
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-]>PLB 137649 137649 S0370-2693(22)00783-3 10.1016/j.physletb.2022.137649 The Author(s) Experiments https://www.hepdata.net/ Fig. 1 Illustration of toward, away and transverse regions with respect to the leading particle in a collision. Fig. 1 Fig. 2 Top panels: transverse momentum spectra of charged particles in the transverse region for different multiplicity classes in pp (left), p–Pb (middle) and Pb–Pb (right) collisions at s NN = 5.02 TeV. The p T spectra are measured at mid pseudorapidity (|η | < 0.8). Lower panels: Ratio of p T spectra in different multiplicity classes to the p T spectrum in the 0−100% multiplicity class for the corresponding collision systems. The statistical and systematic uncertainties are shown by bars and boxes, respectively. Fig. 2 Fig. 3 Transverse momentum spectra of charged particles in Toward-Transverse, d N ch st / d p T (top plot) and Away-Transverse, d N ch sa / d p T (bottom plot) regions for different multiplicity classes in pp (left), p–Pb (middle) and Pb–Pb (right) collisions at s NN = 5.02 TeV. The p T spectra are measured at mid pseudorapidity (|η | < 0.8). The lower panels of both plots show the ratio to minimum bias pp collisions. The statistical and systematic uncertainties are shown by bars and boxes, respectively. Fig. 3 Fig. 4 The I X t (left) and I X a (right) as a function of 〈 N ch T 〉 in 4 <p T < 6 GeV/c for different multiplicity classes in pp, p–Pb and Pb–Pb collisions at s NN = 5.02 TeV. Pb–Pb results are shown assuming a flat background (filled markers), and assuming a v 2 -modulated background (empty markers). The statistical and systematic uncertainties are shown by bars and boxes, respectively. Fig. 4 Fig. 5 Comparison of the measured the I X t (left) and I X a (right) in 4 <p T < 6 GeV/c with model predictions. The results are shown as a function of 〈 N ch T 〉 for different multiplicity classes in pp (top panel), p–Pb (middle panel) and Pb–Pb (bottom panel) collisions at s NN = 5.02 TeV. The red and magenta lines show the PYTHIA 8 (Monash) [28] and PYTHIA 8/Angantyr [28] predictions, respectively. The blue lines show the EPOS-LHC [21] results. The statistical and systematic uncertainties are shown by bars and boxes, respectively. Fig. 5 Table 1 Contributions to the relative (%) systematic uncertainty on the p T spectra of primary charged particles in pp, p–Pb, and Pb–Pb collisions at s NN = 5.02 TeV. Just for illustration, the range in the table corresponds to the lowest and highest relative systematic uncertainty in the considered p T range. The individual contributions are summed in quadrature to obtain the total uncertainty. Table 1 Source of uncertainty pp p–Pb Pb–Pb |
Track selection 2.1–8.2 2.4–5.8 3.0–9.9 |
Particle composition 0.3–1.8 0.5–1.9 0.3–2.4 |
Secondary particles 0.0–0.4 0.0–2.4 0.0–1.9 |
Matching efficiency 2.0–4.2 0.7–3.7 0.6–3.7 |
Total 3.2–8.8 3.6–6.3 3.5–10.0 |
Total (N ch -dependent) 2.0–4.5 1.7–4.0 1.1–3.7 |
Study of charged particle production at high p T using event topology in pp, p–Pb and Pb–Pb collisions at s NN = 5.02 TeV ALICE Collaboration ⋆ S. Acharya 124 131 D. Adamová 86 A. Adler 69 G. Aglieri Rinella 32 M. Agnello 29 N. Agrawal 50 Z. Ahammed 131 S. Ahmad 15 S.U. Ahn 70 I. Ahuja 37 A. Akindinov 139 M. Al-Turany 98 D. Aleksandrov 139 B. Alessandro 55 H.M. Alfanda 6 R. Alfaro Molina 66 B. Ali 15 Y. Ali 13 A. Alici 25 N. Alizadehvandchali 113 A. Alkin 32 J. Alme 20 G. Alocco 51 T. Alt 63 I. Altsybeev 139 M.N. Anaam 6 C. Andrei 45 A. Andronic 134 V. Anguelov 95 F. Antinori 53 P. Antonioli 50 C. Anuj 15 N. Apadula 74 L. Aphecetche 103 H. Appelshäuser 63 S. Arcelli 25 R. Arnaldi 55 I.C. Arsene 19 M. Arslandok 136 A. Augustinus 32 R. Averbeck 98 S. Aziz 72 M.D. Azmi 15 A. Badalà 52 Y.W. Baek 40 X. Bai 98 R. Bailhache 63 Y. Bailung 47 R. Bala 91 A. Balbino 29 A. Baldisseri 127 B. Balis 2 D. Banerjee 4 Z. Banoo 91 R. Barbera 26 L. Barioglio 96 M. Barlou 78 G.G. Barnaföldi 135 L.S. Barnby 85 V. Barret 124 L. Barreto 109 C. Bartels 116 K. Barth 32 E. Bartsch 63 F. Baruffaldi 27 N. Bastid 124 S. Basu 75 G. Batigne 103 D. Battistini 96 B. Batyunya 140 D. Bauri 46 J.L. Bazo Alba 101 I.G. Bearden 83 C. Beattie 136 P. Becht 98 D. Behera 47 I. Belikov 126 A.D.C. Bell Hechavarria 134 F. Bellini 25 R. Bellwied 113 S. Belokurova 139 V. Belyaev 139 G. Bencedi 135 64 S. Beole 24 A. Bercuci 45 Y. Berdnikov 139 A. Berdnikova 95 L. Bergmann 95 M.G. Besoiu 62 L. Betev 32 P.P. Bhaduri 131 A. Bhasin 91 I.R. Bhat 91 M.A. Bhat 4 B. Bhattacharjee 41 L. Bianchi 24 N. Bianchi 48 J. Bielčík 35 J. Bielčíková 86 J. Biernat 106 A. Bilandzic 96 G. Biro 135 S. Biswas 4 J.T. Blair 107 D. Blau 139 M.B. Blidaru 98 N. Bluhme 38 C. Blume 63 G. Boca 21 54 F. Bock 87 T. Bodova 20 A. Bogdanov 139 S. Boi 22 J. Bok 57 L. Boldizsár 135 A. Bolozdynya 139 M. Bombara 37 P.M. Bond 32 G. Bonomi 130 54 H. Borel 127 A. Borissov 139 H. Bossi 136 E. Botta 24 L. Bratrud 63 P. Braun-Munzinger 98 M. Bregant 109 M. Broz 35 G.E. Bruno 97 31 M.D. Buckland 116 D. Budnikov 139 H. Buesching 63 S. Bufalino 29 O. Bugnon 103 P. Buhler 102 Z. Buthelezi 67 120 J.B. Butt 13 A. Bylinkin 115 S.A. Bysiak 106 M. Cai 27 6 H. Caines 136 A. Caliva 98 E. Calvo Villar 101 J.M.M. Camacho 108 R.S. Camacho 44 P. Camerini 23 F.D.M. Canedo 109 M. Carabas 123 F. Carnesecchi 32 R. Caron 125 127 J. Castillo Castellanos 127 F. Catalano 29 C. Ceballos Sanchez 140 I. Chakaberia 74 P. Chakraborty 46 S. Chandra 131 S. Chapeland 32 M. Chartier 116 S. Chattopadhyay 131 S. Chattopadhyay 99 T.G. Chavez 44 T. Cheng 6 C. Cheshkov 125 B. Cheynis 125 V. Chibante Barroso 32 D.D. Chinellato 110 E.S. Chizzali 96 II J. Cho 57 S. Cho 57 P. Chochula 32 P. Christakoglou 84 C.H. Christensen 83 P. Christiansen 75 T. Chujo 122 M. Ciacco 29 C. Cicalo 51 L. Cifarelli 25 F. Cindolo 50 M.R. Ciupek 98 G. Clai 50 III F. Colamaria 49 J.S. Colburn 100 D. Colella 97 31 A. Collu 74 M. Colocci 32 M. Concas 55 IV G. Conesa Balbastre 73 Z. Conesa del Valle 72 G. Contin 23 J.G. Contreras 35 M.L. Coquet 127 T.M. Cormier 87 I P. Cortese 129 55 M.R. Cosentino 111 F. Costa 32 S. Costanza 21 54 P. Crochet 124 R. Cruz-Torres 74 E. Cuautle 64 P. Cui 6 L. Cunqueiro 87 A. Dainese 53 M.C. Danisch 95 A. Danu 62 P. Das 80 P. Das 4 S. Das 4 S. Dash 46 R.M.H. David 44 A. De Caro 28 G. de Cataldo 49 L. De Cilladi 24 J. de Cuveland 38 A. De Falco 22 D. De Gruttola 28 N. De Marco 55 C. De Martin 23 S. De Pasquale 28 S. Deb 47 H.F. Degenhardt 109 K.R. Deja 132 R. Del Grande 96 L. Dello Stritto 28 W. Deng 6 P. Dhankher 18 D. Di Bari 31 A. Di Mauro 32 R.A. Diaz 140 7 T. Dietel 112 Y. Ding 125 6 R. Divià 32 D.U. Dixit 18 Ø. Djuvsland 20 U. Dmitrieva 139 A. Dobrin 62 B. Dönigus 63 A.K. Dubey 131 J.M. Dubinski 132 A. Dubla 98 S. Dudi 90 P. Dupieux 124 M. Durkac 105 N. Dzalaiova 12 T.M. Eder 134 R.J. Ehlers 87 V.N. Eikeland 20 F. Eisenhut 63 D. Elia 49 B. Erazmus 103 F. Ercolessi 25 F. Erhardt 89 M.R. Ersdal 20 B. Espagnon 72 G. Eulisse 32 D. Evans 100 S. Evdokimov 139 L. Fabbietti 96 M. Faggin 27 J. Faivre 73 F. Fan 6 W. Fan 74 A. Fantoni 48 M. Fasel 87 P. Fecchio 29 A. Feliciello 55 G. Feofilov 139 A. Fernández Téllez 44 M.B. Ferrer 32 A. Ferrero 127 A. Ferretti 24 V.J.G. Feuillard 95 J. Figiel 106 V. Filova 35 D. Finogeev 139 F.M. Fionda 51 G. Fiorenza 97 F. Flor 113 A.N. Flores 107 S. Foertsch 67 I. Fokin 95 S. Fokin 139 E. Fragiacomo 56 E. Frajna 135 U. Fuchs 32 N. Funicello 28 C. Furget 73 A. Furs 139 J.J. Gaardhøje 83 M. Gagliardi 24 A.M. Gago 101 A. Gal 126 C.D. Galvan 108 P. Ganoti 78 C. Garabatos 98 J.R.A. Garcia 44 E. Garcia-Solis 9 K. Garg 103 C. Gargiulo 32 A. Garibli 81 K. Garner 134 E.F. Gauger 107 A. Gautam 115 M.B. Gay Ducati 65 M. Germain 103 S.K. Ghosh 4 M. Giacalone 25 P. Gianotti 48 P. Giubellino 98 55 P. Giubilato 27 A.M.C. Glaenzer 127 P. Glässel 95 E. Glimos 119 D.J.Q. Goh 76 V. Gonzalez 133 L.H. González-Trueba 66 S. Gorbunov 38 M. Gorgon 2 L. Görlich 106 S. Gotovac 33 V. Grabski 66 L.K. Graczykowski 132 E. Grecka 86 L. Greiner 74 A. Grelli 58 C. Grigoras 32 V. Grigoriev 139 S. Grigoryan 140 1 F. Grosa 32 J.F. Grosse-Oetringhaus 32 R. Grosso 98 D. Grund 35 G.G. Guardiano 110 R. Guernane 73 M. Guilbaud 103 K. Gulbrandsen 83 T. Gunji 121 W. Guo 6 A. Gupta 91 R. Gupta 91 S.P. Guzman 44 L. Gyulai 135 M.K. Habib 98 C. Hadjidakis 72 H. Hamagaki 76 M. Hamid 6 Y. Han 137 R. Hannigan 107 M.R. Haque 132 A. Harlenderova 98 J.W. Harris 136 A. Harton 9 J.A. Hasenbichler 32 H. Hassan 87 D. Hatzifotiadou 50 P. Hauer 42 L.B. Havener 136 S.T. Heckel 96 E. Hellbär 98 H. Helstrup 34 T. Herman 35 G. Herrera Corral 8 F. Herrmann 134 K.F. Hetland 34 B. Heybeck 63 H. Hillemanns 32 C. Hills 116 B. Hippolyte 126 B. Hofman 58 B. Hohlweger 84 J. Honermann 134 G.H. Hong 137 D. Horak 35 A. Horzyk 2 R. Hosokawa 14 Y. Hou 6 P. Hristov 32 C. Hughes 119 P. Huhn 63 L.M. Huhta 114 C.V. Hulse 72 T.J. Humanic 88 H. Hushnud 99 A. Hutson 113 D. Hutter 38 J.P. Iddon 116 R. Ilkaev 139 H. Ilyas 13 M. Inaba 122 G.M. Innocenti 32 M. Ippolitov 139 A. Isakov 86 T. Isidori 115 M.S. Islam 99 M. Ivanov 98 V. Ivanov 139 V. Izucheev 139 M. Jablonski 2 B. Jacak 74 N. Jacazio 32 P.M. Jacobs 74 S. Jadlovska 105 J. Jadlovsky 105 L. Jaffe 38 C. Jahnke 110 M.A. Janik 132 T. Janson 69 M. Jercic 89 O. Jevons 100 A.A.P. Jimenez 64 F. Jonas 87 134 P.G. Jones 100 J.M. Jowett 32 98 J. Jung 63 M. Jung 63 A. Junique 32 A. Jusko 100 M.J. Kabus 32 132 J. Kaewjai 104 P. Kalinak 59 A.S. Kalteyer 98 A. Kalweit 32 V. Kaplin 139 A. Karasu Uysal 71 D. Karatovic 89 O. Karavichev 139 T. Karavicheva 139 P. Karczmarczyk 132 E. Karpechev 139 V. Kashyap 80 A. Kazantsev 139 U. Kebschull 69 R. Keidel 138 D.L.D. Keijdener 58 M. Keil 32 B. Ketzer 42 A.M. Khan 6 S. Khan 15 A. Khanzadeev 139 Y. Kharlov 139 A. Khatun 15 A. Khuntia 106 B. Kileng 34 B. Kim 16 C. Kim 16 D.J. Kim 114 E.J. Kim 68 J. Kim 137 J.S. Kim 40 J. Kim 95 J. Kim 68 M. Kim 95 S. Kim 17 T. Kim 137 S. Kirsch 63 I. Kisel 38 S. Kiselev 139 A. Kisiel 132 J.P. Kitowski 2 J.L. Klay 5 J. Klein 32 S. Klein 74 C. Klein-Bösing 134 M. Kleiner 63 T. Klemenz 96 A. Kluge 32 A.G. Knospe 113 C. Kobdaj 104 T. Kollegger 98 A. Kondratyev 140 N. Kondratyeva 139 E. Kondratyuk 139 J. Konig 63 S.A. Konigstorfer 96 P.J. Konopka 32 G. Kornakov 132 S.D. Koryciak 2 A. Kotliarov 86 O. Kovalenko 79 V. Kovalenko 139 M. Kowalski 106 I. Králik 59 A. Kravčáková 37 L. Kreis 98 M. Krivda 100 59 F. Krizek 86 K. Krizkova Gajdosova 35 M. Kroesen 95 M. Krüger 63 D.M. Krupova 35 E. Kryshen 139 M. Krzewicki 38 V. Kučera 32 C. Kuhn 126 P.G. Kuijer 84 T. Kumaoka 122 D. Kumar 131 L. Kumar 90 N. Kumar 90 S. Kundu 32 P. Kurashvili 79 A. Kurepin 139 A.B. Kurepin 139 S. Kushpil 86 J. Kvapil 100 M.J. Kweon 57 J.Y. Kwon 57 Y. Kwon 137 S.L. La Pointe 38 P. La Rocca 26 Y.S. Lai 74 A. Lakrathok 104 M. Lamanna 32 R. Langoy 118 P. Larionov 48 E. Laudi 32 L. Lautner 32 96 R. Lavicka 102 T. Lazareva 139 R. Lea 130 54 J. Lehrbach 38 R.C. Lemmon 85 I. León Monzón 108 M.M. Lesch 96 E.D. Lesser 18 M. Lettrich 96 P. Lévai 135 X. Li 10 X.L. Li 6 J. Lien 118 R. Lietava 100 B. Lim 16 S.H. Lim 16 V. Lindenstruth 38 A. Lindner 45 C. Lippmann 98 A. Liu 18 D.H. Liu 6 J. Liu 116 I.M. Lofnes 20 V. Loginov 139 C. Loizides 87 P. Loncar 33 J.A. Lopez 95 X. Lopez 124 E. López Torres 7 P. Lu 98 117 J.R. Luhder 134 M. Lunardon 27 G. Luparello 56 Y.G. Ma 39 A. Maevskaya 139 M. Mager 32 T. Mahmoud 42 A. Maire 126 M. Malaev 139 N.M. Malik 91 Q.W. Malik 19 S.K. Malik 91 L. Malinina 140 VII D. Mal'Kevich 139 D. Mallick 80 N. Mallick 47 G. Mandaglio 30 52 V. Manko 139 F. Manso 124 V. Manzari 49 Y. Mao 6 G.V. Margagliotti 23 A. Margotti 50 A. Marín 98 C. Markert 107 M. Marquard 63 N.A. Martin 95 P. Martinengo 32 J.L. Martinez 113 M.I. Martínez 44 G. Martínez García 103 S. Masciocchi 98 M. Masera 24 A. Masoni 51 L. Massacrier 72 A. Mastroserio 128 49 A.M. Mathis 96 O. Matonoha 75 P.F.T. Matuoka 109 A. Matyja 106 C. Mayer 106 A.L. Mazuecos 32 F. Mazzaschi 24 M. Mazzilli 32 J.E. Mdhluli 120 A.F. Mechler 63 Y. Melikyan 139 A. Menchaca-Rocha 66 E. Meninno 102 28 A.S. Menon 113 M. Meres 12 S. Mhlanga 112 67 Y. Miake 122 L. Micheletti 55 L.C. Migliorin 125 D.L. Mihaylov 96 K. Mikhaylov 140 139 A.N. Mishra 135 D. Miśkowiec 98 A. Modak 4 A.P. Mohanty 58 B. Mohanty 80 M. Mohisin Khan 15 V M.A. Molander 43 Z. Moravcova 83 C. Mordasini 96 D.A. Moreira De Godoy 134 I. Morozov 139 A. Morsch 32 T. Mrnjavac 32 V. Muccifora 48 E. Mudnic 33 S. Muhuri 131 J.D. Mulligan 74 A. Mulliri 22 M.G. Munhoz 109 R.H. Munzer 63 H. Murakami 121 S. Murray 112 L. Musa 32 J. Musinsky 59 J.W. Myrcha 132 B. Naik 120 R. Nair 79 B.K. Nandi 46 R. Nania 50 E. Nappi 49 A.F. Nassirpour 75 A. Nath 95 C. Nattrass 119 A. Neagu 19 A. Negru 123 L. Nellen 64 S.V. Nesbo 34 G. Neskovic 38 D. Nesterov 139 B.S. Nielsen 83 E.G. Nielsen 83 S. Nikolaev 139 S. Nikulin 139 V. Nikulin 139 F. Noferini 50 S. Noh 11 P. Nomokonov 140 J. Norman 116 N. Novitzky 122 P. Nowakowski 132 A. Nyanin 139 J. Nystrand 20 M. Ogino 76 A. Ohlson 75 V.A. Okorokov 139 J. Oleniacz 132 A.C. Oliveira Da Silva 119 M.H. Oliver 136 A. Onnerstad 114 C. Oppedisano 55 A. Ortiz Velasquez 64 A. Oskarsson 75 J. Otwinowski 106 M. Oya 93 K. Oyama 76 Y. Pachmayer 95 S. Padhan 46 D. Pagano 130 54 G. Paić 64 A. Palasciano 49 S. Panebianco 127 J. Park 57 J.E. Parkkila 32 114 S.P. Pathak 113 R.N. Patra 91 B. Paul 22 H. Pei 6 T. Peitzmann 58 X. Peng 6 L.G. Pereira 65 H. Pereira Da Costa 127 D. Peresunko 139 G.M. Perez 7 S. Perrin 127 Y. Pestov 139 V. Petráček 35 V. Petrov 139 M. Petrovici 45 R.P. Pezzi 103 65 S. Piano 56 M. Pikna 12 P. Pillot 103 O. Pinazza 50 32 L. Pinsky 113 C. Pinto 96 26 S. Pisano 48 M. Płoskoń 74 M. Planinic 89 F. Pliquett 63 M.G. Poghosyan 87 S. Politano 29 N. Poljak 89 A. Pop 45 S. Porteboeuf-Houssais 124 J. Porter 74 V. Pozdniakov 140 S.K. Prasad 4 S. Prasad 47 R. Preghenella 50 F. Prino 55 C.A. Pruneau 133 I. Pshenichnov 139 M. Puccio 32 S. Qiu 84 L. Quaglia 24 R.E. Quishpe 113 S. Ragoni 100 A. Rakotozafindrabe 127 L. Ramello 129 55 F. Rami 126 S.A.R. Ramirez 44 T.A. Rancien 73 R. Raniwala 92 S. Raniwala 92 S.S. Räsänen 43 R. Rath 47 I. Ravasenga 84 K.F. Read 87 119 A.R. Redelbach 38 K. Redlich 79 VI A. Rehman 20 P. Reichelt 63 F. Reidt 32 H.A. Reme-Ness 34 Z. Rescakova 37 K. Reygers 95 A. Riabov 139 V. Riabov 139 R. Ricci 28 T. Richert 75 M. Richter 19 W. Riegler 32 F. Riggi 26 C. Ristea 62 M. Rodríguez Cahuantzi 44 K. Røed 19 R. Rogalev 139 E. Rogochaya 140 T.S. Rogoschinski 63 D. Rohr 32 D. Röhrich 20 P.F. Rojas 44 S. Rojas Torres 35 P.S. Rokita 132 F. Ronchetti 48 A. Rosano 30 52 E.D. Rosas 64 A. Rossi 53 A. Roy 47 P. Roy 99 S. Roy 46 N. Rubini 25 O.V. Rueda 75 D. Ruggiano 132 R. Rui 23 B. Rumyantsev 140 P.G. Russek 2 R. Russo 84 A. Rustamov 81 E. Ryabinkin 139 Y. Ryabov 139 A. Rybicki 106 H. Rytkonen 114 W. Rzesa 132 O.A.M. Saarimaki 43 R. Sadek 103 S. Sadovsky 139 J. Saetre 20 K. Šafařík 35 S.K. Saha 131 S. Saha 80 B. Sahoo 46 P. Sahoo 46 R. Sahoo 47 S. Sahoo 60 D. Sahu 47 P.K. Sahu 60 J. Saini 131 K. Sajdakova 37 S. Sakai 122 M.P. Salvan 98 S. Sambyal 91 T.B. Saramela 109 D. Sarkar 133 N. Sarkar 131 P. Sarma 41 V. Sarritzu 22 V.M. Sarti 96 M.H.P. Sas 136 J. Schambach 87 H.S. Scheid 63 C. Schiaua 45 R. Schicker 95 A. Schmah 95 C. Schmidt 98 H.R. Schmidt 94 M.O. Schmidt 32 M. Schmidt 94 N.V. Schmidt 87 63 A.R. Schmier 119 R. Schotter 126 J. Schukraft 32 K. Schwarz 98 K. Schweda 98 G. Scioli 25 E. Scomparin 55 J.E. Seger 14 Y. Sekiguchi 121 D. Sekihata 121 I. Selyuzhenkov 98 139 S. Senyukov 126 J.J. Seo 57 D. Serebryakov 139 L. Šerkšnytė 96 A. Sevcenco 62 T.J. Shaba 67 A. Shabanov 139 A. Shabetai 103 R. Shahoyan 32 W. Shaikh 99 A. Shangaraev 139 A. Sharma 90 D. Sharma 46 H. Sharma 106 M. Sharma 91 N. Sharma 90 S. Sharma 91 U. Sharma 91 A. Shatat 72 O. Sheibani 113 K. Shigaki 93 M. Shimomura 77 S. Shirinkin 139 Q. Shou 39 Y. Sibiriak 139 S. Siddhanta 51 T. Siemiarczuk 79 T.F. Silva 109 D. Silvermyr 75 T. Simantathammakul 104 R. Simeonov 36 G. Simonetti 32 B. Singh 91 B. Singh 96 R. Singh 80 R. Singh 91 R. Singh 47 V.K. Singh 131 V. Singhal 131 T. Sinha 99 B. Sitar 12 M. Sitta 129 55 T.B. Skaali 19 G. Skorodumovs 95 M. Slupecki 43 N. Smirnov 136 R.J.M. Snellings 58 E.H. Solheim 19 C. Soncco 101 J. Song 113 A. Songmoolnak 104 F. Soramel 27 S. Sorensen 119 R. Spijkers 84 I. Sputowska 106 J. Staa 75 J. Stachel 95 I. Stan 62 P.J. Steffanic 119 S.F. Stiefelmaier 95 D. Stocco 103 I. Storehaug 19 M.M. Storetvedt 34 P. Stratmann 134 S. Strazzi 25 C.P. Stylianidis 84 A.A.P. Suaide 109 C. Suire 72 M. Sukhanov 139 M. Suljic 32 V. Sumberia 91 S. Sumowidagdo 82 S. Swain 60 A. Szabo 12 I. Szarka 12 U. Tabassam 13 S.F. Taghavi 96 G. Taillepied 98 124 J. Takahashi 110 G.J. Tambave 20 S. Tang 124 6 Z. Tang 117 J.D. Tapia Takaki 115 N. Tapus 123 L.A. Tarasovicova 134 M.G. Tarzila 45 A. Tauro 32 A. Telesca 32 L. Terlizzi 24 C. Terrevoli 113 G. Tersimonov 3 S. Thakur 131 D. Thomas 107 R. Tieulent 125 A. Tikhonov 139 A.R. Timmins 113 M. Tkacik 105 T. Tkacik 105 A. Toia 63 N. Topilskaya 139 M. Toppi 48 F. Torales-Acosta 18 T. Tork 72 A.G. Torres Ramos 31 A. Trifiró 30 52 A.S. Triolo 30 52 S. Tripathy 50 T. Tripathy 46 S. Trogolo 32 V. Trubnikov 3 W.H. Trzaska 114 T.P. Trzcinski 132 R. Turrisi 53 T.S. Tveter 19 K. Ullaland 20 B. Ulukutlu 96 A. Uras 125 M. Urioni 54 130 G.L. Usai 22 M. Vala 37 N. Valle 21 S. Vallero 55 L.V.R. van Doremalen 58 M. van Leeuwen 84 C.A. van Veen 95 R.J.G. van Weelden 84 P. Vande Vyvre 32 D. Varga 135 Z. Varga 135 M. Varga-Kofarago 135 M. Vasileiou 78 A. Vasiliev 139 O. Vázquez Doce 96 V. Vechernin 139 E. Vercellin 24 S. Vergara Limón 44 L. Vermunt 58 R. Vértesi 135 M. Verweij 58 L. Vickovic 33 Z. Vilakazi 120 O. Villalobos Baillie 100 G. Vino 49 A. Vinogradov 139 T. Virgili 28 V. Vislavicius 83 A. Vodopyanov 140 B. Volkel 32 M.A. Völkl 95 K. Voloshin 139 S.A. Voloshin 133 G. Volpe 31 B. von Haller 32 I. Vorobyev 96 N. Vozniuk 139 J. Vrláková 37 B. Wagner 20 C. Wang 39 D. Wang 39 M. Weber 102 A. Wegrzynek 32 F.T. Weiglhofer 38 S.C. Wenzel 32 J.P. Wessels 134 S.L. Weyhmiller 136 J. Wiechula 63 J. Wikne 19 G. Wilk 79 J. Wilkinson 98 G.A. Willems 134 B. Windelband 95 M. Winn 127 J.R. Wright 107 W. Wu 39 Y. Wu 117 R. Xu 6 A.K. Yadav 131 S. Yalcin 71 Y. Yamaguchi 93 K. Yamakawa 93 S. Yang 20 S. Yano 93 Z. Yin 6 I.-K. Yoo 16 J.H. Yoon 57 S. Yuan 20 A. Yuncu 95 V. Zaccolo 23 C. Zampolli 32 H.J.C. Zanoli 58 F. Zanone 95 N. Zardoshti 32 100 A. Zarochentsev 139 P. Závada 61 N. Zaviyalov 139 M. Zhalov 139 B. Zhang 6 S. Zhang 39 X. Zhang 6 Y. Zhang 117 M. Zhao 10 V. Zherebchevskii 139 Y. Zhi 10 N. Zhigareva 139 D. Zhou 6 Y. Zhou 83 J. Zhu 98 6 Y. Zhu 6 G. Zinovjev 3 I N. Zurlo 130 54 1 A.I. Alikhanyan National Science Laboratory (Yerevan Physics Institute) Foundation, Yerevan, Armenia A.I. Alikhanyan National Science Laboratory (Yerevan Physics Institute) Foundation Yerevan Armenia A.I. Alikhanyan National Science Laboratory (Yerevan Physics Institute) Foundation, Yerevan, Armenia 2 AGH University of Science and Technology, Cracow, Poland AGH University of Science and Technology Cracow Poland AGH University of Science and Technology, Cracow, Poland 3 Bogolyubov Institute for Theoretical Physics, National Academy of Sciences of Ukraine, Kiev, Ukraine Bogolyubov Institute for Theoretical Physics National Academy of Sciences of Ukraine Kiev Ukraine Bogolyubov Institute for Theoretical Physics, National Academy of Sciences of Ukraine, Kiev, Ukraine 4 Bose Institute, Department of Physics and Centre for Astroparticle Physics and Space Science (CAPSS), Kolkata, India Bose Institute Department of Physics Centre for Astroparticle Physics and Space Science (CAPSS) Kolkata India Bose Institute, Department of Physics and Centre for Astroparticle Physics and Space Science (CAPSS), Kolkata, India 5 California Polytechnic State University, San Luis Obispo, CA, United States California Polytechnic State University San Luis Obispo CA United States California Polytechnic State University, San Luis Obispo, California, United States 6 Central China Normal University, Wuhan, China Central China Normal University Wuhan China Central China Normal University, Wuhan, China 7 Centro de Aplicaciones Tecnológicas y Desarrollo Nuclear (CEADEN), Havana, Cuba Centro de Aplicaciones Tecnológicas y Desarrollo Nuclear (CEADEN) Havana Cuba Centro de Aplicaciones Tecnológicas y Desarrollo Nuclear (CEADEN), Havana, Cuba 8 Centro de Investigación y de Estudios Avanzados (CINVESTAV), Mexico City and Mérida, Mexico Centro de Investigación y de Estudios Avanzados (CINVESTAV) Mexico City and Mérida Mexico Centro de Investigación y de Estudios Avanzados (CINVESTAV), Mexico City and Mérida, Mexico 9 Chicago State University, Chicago, IL, United States Chicago State University Chicago IL United States Chicago State University, Chicago, Illinois, United States 10 China Institute of Atomic Energy, Beijing, China China Institute of Atomic Energy Beijing China China Institute of Atomic Energy, Beijing, China 11 Chungbuk National University, Cheongju, Republic of Korea Chungbuk National University Cheongju Republic of Korea Chungbuk National University, Cheongju, Republic of Korea 12 Comenius University Bratislava, Faculty of Mathematics, Physics and Informatics, Bratislava, Slovak Republic Comenius University Bratislava Faculty of Mathematics, Physics and Informatics Bratislava Slovak Republic Comenius University Bratislava, Faculty of Mathematics, Physics and Informatics, Bratislava, Slovak Republic 13 COMSATS University Islamabad, Islamabad, Pakistan COMSATS University Islamabad Islamabad Pakistan COMSATS University Islamabad, Islamabad, Pakistan 14 Creighton University, Omaha, NE, United States Creighton University Omaha NE United States Creighton University, Omaha, Nebraska, United States 15 Department of Physics, Aligarh Muslim University, Aligarh, India Department of Physics Aligarh Muslim University Aligarh India Department of Physics, Aligarh Muslim University, Aligarh, India 16 Department of Physics, Pusan National University, Pusan, Republic of Korea Department of Physics Pusan National University Pusan Republic of Korea Department of Physics, Pusan National University, Pusan, Republic of Korea 17 Department of Physics, Sejong University, Seoul, Republic of Korea Department of Physics Sejong University Seoul Republic of Korea Department of Physics, Sejong University, Seoul, Republic of Korea 18 Department of Physics, University of California, Berkeley, CA, United States Department of Physics University of California Berkeley CA United States Department of Physics, University of California, Berkeley, California, United States 19 Department of Physics, University of Oslo, Oslo, Norway Department of Physics University of Oslo Oslo Norway Department of Physics, University of Oslo, Oslo, Norway 20 Department of Physics and Technology, University of Bergen, Bergen, Norway Department of Physics and Technology University of Bergen Bergen Norway Department of Physics and Technology, University of Bergen, Bergen, Norway 21 Dipartimento di Fisica, Università di Pavia, Pavia, Italy Dipartimento di Fisica Università di Pavia Pavia Italy Dipartimento di Fisica, Università di Pavia, Pavia, Italy 22 Dipartimento di Fisica dell'Università and Sezione INFN, Cagliari, Italy Dipartimento di Fisica dell'Università Sezione INFN Cagliari Italy Dipartimento di Fisica dell'Università and Sezione INFN, Cagliari, Italy 23 Dipartimento di Fisica dell'Università and Sezione INFN, Trieste, Italy Dipartimento di Fisica dell'Università Sezione INFN Trieste Italy Dipartimento di Fisica dell'Università and Sezione INFN, Trieste, Italy 24 Dipartimento di Fisica dell'Università and Sezione INFN, Turin, Italy Dipartimento di Fisica dell'Università Sezione INFN Turin Italy Dipartimento di Fisica dell'Università and Sezione INFN, Turin, Italy 25 Dipartimento di Fisica e Astronomia dell'Università and Sezione INFN, Bologna, Italy Dipartimento di Fisica e Astronomia dell'Università Sezione INFN Bologna Italy Dipartimento di Fisica e Astronomia dell'Università and Sezione INFN, Bologna, Italy 26 Dipartimento di Fisica e Astronomia dell'Università and Sezione INFN, Catania, Italy Dipartimento di Fisica e Astronomia dell'Università Sezione INFN Catania Italy Dipartimento di Fisica e Astronomia dell'Università and Sezione INFN, Catania, Italy 27 Dipartimento di Fisica e Astronomia dell'Università and Sezione INFN, Padova, Italy Dipartimento di Fisica e Astronomia dell'Università Sezione INFN Padova Italy Dipartimento di Fisica e Astronomia dell'Università and Sezione INFN, Padova, Italy 28 Dipartimento di Fisica ‘E.R. Caianiello’ dell'Università and Gruppo Collegato INFN, Salerno, Italy Dipartimento di Fisica ‘E.R. Caianiello’ dell'Università Gruppo Collegato INFN Salerno Italy Dipartimento di Fisica ‘E.R. Caianiello’ dell'Università and Gruppo Collegato INFN, Salerno, Italy 29 Dipartimento DISAT del Politecnico and Sezione INFN, Turin, Italy Dipartimento DISAT del Politecnico Sezione INFN Turin Italy Dipartimento DISAT del Politecnico and Sezione INFN, Turin, Italy 30 Dipartimento di Scienze MIFT, Università di Messina, Messina, Italy Dipartimento di Scienze MIFT Università di Messina Messina Italy Dipartimento di Scienze MIFT, Università di Messina, Messina, Italy 31 Dipartimento Interateneo di Fisica ‘M. Merlin’ and Sezione INFN, Bari, Italy Dipartimento Interateneo di Fisica ‘M. Merlin’ Sezione INFN Bari Italy Dipartimento Interateneo di Fisica ‘M. Merlin’ and Sezione INFN, Bari, Italy 32 European Organization for Nuclear Research (CERN), Geneva, Switzerland European Organization for Nuclear Research (CERN) Geneva Switzerland European Organization for Nuclear Research (CERN), Geneva, Switzerland 33 Faculty of Electrical Engineering, Mechanical Engineering and Naval Architecture, University of Split, Split, Croatia Faculty of Electrical Engineering, Mechanical Engineering and Naval Architecture University of Split Split Croatia Faculty of Electrical Engineering, Mechanical Engineering and Naval Architecture, University of Split, Split, Croatia 34 Faculty of Engineering and Science, Western Norway University of Applied Sciences, Bergen, Norway Faculty of Engineering and Science Western Norway University of Applied Sciences Bergen Norway Faculty of Engineering and Science, Western Norway University of Applied Sciences, Bergen, Norway 35 Faculty of Nuclear Sciences and Physical Engineering, Czech Technical University in Prague, Prague, Czech Republic Faculty of Nuclear Sciences and Physical Engineering Czech Technical University in Prague Prague Czech Republic Faculty of Nuclear Sciences and Physical Engineering, Czech Technical University in Prague, Prague, Czech Republic 36 Faculty of Physics, Sofia University, Sofia, Bulgaria Faculty of Physics Sofia University Sofia Bulgaria Faculty of Physics, Sofia University, Sofia, Bulgaria 37 Faculty of Science, P.J. Šafárik University, Košice, Slovak Republic Faculty of Science P.J. Šafárik University Košice Slovak Republic Faculty of Science, P.J. Šafárik University, Košice, Slovak Republic 38 Frankfurt Institute for Advanced Studies, Johann Wolfgang Goethe-Universität Frankfurt, Frankfurt, Germany Frankfurt Institute for Advanced Studies Johann Wolfgang Goethe-Universität Frankfurt Frankfurt Germany Frankfurt Institute for Advanced Studies, Johann Wolfgang Goethe-Universität Frankfurt, Frankfurt, Germany 39 Fudan University, Shanghai, China Fudan University Shanghai China Fudan University, Shanghai, China 40 Gangneung-Wonju National University, Gangneung, Republic of Korea Gangneung-Wonju National University Gangneung Republic of Korea Gangneung-Wonju National University, Gangneung, Republic of Korea 41 Gauhati University, Department of Physics, Guwahati, India Gauhati University Department of Physics Guwahati India Gauhati University, Department of Physics, Guwahati, India 42 Helmholtz-Institut für Strahlen- und Kernphysik, Rheinische Friedrich-Wilhelms-Universität Bonn, Bonn, Germany Helmholtz-Institut für Strahlen- und Kernphysik Rheinische Friedrich-Wilhelms-Universität Bonn Bonn Germany Helmholtz-Institut für Strahlen- und Kernphysik, Rheinische Friedrich-Wilhelms-Universität Bonn, Bonn, Germany 43 Helsinki Institute of Physics (HIP), Helsinki, Finland Helsinki Institute of Physics (HIP) Helsinki Finland Helsinki Institute of Physics (HIP), Helsinki, Finland 44 High Energy Physics Group, Universidad Autónoma de Puebla, Puebla, Mexico High Energy Physics Group Universidad Autónoma de Puebla Puebla Mexico High Energy Physics Group, Universidad Autónoma de Puebla, Puebla, Mexico 45 Horia Hulubei National Institute of Physics and Nuclear Engineering, Bucharest, Romania Horia Hulubei National Institute of Physics and Nuclear Engineering Bucharest Romania Horia Hulubei National Institute of Physics and Nuclear Engineering, Bucharest, Romania 46 Indian Institute of Technology Bombay (IIT), Mumbai, India Indian Institute of Technology Bombay (IIT) Mumbai India Indian Institute of Technology Bombay (IIT), Mumbai, India 47 Indian Institute of Technology Indore, Indore, India Indian Institute of Technology Indore Indore India Indian Institute of Technology Indore, Indore, India 48 INFN, Laboratori Nazionali di Frascati, Frascati, Italy INFN, Laboratori Nazionali di Frascati Frascati Italy INFN, Laboratori Nazionali di Frascati, Frascati, Italy 49 INFN, Sezione di Bari, Bari, Italy INFN, Sezione di Bari Bari Italy INFN, Sezione di Bari, Bari, Italy 50 INFN, Sezione di Bologna, Bologna, Italy INFN, Sezione di Bologna Bologna Italy INFN, Sezione di Bologna, Bologna, Italy 51 INFN, Sezione di Cagliari, Cagliari, Italy INFN, Sezione di Cagliari Cagliari Italy INFN, Sezione di Cagliari, Cagliari, Italy 52 INFN, Sezione di Catania, Catania, Italy INFN, Sezione di Catania Catania Italy INFN, Sezione di Catania, Catania, Italy 53 INFN, Sezione di Padova, Padova, Italy INFN, Sezione di Padova Padova Italy INFN, Sezione di Padova, Padova, Italy 54 INFN, Sezione di Pavia, Pavia, Italy INFN, Sezione di Pavia Pavia Italy INFN, Sezione di Pavia, Pavia, Italy 55 INFN, Sezione di Torino, Turin, Italy INFN, Sezione di Torino Turin Italy INFN, Sezione di Torino, Turin, Italy 56 INFN, Sezione di Trieste, Trieste, Italy INFN, Sezione di Trieste Trieste Italy INFN, Sezione di Trieste, Trieste, Italy 57 Inha University, Incheon, Republic of Korea Inha University Incheon Republic of Korea Inha University, Incheon, Republic of Korea 58 Institute for Gravitational and Subatomic Physics (GRASP), Utrecht University/Nikhef, Utrecht, Netherlands Institute for Gravitational and Subatomic Physics (GRASP) Utrecht University/Nikhef Utrecht Netherlands Institute for Gravitational and Subatomic Physics (GRASP), Utrecht University/Nikhef, Utrecht, Netherlands 59 Institute of Experimental Physics, Slovak Academy of Sciences, Košice, Slovak Republic Institute of Experimental Physics Slovak Academy of Sciences Košice Slovak Republic Institute of Experimental Physics, Slovak Academy of Sciences, Košice, Slovak Republic 60 Institute of Physics, Homi Bhabha National Institute, Bhubaneswar, India Institute of Physics Homi Bhabha National Institute Bhubaneswar India Institute of Physics, Homi Bhabha National Institute, Bhubaneswar, India 61 Institute of Physics of the Czech Academy of Sciences, Prague, Czech Republic Institute of Physics of the Czech Academy of Sciences Prague Czech Republic Institute of Physics of the Czech Academy of Sciences, Prague, Czech Republic 62 Institute of Space Science (ISS), Bucharest, Romania Institute of Space Science (ISS) Bucharest Romania Institute of Space Science (ISS), Bucharest, Romania 63 Institut für Kernphysik, Johann Wolfgang Goethe-Universität Frankfurt, Frankfurt, Germany Institut für Kernphysik Johann Wolfgang Goethe-Universität Frankfurt Frankfurt Germany Institut für Kernphysik, Johann Wolfgang Goethe-Universität Frankfurt, Frankfurt, Germany 64 Instituto de Ciencias Nucleares, Universidad Nacional Autónoma de México, Mexico City, Mexico Instituto de Ciencias Nucleares Universidad Nacional Autónoma de México Mexico City Mexico Instituto de Ciencias Nucleares, Universidad Nacional Autónoma de México, Mexico City, Mexico 65 Instituto de Física, Universidade Federal do Rio Grande do Sul (UFRGS), Porto Alegre, Brazil Instituto de Física Universidade Federal do Rio Grande do Sul (UFRGS) Porto Alegre Brazil Instituto de Física, Universidade Federal do Rio Grande do Sul (UFRGS), Porto Alegre, Brazil 66 Instituto de Física, Universidad Nacional Autónoma de México, Mexico City, Mexico Instituto de Física Universidad Nacional Autónoma de México Mexico City Mexico Instituto de Física, Universidad Nacional Autónoma de México, Mexico City, Mexico 67 iThemba LABS, National Research Foundation, Somerset West, South Africa iThemba LABS National Research Foundation Somerset West South Africa iThemba LABS, National Research Foundation, Somerset West, South Africa 68 Jeonbuk National University, Jeonju, Republic of Korea Jeonbuk National University Jeonju Republic of Korea Jeonbuk National University, Jeonju, Republic of Korea 69 Johann-Wolfgang-Goethe Universität Frankfurt Institut für Informatik, Fachbereich Informatik und Mathematik, Frankfurt, Germany Johann-Wolfgang-Goethe Universität Frankfurt Institut für Informatik Fachbereich Informatik und Mathematik Frankfurt Germany Johann-Wolfgang-Goethe Universität Frankfurt Institut für Informatik, Fachbereich Informatik und Mathematik, Frankfurt, Germany 70 Korea Institute of Science and Technology Information, Daejeon, Republic of Korea Korea Institute of Science and Technology Information Daejeon Republic of Korea Korea Institute of Science and Technology Information, Daejeon, Republic of Korea 71 KTO Karatay University, Konya, Turkey KTO Karatay University Konya Turkey KTO Karatay University, Konya, Turkey 72 Laboratoire de Physique des 2 Infinis, Irène Joliot-Curie, Orsay, France Laboratoire de Physique des 2 Infinis, Irène Joliot-Curie Orsay France Laboratoire de Physique des 2 Infinis, Irène Joliot-Curie, Orsay, France 73 Laboratoire de Physique Subatomique et de Cosmologie, Université Grenoble-Alpes, CNRS-IN2P3, Grenoble, France Laboratoire de Physique Subatomique et de Cosmologie Université Grenoble-Alpes CNRS-IN2P3 Grenoble France Laboratoire de Physique Subatomique et de Cosmologie, Université Grenoble-Alpes, CNRS-IN2P3, Grenoble, France 74 Lawrence Berkeley National Laboratory, Berkeley, CA, United States Lawrence Berkeley National Laboratory Berkeley CA United States Lawrence Berkeley National Laboratory, Berkeley, California, United States 75 Lund University Department of Physics, Division of Particle Physics, Lund, Sweden Lund University Department of Physics Division of Particle Physics Lund Sweden Lund University Department of Physics, Division of Particle Physics, Lund, Sweden 76 Nagasaki Institute of Applied Science, Nagasaki, Japan Nagasaki Institute of Applied Science Nagasaki Japan Nagasaki Institute of Applied Science, Nagasaki, Japan 77 Nara Women's University (NWU), Nara, Japan Nara Women's University (NWU) Nara Japan Nara Women's University (NWU), Nara, Japan 78 National and Kapodistrian University of Athens, School of Science, Department of Physics , Athens, Greece National and Kapodistrian University of Athens School of Science Department of Physics Athens Greece National and Kapodistrian University of Athens, School of Science, Department of Physics , Athens, Greece 79 National Centre for Nuclear Research, Warsaw, Poland National Centre for Nuclear Research Warsaw Poland National Centre for Nuclear Research, Warsaw, Poland 80 National Institute of Science Education and Research, Homi Bhabha National Institute, Jatni, India National Institute of Science Education and Research Homi Bhabha National Institute Jatni India National Institute of Science Education and Research, Homi Bhabha National Institute, Jatni, India 81 National Nuclear Research Center, Baku, Azerbaijan National Nuclear Research Center Baku Azerbaijan National Nuclear Research Center, Baku, Azerbaijan 82 National Research and Innovation Agency - BRIN, Jakarta, Indonesia National Research and Innovation Agency - BRIN Jakarta Indonesia National Research and Innovation Agency - BRIN, Jakarta, Indonesia 83 Niels Bohr Institute, University of Copenhagen, Copenhagen, Denmark Niels Bohr Institute University of Copenhagen Copenhagen Denmark Niels Bohr Institute, University of Copenhagen, Copenhagen, Denmark 84 Nikhef, National institute for subatomic physics, Amsterdam, Netherlands Nikhef, National institute for subatomic physics Amsterdam Netherlands Nikhef, National institute for subatomic physics, Amsterdam, Netherlands 85 Nuclear Physics Group, STFC Daresbury Laboratory, Daresbury, United Kingdom Nuclear Physics Group STFC Daresbury Laboratory Daresbury United Kingdom Nuclear Physics Group, STFC Daresbury Laboratory, Daresbury, United Kingdom 86 Nuclear Physics Institute of the Czech Academy of Sciences, Husinec-Řež, Czech Republic Nuclear Physics Institute of the Czech Academy of Sciences Husinec-Řež Czech Republic Nuclear Physics Institute of the Czech Academy of Sciences, Husinec-Řež, Czech Republic 87 Oak Ridge National Laboratory, Oak Ridge, TN, United States Oak Ridge National Laboratory Oak Ridge TN United States Oak Ridge National Laboratory, Oak Ridge, Tennessee, United States 88 Ohio State University, Columbus, OH, United States Ohio State University Columbus OH United States Ohio State University, Columbus, Ohio, United States 89 Physics department, Faculty of science, University of Zagreb, Zagreb, Croatia Physics department Faculty of science, University of Zagreb Zagreb Croatia Physics department, Faculty of science, University of Zagreb, Zagreb, Croatia 90 Physics Department, Panjab University, Chandigarh, India Physics Department Panjab University Chandigarh India Physics Department, Panjab University, Chandigarh, India 91 Physics Department, University of Jammu, Jammu, India Physics Department University of Jammu Jammu India Physics Department, University of Jammu, Jammu, India 92 Physics Department, University of Rajasthan, Jaipur, India Physics Department University of Rajasthan Jaipur India Physics Department, University of Rajasthan, Jaipur, India 93 Physics Program and International Institute for Sustainability with Knotted Chiral Meta Matter (SKCM2), Hiroshima University, Hiroshima, Japan Physics Program and International Institute for Sustainability with Knotted Chiral Meta Matter (SKCM2) Hiroshima University Hiroshima Japan Physics Program and International Institute for Sustainability with Knotted Chiral Meta Matter (SKCM2), Hiroshima University, Hiroshima, Japan 94 Physikalisches Institut, Eberhard-Karls-Universität Tübingen, Tübingen, Germany Physikalisches Institut Eberhard-Karls-Universität Tübingen Tübingen Germany Physikalisches Institut, Eberhard-Karls-Universität Tübingen, Tübingen, Germany 95 Physikalisches Institut, Ruprecht-Karls-Universität Heidelberg, Heidelberg, Germany Physikalisches Institut Ruprecht-Karls-Universität Heidelberg Heidelberg Germany Physikalisches Institut, Ruprecht-Karls-Universität Heidelberg, Heidelberg, Germany 96 Physik Department, Technische Universität München, Munich, Germany Physik Department Technische Universität München Munich Germany Physik Department, Technische Universität München, Munich, Germany 97 Politecnico di Bari and Sezione INFN, Bari, Italy Politecnico di Bari Sezione INFN Bari Italy Politecnico di Bari and Sezione INFN, Bari, Italy 98 Research Division and ExtreMe Matter Institute EMMI, GSI Helmholtzzentrum für Schwerionenforschung GmbH, Darmstadt, Germany Research Division ExtreMe Matter Institute EMMI GSI Helmholtzzentrum für Schwerionenforschung GmbH Darmstadt Germany Research Division and ExtreMe Matter Institute EMMI, GSI Helmholtzzentrum für Schwerionenforschung GmbH, Darmstadt, Germany 99 Saha Institute of Nuclear Physics, Homi Bhabha National Institute, Kolkata, India Saha Institute of Nuclear Physics Homi Bhabha National Institute Kolkata India Saha Institute of Nuclear Physics, Homi Bhabha National Institute, Kolkata, India 100 School of Physics and Astronomy, University of Birmingham, Birmingham, United Kingdom School of Physics and Astronomy University of Birmingham Birmingham United Kingdom School of Physics and Astronomy, University of Birmingham, Birmingham, United Kingdom 101 Sección Física, Departamento de Ciencias, Pontificia Universidad Católica del Perú, Lima, Peru Sección Física Departamento de Ciencias Pontificia Universidad Católica del Perú Lima Peru Sección Física, Departamento de Ciencias, Pontificia Universidad Católica del Perú, Lima, Peru 102 Stefan Meyer Institut für Subatomare Physik (SMI), Vienna, Austria Stefan Meyer Institut für Subatomare Physik (SMI) Vienna Austria Stefan Meyer Institut für Subatomare Physik (SMI), Vienna, Austria 103 SUBATECH, IMT Atlantique, Nantes Université, CNRS-IN2P3, Nantes, France SUBATECH IMT Atlantique Nantes Université CNRS-IN2P3 Nantes France SUBATECH, IMT Atlantique, Nantes Université, CNRS-IN2P3, Nantes, France 104 Suranaree University of Technology, Nakhon Ratchasima, Thailand Suranaree University of Technology Nakhon Ratchasima Thailand Suranaree University of Technology, Nakhon Ratchasima, Thailand 105 Technical University of Košice, Košice, Slovak Republic Technical University of Košice Košice Slovak Republic Technical University of Košice, Košice, Slovak Republic 106 The Henryk Niewodniczanski Institute of Nuclear Physics, Polish Academy of Sciences, Cracow, Poland The Henryk Niewodniczanski Institute of Nuclear Physics Polish Academy of Sciences Cracow Poland The Henryk Niewodniczanski Institute of Nuclear Physics, Polish Academy of Sciences, Cracow, Poland 107 The University of Texas at Austin, Austin, TX, United States The University of Texas at Austin Austin TX United States The University of Texas at Austin, Austin, Texas, United States 108 Universidad Autónoma de Sinaloa, Culiacán, Mexico Universidad Autónoma de Sinaloa Culiacán Mexico Universidad Autónoma de Sinaloa, Culiacán, Mexico 109 Universidade de São Paulo (USP), São Paulo, Brazil Universidade de São Paulo (USP) São Paulo Brazil Universidade de São Paulo (USP), São Paulo, Brazil 110 Universidade Estadual de Campinas (UNICAMP), Campinas, Brazil Universidade Estadual de Campinas (UNICAMP) Campinas Brazil Universidade Estadual de Campinas (UNICAMP), Campinas, Brazil 111 Universidade Federal do ABC, Santo Andre, Brazil Universidade Federal do ABC Santo Andre Brazil Universidade Federal do ABC, Santo Andre, Brazil 112 University of Cape Town, Cape Town, South Africa University of Cape Town Cape Town South Africa University of Cape Town, Cape Town, South Africa 113 University of Houston, Houston, TX, United States University of Houston Houston TX United States University of Houston, Houston, Texas, United States 114 University of Jyväskylä, Jyväskylä, Finland University of Jyväskylä Jyväskylä Finland University of Jyväskylä, Jyväskylä, Finland 115 University of Kansas, Lawrence, KS, United States University of Kansas Lawrence KS United States University of Kansas, Lawrence, Kansas, United States 116 University of Liverpool, Liverpool, United Kingdom University of Liverpool Liverpool United Kingdom University of Liverpool, Liverpool, United Kingdom 117 University of Science and Technology of China, Hefei, China University of Science and Technology of China Hefei China University of Science and Technology of China, Hefei, China 118 University of South-Eastern Norway, Kongsberg, Norway University of South-Eastern Norway Kongsberg Norway University of South-Eastern Norway, Kongsberg, Norway 119 University of Tennessee, Knoxville, TN, United States University of Tennessee Knoxville TN United States University of Tennessee, Knoxville, Tennessee, United States 120 University of the Witwatersrand, Johannesburg, South Africa University of the Witwatersrand Johannesburg South Africa University of the Witwatersrand, Johannesburg, South Africa 121 University of Tokyo, Tokyo, Japan University of Tokyo Tokyo Japan University of Tokyo, Tokyo, Japan 122 University of Tsukuba, Tsukuba, Japan University of Tsukuba Tsukuba Japan University of Tsukuba, Tsukuba, Japan 123 University Politehnica of Bucharest, Bucharest, Romania University Politehnica of Bucharest Bucharest Romania University Politehnica of Bucharest, Bucharest, Romania 124 Université Clermont Auvergne, CNRS/IN2P3, LPC, Clermont-Ferrand, France Université Clermont Auvergne CNRS/IN2P3 LPC Clermont-Ferrand France Université Clermont Auvergne, CNRS/IN2P3, LPC, Clermont-Ferrand, France 125 Université de Lyon, CNRS/IN2P3, Institut de Physique des 2 Infinis de Lyon, Lyon, France Université de Lyon CNRS/IN2P3 Institut de Physique des 2 Infinis de Lyon Lyon France Université de Lyon, CNRS/IN2P3, Institut de Physique des 2 Infinis de Lyon, Lyon, France 126 Université de Strasbourg, CNRS, IPHC UMR 7178, F-67000 Strasbourg, France Université de Strasbourg CNRS IPHC UMR 7178 Strasbourg F-67000 France Université de Strasbourg, CNRS, IPHC UMR 7178, F-67000 Strasbourg, France, Strasbourg, France 127 Université Paris-Saclay Centre d'Etudes de Saclay (CEA), IRFU, Départment de Physique Nucléaire (DPhN), Saclay, France Université Paris-Saclay Centre d'Etudes de Saclay (CEA) IRFU Départment de Physique Nucléaire (DPhN) Saclay France Université Paris-Saclay Centre d'Etudes de Saclay (CEA), IRFU, Départment de Physique Nucléaire (DPhN), Saclay, France 128 Università degli Studi di Foggia, Foggia, Italy Università degli Studi di Foggia Foggia Italy Università degli Studi di Foggia, Foggia, Italy 129 Università del Piemonte Orientale, Vercelli, Italy Università del Piemonte Orientale Vercelli Italy Università del Piemonte Orientale, Vercelli, Italy 130 Università di Brescia, Brescia, Italy Università di Brescia Brescia Italy Università di Brescia, Brescia, Italy 131 Variable Energy Cyclotron Centre, Homi Bhabha National Institute, Kolkata, India Variable Energy Cyclotron Centre Homi Bhabha National Institute Kolkata India Variable Energy Cyclotron Centre, Homi Bhabha National Institute, Kolkata, India 132 Warsaw University of Technology, Warsaw, Poland Warsaw University of Technology Warsaw Poland Warsaw University of Technology, Warsaw, Poland 133 Wayne State University, Detroit, MI, United States Wayne State University Detroit MI United States Wayne State University, Detroit, Michigan, United States 134 Westfälische Wilhelms-Universität Münster, Institut für Kernphysik, Münster, Germany Westfälische Wilhelms-Universität Münster Institut für Kernphysik Münster Germany Westfälische Wilhelms-Universität Münster, Institut für Kernphysik, Münster, Germany 135 Wigner Research Centre for Physics, Budapest, Hungary Wigner Research Centre for Physics Budapest Hungary Wigner Research Centre for Physics, Budapest, Hungary 136 Yale University, New Haven, CT, United States Yale University New Haven CT United States Yale University, New Haven, Connecticut, United States 137 Yonsei University, Seoul, Republic of Korea Yonsei University Seoul Republic of Korea Yonsei University, Seoul, Republic of Korea 138 Zentrum für Technologie und Transfer (ZTT), Worms, Germany Zentrum für Technologie und Transfer (ZTT) Worms Germany Zentrum für Technologie und Transfer (ZTT), Worms, Germany 139 Affiliated with an institute covered by a cooperation agreement with CERN Affiliated with an institute covered by a cooperation agreement with CERN Affiliated with an institute covered by a cooperation agreement with CERN 140 Affiliated with an international laboratory covered by a cooperation agreement with CERN Affiliated with an international laboratory covered by a cooperation agreement with CERN Affiliated with an international laboratory covered by a cooperation agreement with CERN I Deceased. II Also at: Max-Planck-Institut für Physik, Munich, Germany. III Also at: Italian National Agency for New Technologies, Energy and Sustainable Economic Development (ENEA), Bologna, Italy. IV Also at: Dipartimento DET del Politecnico di Torino, Turin, Italy. V Also at: Department of Applied Physics, Aligarh Muslim University, Aligarh, India. VI Also at: Institute of Theoretical Physics, University of Wroclaw, Poland. VII Also at: An institution covered by a cooperation agreement with CERN. ⋆ E-mail address: alice-publications@cern.ch .Editor: M. Pierini Abstract This letter reports measurements which characterize the underlying event associated with hard scatterings at mid-pseudorapidity (| η | < 0.8 ) in pp, p–Pb and Pb–Pb collisions at centre-of-mass energy per nucleon pair, s NN = 5.02 TeV. The measurements are performed with ALICE at the LHC. Different multiplicity classes are defined based on the event activity measured at forward rapidities. The hard scatterings are identified by the leading particle defined as the charged particle with the largest transverse momentum (p T ) in the collision and having 8 < p T < 15 GeV/c . The p T spectra of associated particles (0.5 ≤ p T < 6 GeV/c ) are measured in different azimuthal regions defined with respect to the leading particle direction: toward, transverse, and away. The associated charged particle yields in the transverse region are subtracted from those of the away and toward regions. The remaining jet-like yields are reported as a function of the multiplicity measured in the transverse region. The measurements show a suppression of the jet-like yield in the away region and an enhancement of high-p T associated particles in the toward region in central Pb–Pb collisions, as compared to minimum-bias pp collisions. These observations are consistent with previous measurements that used two-particle correlations, and with an interpretation in terms of parton energy loss in a high-density quark gluon plasma. These yield modifications vanish in peripheral Pb–Pb collisions and are not observed in either high-multiplicity pp or p–Pb collisions. Data availability This manuscript has associated data in a HEPData repository at https://www.hepdata.net/ . 1 Introduction In proton-proton (pp) collisions, jets, originating from partonic scatterings with large momentum transfer, are accompanied by particles produced by initial- and final-state radiation (ISR and FSR, respectively), as well as, by a plethora of other mechanisms. These include proton break-up, and, in a scenario incorporating multi-parton interactions (MPI) [1,2] , several semi-hard parton-parton scatterings in a single pp collision. These jet-accompanying particles experimentally make up the underlying event (UE) and are commonly studied via azimuthal separations from the jets to minimise the influence of hard scatterings. The present study follows the strategy originally introduced by the CDF collaboration [3] . First, the leading charged particle in the event is found, i.e., the charged particle with the highest transverse momentum in the collision (p T trig ). Secondly, the associated particles (p T < p T trig ) are measured in three topological regions depending on their azimuthal angle relative to the leading particle, | Δ φ | = | φ assoc − φ trig | , see Fig. 1 . The toward region contains the primary jet within the acceptance of the detector, while the away region contains the back-scattered particles of the recoil jet [4] . In contrast, the transverse region is dominated by the underlying-event dynamics, but it also includes contributions from ISR and FSR [5] . The measurements performed at RHIC and LHC in small systems (pp, p–A, and d–A collisions) have shown for high particle multiplicities similar phenomena as were originally observed only in A–A collisions and have been attributed there to the formation of the strongly interacting quark gluon plasma [6,7] , namely, long range angular correlations and collectivity [8] . The origin of these effects in small systems is still an open question; on one hand, hydrodynamical calculations describe some aspects of the data [9] ; on the other hand, mechanisms like colour reconnection [10] , rope hadronisation [11] , and string shoving [12] can produce collective-like effects in Monte Carlo event generators such as PYTHIA 8 [13] . Thus, investigating pp collisions as a function of the charged particle multiplicity has become ever more pertinent [9,14–18] . The interpretation of the results from the analysis of high-multiplicity pp collisions is challenging due to the selection biases of the sample towards events in which partonic scatterings with large momentum transfer (hard scatterings) occurred. To mitigate this inherent bias, Martin et al. [19] suggested to use the charged-particle multiplicity in the transverse region (N ch T ) as a classifier of the activity in the collisions, since the correlation between N ch T and the hardest scattering in the collision is small. The ALICE collaboration has reported the first N ch T spectra measured in pp collisions at centre-of-mass energy, s = 13 TeV [20] . Event generators, such as PYTHIA 8 [13] and EPOS-LHC [21] , do not provide a good description of the measured distribution of the ratio N ch T /〈 N ch T 〉 , where 〈 N ch T 〉 is the event-averaged charged-particle multiplicity in the transverse region, underestimating in particular the number of collisions with large N ch T ( > 3 × 〈 N ch T 〉 ) . In the framework of MPI-based models, like those implemented in PYTHIA 8 and HERWIG 7 [22] , the probability for a hard scattering in the collision increases with decreasing impact parameterVIII VIII In event generators like PYTHIA 8 the impact parameter profile is described by an overlap matter distribution of the two incoming hadrons. between the colliding protons. Thus, requiring a high-p T particle (e.g., p T trig > 8 GeV/c ) in a given pp collision biases the selection of collisions towards those with a smaller impact parameter [23] , which in turn biases the selection towards pp collisions with more MPI [20] . This feature of the N ch T -based analysis is important for the isolation of potential MPI and colour reconnection effects, which according to PYTHIA 8, produce effects resembling collective behaviour [10] . By construction, MPI and colour reconnection effects are expected to be more relevant in the transverse region than in the away and toward regions [24] . It is worth mentioning that the MPI picture has been used to explain the p T spectra in p–Pb collisions and peripheral Pb–Pb collisions [25–27] . Studies, as a function of N ch T , are therefore important to the understanding of the effects observed in high-multiplicity pp collisions. Last but not least, measurements of UE observables are also important to tune event generators [28] that include hard partonic scatterings and MPI. This letter reports the inclusive charged-particle transverse momentum spectra in pp, p–Pb and Pb–Pb collisions at centre-of-mass energy per nucleon pair s NN = 5.02 TeV containing a high-p T leading particle within the kinematic intervals 8 ≤ p T trig < 15 GeV/c and | η | < 0.8 . This guarantees the selection of collisions in which the average activity in the transverse region is roughly flat as a function of p T trig [20] , and therefore, any additional selection on the charged particle multiplicity will only modulate the UE activity. The measurements are performed considering different event classes defined in terms of the multiplicity registered in the forward detectors. The p T spectra of associated charged particles (0.5 ≤ p T < 6 GeV/c and | η | < 0.8 ) are measured in the toward, away, and transverse regions as a function of the average charged particle multiplicity in the transverse region. To further investigate the possible modification of the particles produced in the hard scattering in pp, p–Pb, and Pb–Pb collisions, the p T distributions in the toward (d N ch t / d p T ) and away (d N ch a / d p T ) regions obtained after the subtraction of the p T spectra in the transverse region (d N ch T / d p T ) are also reported. The subtracted yields (d N ch st , sa / d p T ) are further normalised to those measured in minimum-bias (MB) pp collisions,(1) I X t , a ≡ ( d N ch t , a / d p T − d N ch T / d p T ) | X ( d N ch t , a / d p T − d N ch T / d p T ) | pp , MB = ( d N ch st , sa / d p T ) | X ( d N ch st , sa / d p T ) | pp , MB , where X indicates the collision system and the event multiplicity class. In this way, the hard process p T spectra in the toward and away regions are isolated, and thus allowing us to study possible modifications to the produced particles due to medium effects in high-multiplicity pp, p–Pb, and Pb–Pb collisions. In heavy-ion collisions, this ratio is sensitive to the same effects which were studied using the I AA quantity [29–31] , where jets produced in the early stage of the collision propagate through the hot and dense quark–gluon plasma. Their interaction with the coloured medium lead to parton-energy loss (jet quenching) [32] which, for example, results in the suppression of the charged-particle yield at high p T [33] , and the suppression of the high-p T yield in the away region [29,30] . It is worth mentioning that jet quenching effects have not been observed so far in small systems [33,34] . 2 Experiment and data analysis This analysis is based on the data recorded by the ALICE apparatus during the pp and Pb–Pb runs at s NN = 5.02 TeV in 2015, and the p–Pb run at s NN = 5.02 TeV in 2016. The present study uses the V0 detector, and the Silicon Pixel Detector (SPD) for triggering and background rejection. The V0 consists of two arrays of scintillating tiles placed on each side of the interaction point covering the full azimuthal acceptance and the pseudorapidity intervals of 2.8 < η < 5.1 (V0A) and − 3.7 < η < − 1.7 (V0C). The SPD is the innermost part of the Inner Tracking System (ITS) and it is the closest detector to the interaction point. It consists of two cylindrical silicon pixel layers at radial distances of 3.9 and 7.6 cm from the beam line and the pseudorapidity coverages of the two layers are | η | < 2 and | η | < 1.4 , respectively. The data were collected using a minimum-bias trigger, which required a signal in both V0A and V0C detectors. The offline event selection was optimised to reject beam-induced background in all collision systems by utilising the timing signals in the two V0 detectors. In Pb–Pb collisions, the beam-induced background is further suppressed by correlating the timing signals of the neutron zero degree calorimeters, which are positioned on both sides of the interaction point at 112.5 m distance along the beam axis [35] . The signals from the zero degree calorimeters are also used to suppress the contamination from electromagnetic interactions. This is performed by requesting the coincidence of the signals coming from both side zero degree calorimeters by which the background due to single nucleus electromagnetic dissociation processes is excluded. A criterion based on the offline reconstruction of multiple primary vertices in the SPD is applied to reduce the pileup caused by multiple interactions in the same bunch crossing [36] . The results presented in this letter are for minimum-bias triggered pp collisions having at least one charged particle in the pseudorapidity interval | η | < 1 (INEL>0). The INEL>0 event class corresponds to about 75% of the total inelastic cross section [37] . For pp and Pb–Pb collisions, the sample is subdivided into different multiplicity classes based on the total charge deposited in both V0 sub-detectors, which is termed as V0M amplitude [38] . For p–Pb collisions, the sample is subdivided based on the total charge deposited in V0A sub-detector (V0A amplitude) [39] , which is located in the Pb-going direction. The V0A estimator has been implemented in previous measurements that used p–Pb data (see e.g. [40] ). This allows for comparisons with other observables for similar V0A multiplicity classes. To ensure that a hard scattering took place in the collision, events are required to have a trigger particle within 8 ≤ p T trig < 15 GeV/c . In this p T trig interval, the momentum resolution effects are negligible on the extracted yields, and therefore, no p T trig resolution correction is applied. The total number of analysed collisions before the trigger particle selection are about 108 , 108 , and 107 for pp, p–Pb, and Pb–Pb collisions, respectively. The transverse momentum of particles is determined from measurements in the central barrel with the ITS and the Time Projection Chamber (TPC). The ITS is a tracking detector which consists of six cylindrical layers of silicon detectors. The TPC is a cylindrical drift detector which covers a radial distance of 85-247 cm from the beam axis and it has longitudinal dimension extending from about -250 cm to +250 cm around the nominal interaction point. Primary charged particles are measured in the pseudorapidity range of | η | < 0.8 and with p T > 0.5 GeV/c , where η is measured in the laboratory frame for the three collision systems. The configuration for p–Pb collisions with protons at 4 TeV energy colliding with Pb ions that have per-nucleon energies of (Z / A ) × 4 TeV ∼ 1.58 TeV results in a shift in the rapidity of the nucleon–nucleon centre-of-mass system by 0.465 in the direction of the proton beam (negative z-direction). Here Z and A are the atomic and mass numbers of the Pb ion, respectively. Therefore, the detector coverage | η | < 0.8 corresponds to roughly − 0.3 < | η cms | < 1.3 for p–Pb collisions. The particles with mean proper lifetime larger than 1 cm/c , which are either produced directly in the interaction or from decays of particles with mean proper lifetime smaller than 1 cm/c are termed as primary particles [41] . The track selection follows a procedure similar to the one described in Ref. [42] and only few specific details are reported here. Tracks (N tracks ) are required to have two hits in the ITS, out of which at least one should be in either of the two innermost layers. The geometrical track length L is calculated in the TPC readout plane, excluding the information from the pads at the sector boundaries (≈3 cm from the sector edges). The trajectory lengths built from radial segments, i.e. the crossed TPC pad rows, traversed in the TPC by a particle are required to be larger than 85% of the geometrical track length. The pad rows are made of at least 3 neighbouring individual observations (clusters), and their height varies from 7.5 mm to 15 mm [43] . The trajectory lengths built from clusters (one cluster per pad row) is required to be larger than 0.7 × L . The fraction of TPC clusters shared with another track is required to be lower than 0.4. The fit quality for the ITS and TPC track points must satisfy χ ITS 2 / N hits < 36 and χ TPC 2 / N clusters < 4 , respectively, where N hits and N clusters are the numbers of hits in the ITS and the number of clusters in the TPC, respectively. Only tracks with χ TPC − ITS 2 < 36 are included in the analysis, where χ TPC − ITS 2 is calculated comparing the track parameters from the combined ITS and TPC track reconstruction to that derived only from the TPC and constrained to the interaction point. The definition of χ TPC − ITS 2 can be found in Ref. [44] . To reduce the contamination from secondary particles, tracks are accepted if their distance-of-closest-approach (DCA) to the reconstructed primary interaction vertex satisfies in the longitudinal (d z ) and transverse (d xy ) directions the conditions d z < 2 cm and d xy < 0.018 cm + 0.035 (cm×GeV/c )/ p T . The measurement of the transverse momentum spectra of charged particles follows the standard procedure of the ALICE collaboration [42,45] . The raw yields are corrected for efficiency and contamination from secondary particles. The efficiency correction is calculated from Monte Carlo simulations with GEANT3 [46] transport code, which made use of PYTHIA 8 (Monash) [28] , EPOS-LHC [21] and HIJING [47] event generators for pp, p–Pb and Pb–Pb collisions, respectively and incorporated a detailed description of the detector material, geometry and response. Since the event generators do not reproduce the relative abundances of different particle species in the real data, the efficiency obtained from the simulations is re-weighted considering the particle composition from data as outlined in [42] . A multi-component template fit based on the DCA distributions from the simulation is used for the estimation of secondary contamination [42] . The p T spectra for the toward and away regions include contributions from the jet fragmentation, ISR, and FSR, as well as, the contribution from the underlying event. In order to increase the sensitivity to the hardest process of the event, the particle yields measured in the transverse region are subtracted from the corresponding yields in both the toward and away regions: d N ch t , a / d p T − d N ch T / d p T . This approach assumes that the background (UE, ISR, and FSR) in the toward and away regions is similar to the activity in the transverse region. However, one has to keep in mind that in Pb–Pb collisions two-particle correlations are affected by anisotropic transverse flow. In particular, the main contribution is due to the elliptic flow, v 2 , which is the second order coefficient in the Fourier expansion of the azimuthal distribution of the particle momenta [48] . This elliptic azimuthal anisotropy modulates the background according to:(2) B ( Δ φ ) = B 0 ( 1 + 2 V 2 cos ( 2 Δ φ ) ) , where V 2 is approximately given by the product of anisotropic flow coefficients for trigger and associated particles at their respective momenta i.e. V 2 ≈ v 2 trig v 2 assoc . The existing v 2 measurements over a broad transverse momentum range [49] suggest that the effect of the v 2 modulation of background should be more relevant in semi-central Pb–Pb collisions. The effect is expected to be important at low and intermediate transverse momenta and decreases for high transverse momentum particles [30] . In the high-p T region of interest for the jet quenching studies, namely p T > 4 GeV/c , the effect of the v 2 modulation is estimated to be small (about 5%) for Pb–Pb collisions. Given that the v 2 effect is larger in Pb–Pb collisions than in pp and p–Pb collisions, no correction for the v 2 modulation is applied for pp and p–Pb collisions since its effect is smaller than the other sources of systematic uncertainty. The results are shown as a function of the average number of charged particles in the transverse region 〈 N ch T 〉 . The values of 〈 N ch T 〉 are extracted in each multiplicity class from the N tracks distributions in the transverse region that are corrected for detector effects using a Bayesian unfolding [50] . The Bayesian unfolding requires the multiplicity response matrix, which is built from the correlation between the measured multiplicity and the multiplicity at generator level (without detector effects) in the transverse region. This has been obtained from MC simulations which include the propagation of particles through the detector using GEANT 3. As a crosscheck, the 〈 N ch T 〉 values are also calculated by integrating the transverse momentum distributions in the interval 0.5 ≤ p T < 8 GeV/c . The difference between the results from the two strategies is assigned as the systematic uncertainty on 〈 N ch T 〉 , where the effects related to the discrepancy between data and MC in the particle composition and secondary contamination are considered. This uncertainty amounts up to 3.5%, 4% and 6.5% for pp, p–Pb and Pb–Pb collisions, respectively. The systematic uncertainties related to the track selection criteria were studied by repeating the analysis varying one-by-one the track selection criteria [42,45] . In particular, the upper limits of the track fit quality parameters in the ITS (χ ITS 2 / N hits ) and in the TPC (χ TPC 2 / N clusters ) were varied in the ranges of 25–49 and 3–5, respectively. The maximum fraction of shared TPC clusters was varied between 0.2 to 1 and the maximum d z was varied between 1 and 5 cm [42] . We have also quantified the impact of not including the ITS hit requirement in the track selection. The systematic uncertainty on the primary particle composition was estimated using a procedure similar to the one described in [42] . To quantify the uncertainty due to the imperfect simulation of the detector response, the track matching between the TPC and the ITS information in the data and in the simulation were compared. To achieve this, the fraction of secondary particles was rescaled according to fits to the measured DCA distributions. After this rescaling, the agreement between data and model was found to be within 3% for all collision systems. This value was assigned as an additional systematic uncertainty [42] . The systematic uncertainty on the secondary particle contamination considers the imperfection of the method (multi-component template fit) used to extract the correction. The fit ranges were varied and the fit was repeated using templates with two (primaries, secondaries) or three (primaries, secondaries from material, secondaries from weak decays) components. The maximum spread among these variations was assigned as the systematic uncertainty on the secondary contamination. This contribution dominates at low p T . The density of materials used in simulations of the experimental setup was varied by ± 4.5% [35] , resulting in a negligible systematic uncertainty in the considered p T range of 0.5 to 6.0 GeV/c . For the estimation of total systematic uncertainty, all the above listed contributions were summed in quadrature. The systematic uncertainties are independent of the difference between the azimuthal angle of the associated particle and that of the trigger particle. The estimated systematic uncertainties on the p T spectra significantly depend on p T , while the dependence on the multiplicity classes is mild. The ranges of systematic uncertainties in the three considered collision systems are reported in Table 1 for the various sources described above. 3 Results and discussion The p T spectra measured in the transverse region for pp, p–Pb, and Pb–Pb collisions are shown in Fig. 2 (top panel). Results are presented for different multiplicity classes. The ratios between the spectra in the individual multiplicity classes and the MB (0−100%) one are shown in the bottom panel. In the p T range 0.5 − 6 GeV/c , the ratios for the highest multiplicity class (0−5%) are larger than unity and show an increasing trend with increasing p T at low p T (< 2 − 3 GeV/c ) followed at higher p T by a slow decrease. Instead, for the lowest multiplicity classes (40−60% and 60−90%) the ratios are lower than unity and follow an opposite trend with p T , decreasing at low p T and increasing for p T > 3 GeV/c . The behaviour of the ratios as a function of the event activity is reminiscent of analogous ratios as a function of the number of MPI in pp collisions simulated with PYTHIA 8, including colour reconnection [51] . In particular, at p T ≈ 2 − 3 GeV/c the p T spectrum of pp collisions with large MPI activity exhibits an enhancement with respect to the p T spectrum of MB pp collisions. The effect was not observed before in data because, in contrast to the present analysis, the jet contribution was included in the p T spectra [45] . The top (bottom) panel of Fig. 3 shows the charged particle yields for the toward (away) region after the subtraction of the yields measured in the transverse region in pp, p–Pb and Pb–Pb collisions. Results are compared with the p T spectra measured for MB pp collisions (0−100% V0M pp event class) quantified with the ratio I X t , a , as defined in Eq. (1) . At low transverse momenta, p T < 4 GeV/c , I X t , a is close to unity in pp and p–Pb collisions. In contrast, I X t , a in Pb–Pb collisions exhibits a strong multiplicity dependence over the whole measured p T interval. The I X t , a magnitude is larger for semi-peripheral Pb–Pb collisions, the maximum is observed for 20−40% Pb–Pb collisions, and is smaller for the most central and most peripheral classes. Given that the v 2 contribution is not subtracted from the jet-like yields reported in Fig. 3 , the centrality dependence of I X t , a follows the behaviour of v 2 as a function of collision centrality and particle p T in Pb–Pb collisions at LHC energies [52] . Fig. 4 shows the measured values of I X t , a in the transverse momentum interval 4 < p T < 6 GeV/c as a function of the average multiplicity in the transverse region for all the multiplicity classes considered in pp, p–Pb and Pb–Pb collisions. The figure shows that, within uncertainties, the I X t , a values are close to unity for all the multiplicity classes measured in pp and p–Pb collisions. This indicates that effects induced by possible energy loss in these systems are not observed within uncertainties. This result is consistent with previous studies of nuclear modification factor [33] and hadron-jet recoil measurements [34] . By contrast, for Pb–Pb collisions the I X t , a values are compatible to unity for peripheral collisions, and show a gradual enhancement (reduction) with the increase in multiplicity for the toward (away) region. The behaviour is the same for the I X t , a values measured either assuming a flat background or a v 2 -modulated background. The v 2 -modulated background was estimated following the approach depicted in Eq. (2) and using the v 2 data reported in [49] . This behaviour is qualitatively similar to the di-hadron correlation results reported by the STAR and ALICE collaborations [29,30] . In Pb–Pb collisions, I X t provides information about the fragmenting jet leaving the medium, while on the away side, I X a reflects the survival probability of the recoiling parton during passage through the medium. Thus a suppression of I X a would indicate that fewer partons survive the passage through the medium and is expected from the strong in-medium energy loss. On the other hand, the enhancement observed in the toward region is also subject to medium effects. The ratio is sensitive to a) a possible change of the fragmentation functions, b) a possible modification of the quark to gluon jet ratio in the final state due to different coupling with medium, and c) a possible bias on the parton spectrum due to trigger particle selection. Moreover, given that I X t , a is sensitive to the same effects as I AA , the interpretation of the results is similar to that reported in [30] . It is likely that all three effects play a role [30] . A detailed quantification of the contribution of each effect is beyond the scope of the present paper.In order to get further insight into the effect, the measured I X t , a values are compared in Fig. 5 with model predictions. Following the similar treatment of the experimental data, for the models, the total sample is subdivided into different V0M classes and the 〈 N ch T 〉 is calculated for each class. For high-multiplicity pp collisions, although I X t , a is close to unity, a small trend with multiplicity is visible, which is not seen at similar multiplicities (20−90% V0A) in p–Pb data. To understand the source of these slight deviations from unity, the data are compared with the predictions from the PYTHIA 8 (Monash tune [28] ) and EPOS-LHC [21] event generators. In PYTHIA, the hadronization of quarks is simulated using the Lund string fragmentation model [53] . Various PYTHIA tunes have been developed through extensive comparison of Monte Carlo distributions with the minimum-bias data from different experiments. The Monash tune of PYTHIA 8 is tuned to LHC data and uses an updated set of hadronization parameters compared to the previous tunes [28] . EPOS-LHC is built on the Parton-Based Gribov Regge Theory. Utilising the colour exchange mechanism of string excitation, the model is tuned to LHC data [21] . In this model, a part of the collision system which has high parton densities becomes a “core” region that evolves hydrodynamically as a quark–gluon plasma and it is surrounded by a more dilute “corona” for which fragmentation occurs in the vacuum. The upper panel of Fig. 5 shows I X t , a for different multiplicity classes. The observed deviations from unity are reproduced by PYTHIA 8 for both the toward and away regions. Given that PYTHIA 8 does not incorporate any jet quenching mechanism, the origin of the effect in high 〈 N ch T 〉 collisions is related to a remaining bias towards harder fragmentation and more activity from initial and final state radiation [54] . These effects enhance the high-p T yield in the toward region, and produce a broadening in the away region [55] . The EPOS-LHC results in the away region are similar to both data and PYTHIA 8. However, for I X t EPOS-LHC exhibits a trend with a maximum at intermediate multiplicity and a reduction toward low and high multiplicities, which is not consistent with the measurements. The middle and bottom panels of Fig. 5 show I X t , a measured for p–Pb and Pb–Pb collisions, respectively. The data are compared to PYTHIA 8/Angantyr [56] and EPOS-LHC predictions. The Angantyr model in PYTHIA 8 extrapolates the dynamics from pp collisions to p–Pb and Pb–Pb collisions, generalising the formalism adopted for pp collisions by including a description of the nucleon positions within the colliding nuclei and utilising the Glauber model to calculate the number of interacting nucleons and binary nucleon–nucleon collisions. PYTHIA 8/Angantyr, which does not include jet quenching effects, predicts I X t , a values consistent with unity for all the multiplicity classes in Pb–Pb collisions. Whereas for p–Pb collisions I X a is consistent with unity, and I X t is slightly below unity. In EPOS-LHC, a certain p T cutoff is defined in such a way that, above this cutoff, a particle loses part of its momentum in the core but survives as an independent particle produced by a flux tube. Soft particles, which are below the p T cutoff, get completely absorbed and form the core. This sort of energy loss mechanism implemented in EPOS-LHC depends on the system size [21,57,58] . Fig. 5 (middle) shows that for p–Pb collisions, EPOS-LHC does not describe either the magnitude or the trend of the multiplicity dependence of the measured ratio in the toward region, I X t . However, the model is in reasonable agreement with data in the away region. For Pb–Pb collisions, EPOS-LHC predicts a significant enhancement of I X t , a for low 〈 N ch T 〉 ranges and deviates significantly from the experimental results. In summary, while the data from Pb–Pb collisions are in qualitative agreement with expectations from parton energy loss due to the presence of a hot and dense medium, pp and p–Pb data do not show any hint of medium effects in the multiplicity range which is reported. 4 Summary The transverse momentum spectra (0.5 ≤ p T < 6 GeV/c ) of primary charged particles in three azimuthal regions (toward, away and transverse) defined with respect to the direction of the particle with the highest transverse momentum in the event (8 ≤ p T trig < 15 GeV/c ) are reported. The spectra are studied in intervals of the multiplicity measured at forward pseudorapidities for pp, p–Pb, and Pb–Pb collisions at s NN = 5.02 TeV. The p T spectra in the transverse region are subtracted from those of the away and toward regions. This is based on the assumption that the transverse side provides a good estimation of the underlying event contribution in both the toward and away regions. However, for the interpretation of the results one has to keep in mind that v 2 modulates the background and this effect is important for semi-central Pb–Pb collisions and for p T > 4 GeV/c the effect is less than 5% in central and peripheral Pb–Pb collisions. Ratios to MB pp (I X t , a ), i.e., the multiplicity dependent yields normalised to the yield measured in MB pp collisions, are reported. At low transverse momentum (p T < 2 GeV/c ), within 20%, the I X t , a values are multiplicity independent for both the toward and away regions in pp and p–Pb collisions. In contrast, in Pb–Pb collisions for both toward and away regions the I X t , a values exhibit a centrality dependence which is expected given the residual presence of elliptic flow. In the highest transverse momentum interval (4 < p T < 6 GeV/c ), the I X t , a values in pp collisions are closer to unity but they exhibit a small reduction (increase) towards high V0 activity in pp collisions. This trend is well reproduced by PYTHIA 8. In the model, it is due to a selection bias towards pp collisions with harder fragmentation and larger activity from initial and final state radiation. For p–Pb collisions, within uncertainties, the I X t , a values are consistent with unity and do not show a multiplicity dependence. PYTHIA 8/Angantyr fairly describes I X a , but it underestimates by about 10% the I X t values in the low multiplicity classes (40−90% V0A event class). For Pb–Pb collisions, the I X t , a values are close to unity for peripheral collisions, and show a gradual increase (reduction) in the toward (away) region with increasing multiplicity. A similar observable, I AA , based on the per-trigger yield of associated particles in di-hadron correlation has been studied for central and peripheral Pb–Pb collisions at s NN = 2.76 TeV. The behaviour of I X t , a exhibits the same features as I AA : in central collisions, on the away-side, a suppression is observed as expected from strong in-medium energy loss. In the toward region, an enhancement is observed. PYTHIA 8/Angantyr predicts I X t , a ≈ 1 for all multiplicity intervals, and it does not reproduce the observed away-side suppression or toward-side enhancement. Generally, EPOS-LHC does not describe the measured I X t , a ratios. In summary, within the multiplicity reach reported in this paper, no jet quenching effects are observed in pp and p–Pb collisions within uncertainties. Further studies are required to extend the present analysis to higher multiplicities, which are currently limited by the event selection based on the forward V0 detector. The analysis of future pp and p–Pb collisions with much larger integrated luminosity may remove this limitation. Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgements The ALICE Collaboration would like to thank all its engineers and technicians for their invaluable contributions to the construction of the experiment and the CERN accelerator teams for the outstanding performance of the LHC complex. The ALICE Collaboration gratefully acknowledges the resources and support provided by all Grid centres and the Worldwide LHC Computing Grid (WLCG) collaboration. The ALICE Collaboration acknowledges the following funding agencies for their support in building and running the ALICE detector: A.I. Alikhanyan National Science Laboratory (Yerevan Physics Institute) Foundation (ANSL) , State Committee of Science and World Federation of Scientists (WFS) , Armenia; Austrian Academy of Sciences , Austrian Science Fund (FWF): [M 2467-N36 ] and Nationalstiftung für Forschung, Technologie und Entwicklung , Austria; Ministry of Communications and High Technologies, National Nuclear Research Center , Azerbaijan; Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq ), Financiadora de Estudos e Projetos (Finep), Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP ) and Universidade Federal do Rio Grande do Sul (UFRGS ), Brazil; Bulgarian Ministry of Education and Science , within the National Roadmap for Research Infrastructures 2020–2027 (object CERN), Bulgaria; Ministry of Education of China (MOEC), Ministry of Science & Technology of China (MSTC) and National Natural Science Foundation of China (NSFC), China; Ministry of Science and Education and Croatian Science Foundation , Croatia; Centro de Aplicaciones Tecnológicas y Desarrollo Nuclear (CEADEN), Cubaenergía , Cuba; Ministry of Education, Youth and Sports of the Czech Republic , Czech Republic; The Danish Council for Independent Research | Natural Sciences , the Villum Fonden and Danish National Research Foundation (DNRF), Denmark; Helsinki Institute of Physics (HIP), Finland; Commissariat à l'Energie Atomique (CEA ) and Institut National de Physique Nucléaire et de Physique des Particules (IN2P3) and Centre National de la Recherche Scientifique (CNRS), France; Bundesministerium für Bildung und Forschung (BMBF ) and GSI Helmholtzzentrum für Schwerionenforschung GmbH , Germany; General Secretariat for Research and Technology , Ministry of Education, Research and Religions, Greece; National Research, Development and Innovation Office , Hungary; Department of Atomic Energy Government of India (DAE ), Department of Science and Technology, Government of India (DST ), University Grants Commission , Government of India (UGC) and Council of Scientific and Industrial Research (CSIR), India; National Research and Innovation Agency - BRIN , Indonesia; Istituto Nazionale di Fisica Nucleare (INFN ), Italy; Japanese Ministry of Education, Culture, Sports, Science and Technology (MEXT) and Japan Society for the Promotion of Science (JSPS) KAKENHI, Japan; Consejo Nacional de Ciencia (CONACYT ) y Tecnología, through Fondo de Cooperación Internacional en Ciencia y Tecnología (FONCICYT) and Dirección General de Asuntos del Personal Académico (DGAPA), Mexico; Nederlandse Organisatie voor Wetenschappelijk Onderzoek (NWO), Netherlands; The Research Council of Norway , Norway; Commission on Science and Technology for Sustainable Development in the South (COMSATS), Pakistan; Pontificia Universidad Católica del Perú , Peru; Ministry of Education and Science , National Science Centre and WUT ID-UB , Poland; Korea Institute of Science and Technology Information and National Research Foundation of Korea (NRF), Republic of Korea; Ministry of Education and Scientific Research , Institute of Atomic Physics , Ministry of Research and Innovation and Institute of Atomic Physics and University Politehnica of Bucharest , Romania; Ministry of Education, Science, Research and Sport of the Slovak Republic , Slovakia; National Research Foundation of South Africa , South Africa; Swedish Research Council (VR) and Knut & Alice Wallenberg Foundation (KAW), Sweden; European Organization for Nuclear Research , Switzerland; Suranaree University of Technology (SUT), National Science and Technology Development Agency (NSTDA), Thailand Science Research and Innovation (TSRI) and National Science, Research and Innovation Fund (NSRF), Thailand; Turkish Energy, Nuclear and Mineral Research Agency (TENMAK), Turkey; National Academy of Sciences of Ukraine , Ukraine; Science and Technology Facilities Council (STFC), United Kingdom; National Science Foundation of the United States of America (NSF ) and United States Department of Energy , Office of Nuclear Physics (DOE NP), United States of America. In addition, individual groups or members have received support from: Marie Skłodowska Curie, Strong 2020 - Horizon 2020 , European Research Council (grant nos. 824093 , 896850 , 950692 ), European Union ; Academy of Finland (Center of Excellence in Quark Matter) (grant nos. 346327 , 346328 ), Finland; Programa de Apoyos para la Superación del Personal Académico , UNAM , Mexico. References [1] T. Sjöstrand M. van Zijl A multiple interaction model for the event structure in hadron collisions Phys. Rev. D 36 1987 2019 T. Sjöstrand and M. van Zijl, “A Multiple Interaction Model for the Event Structure in Hadron Collisions”, Phys. Rev. D 36 (1987) 2019. [2] P. Bartalini J.R. Gaunt Multiple Parton Interactions at the LHC, vol. 29 2019 WSP P. Bartalini and J. R. Gaunt, eds., Multiple Parton Interactions at the LHC, vol. 29. WSP, 2019. [3] CDF Collaboration T. Affolder Charged jet evolution and the underlying event in p p ¯ collisions at 1.8 TeV Phys. Rev. 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+]>PLB 137649 137649 S0370-2693(22)00783-3 10.1016/j.physletb.2022.137649 The Author(s) Experiments https://www.hepdata.net/ Fig. 1 Illustration of toward, away and transverse regions with respect to the leading particle in a collision. Fig. 1 Fig. 2 Top panels: transverse momentum spectra of charged particles in the transverse region for different multiplicity classes in pp (left), p–Pb (middle) and Pb–Pb (right) collisions at s NN = 5.02 TeV. The p T spectra are measured at mid pseudorapidity (|η | < 0.8). Lower panels: Ratio of p T spectra in different multiplicity classes to the p T spectrum in the 0−100% multiplicity class for the corresponding collision systems. The statistical and systematic uncertainties are shown by bars and boxes, respectively. Fig. 2 Fig. 3 Transverse momentum spectra of charged particles in Toward-Transverse, d N ch st / d p T (top plot) and Away-Transverse, d N ch sa / d p T (bottom plot) regions for different multiplicity classes in pp (left), p–Pb (middle) and Pb–Pb (right) collisions at s NN = 5.02 TeV. The p T spectra are measured at mid pseudorapidity (|η | < 0.8). The lower panels of both plots show the ratio to minimum bias pp collisions. The statistical and systematic uncertainties are shown by bars and boxes, respectively. Fig. 3 Fig. 4 The I X t (left) and I X a (right) as a function of 〈 N ch T 〉 in 4 <p T < 6 GeV/c for different multiplicity classes in pp, p–Pb and Pb–Pb collisions at s NN = 5.02 TeV. Pb–Pb results are shown assuming a flat background (filled markers), and assuming a v 2 -modulated background (empty markers). The statistical and systematic uncertainties are shown by bars and boxes, respectively. Fig. 4 Fig. 5 Comparison of the measured the I X t (left) and I X a (right) in 4 <p T < 6 GeV/c with model predictions. The results are shown as a function of 〈 N ch T 〉 for different multiplicity classes in pp (top panel), p–Pb (middle panel) and Pb–Pb (bottom panel) collisions at s NN = 5.02 TeV. The red and magenta lines show the PYTHIA 8 (Monash) [28] and PYTHIA 8/Angantyr [28] predictions, respectively. The blue lines show the EPOS-LHC [21] results. The statistical and systematic uncertainties are shown by bars and boxes, respectively. Fig. 5 Table 1 Contributions to the relative (%) systematic uncertainty on the p T spectra of primary charged particles in pp, p–Pb, and Pb–Pb collisions at s NN = 5.02 TeV. Just for illustration, the range in the table corresponds to the lowest and highest relative systematic uncertainty in the considered p T range. The individual contributions are summed in quadrature to obtain the total uncertainty. Table 1 Source of uncertainty pp p–Pb Pb–Pb |
Track selection 2.1–8.2 2.4–5.8 3.0–9.9 |
Particle composition 0.3–1.8 0.5–1.9 0.3–2.4 |
Secondary particles 0.0–0.4 0.0–2.4 0.0–1.9 |
Matching efficiency 2.0–4.2 0.7–3.7 0.6–3.7 |
Total 3.2–8.8 3.6–6.3 3.5–10.0 |
Total (N ch -dependent) 2.0–4.5 1.7–4.0 1.1–3.7 |
Study of charged particle production at high p T using event topology in pp, p–Pb and Pb–Pb collisions at s NN = 5.02 TeV ALICE Collaboration ⋆ S. Acharya 124 131 D. Adamová 86 A. Adler 69 G. Aglieri Rinella 32 M. Agnello 29 N. Agrawal 50 Z. Ahammed 131 S. Ahmad 15 S.U. Ahn 70 I. Ahuja 37 A. Akindinov 139 M. Al-Turany 98 D. Aleksandrov 139 B. Alessandro 55 H.M. Alfanda 6 R. Alfaro Molina 66 B. Ali 15 Y. Ali 13 A. Alici 25 N. Alizadehvandchali 113 A. Alkin 32 J. Alme 20 G. Alocco 51 T. Alt 63 I. Altsybeev 139 M.N. Anaam 6 C. Andrei 45 A. Andronic 134 V. Anguelov 95 F. Antinori 53 P. Antonioli 50 C. Anuj 15 N. Apadula 74 L. Aphecetche 103 H. Appelshäuser 63 S. Arcelli 25 R. Arnaldi 55 I.C. Arsene 19 M. Arslandok 136 A. Augustinus 32 R. Averbeck 98 S. Aziz 72 M.D. Azmi 15 A. Badalà 52 Y.W. Baek 40 X. Bai 98 R. Bailhache 63 Y. Bailung 47 R. Bala 91 A. Balbino 29 A. Baldisseri 127 B. Balis 2 D. Banerjee 4 Z. Banoo 91 R. Barbera 26 L. Barioglio 96 M. Barlou 78 G.G. Barnaföldi 135 L.S. Barnby 85 V. Barret 124 L. Barreto 109 C. Bartels 116 K. Barth 32 E. Bartsch 63 F. Baruffaldi 27 N. Bastid 124 S. Basu 75 G. Batigne 103 D. Battistini 96 B. Batyunya 140 D. Bauri 46 J.L. Bazo Alba 101 I.G. Bearden 83 C. Beattie 136 P. Becht 98 D. Behera 47 I. Belikov 126 A.D.C. Bell Hechavarria 134 F. Bellini 25 R. Bellwied 113 S. Belokurova 139 V. Belyaev 139 G. Bencedi 135 64 S. Beole 24 A. Bercuci 45 Y. Berdnikov 139 A. Berdnikova 95 L. Bergmann 95 M.G. Besoiu 62 L. Betev 32 P.P. Bhaduri 131 A. Bhasin 91 I.R. Bhat 91 M.A. Bhat 4 B. Bhattacharjee 41 L. Bianchi 24 N. Bianchi 48 J. Bielčík 35 J. Bielčíková 86 J. Biernat 106 A. Bilandzic 96 G. Biro 135 S. Biswas 4 J.T. Blair 107 D. Blau 139 M.B. Blidaru 98 N. Bluhme 38 C. Blume 63 G. Boca 21 54 F. Bock 87 T. Bodova 20 A. Bogdanov 139 S. Boi 22 J. Bok 57 L. Boldizsár 135 A. Bolozdynya 139 M. Bombara 37 P.M. Bond 32 G. Bonomi 130 54 H. Borel 127 A. Borissov 139 H. Bossi 136 E. Botta 24 L. Bratrud 63 P. Braun-Munzinger 98 M. Bregant 109 M. Broz 35 G.E. Bruno 97 31 M.D. Buckland 116 D. Budnikov 139 H. Buesching 63 S. Bufalino 29 O. Bugnon 103 P. Buhler 102 Z. Buthelezi 67 120 J.B. Butt 13 A. Bylinkin 115 S.A. Bysiak 106 M. Cai 27 6 H. Caines 136 A. Caliva 98 E. Calvo Villar 101 J.M.M. Camacho 108 R.S. Camacho 44 P. Camerini 23 F.D.M. Canedo 109 M. Carabas 123 F. Carnesecchi 32 R. Caron 125 127 J. Castillo Castellanos 127 F. Catalano 29 C. Ceballos Sanchez 140 I. Chakaberia 74 P. Chakraborty 46 S. Chandra 131 S. Chapeland 32 M. Chartier 116 S. Chattopadhyay 131 S. Chattopadhyay 99 T.G. Chavez 44 T. Cheng 6 C. Cheshkov 125 B. Cheynis 125 V. Chibante Barroso 32 D.D. Chinellato 110 E.S. Chizzali 96 II J. Cho 57 S. Cho 57 P. Chochula 32 P. Christakoglou 84 C.H. Christensen 83 P. Christiansen 75 T. Chujo 122 M. Ciacco 29 C. Cicalo 51 L. Cifarelli 25 F. Cindolo 50 M.R. Ciupek 98 G. Clai 50 III F. Colamaria 49 J.S. Colburn 100 D. Colella 97 31 A. Collu 74 M. Colocci 32 M. Concas 55 IV G. Conesa Balbastre 73 Z. Conesa del Valle 72 G. Contin 23 J.G. Contreras 35 M.L. Coquet 127 T.M. Cormier 87 I P. Cortese 129 55 M.R. Cosentino 111 F. Costa 32 S. Costanza 21 54 P. Crochet 124 R. Cruz-Torres 74 E. Cuautle 64 P. Cui 6 L. Cunqueiro 87 A. Dainese 53 M.C. Danisch 95 A. Danu 62 P. Das 80 P. Das 4 S. Das 4 S. Dash 46 R.M.H. David 44 A. De Caro 28 G. de Cataldo 49 L. De Cilladi 24 J. de Cuveland 38 A. De Falco 22 D. De Gruttola 28 N. De Marco 55 C. De Martin 23 S. De Pasquale 28 S. Deb 47 H.F. Degenhardt 109 K.R. Deja 132 R. Del Grande 96 L. Dello Stritto 28 W. Deng 6 P. Dhankher 18 D. Di Bari 31 A. Di Mauro 32 R.A. Diaz 140 7 T. Dietel 112 Y. Ding 125 6 R. Divià 32 D.U. Dixit 18 Ø. Djuvsland 20 U. Dmitrieva 139 A. Dobrin 62 B. Dönigus 63 A.K. Dubey 131 J.M. Dubinski 132 A. Dubla 98 S. Dudi 90 P. Dupieux 124 M. Durkac 105 N. Dzalaiova 12 T.M. Eder 134 R.J. Ehlers 87 V.N. Eikeland 20 F. Eisenhut 63 D. Elia 49 B. Erazmus 103 F. Ercolessi 25 F. Erhardt 89 M.R. Ersdal 20 B. Espagnon 72 G. Eulisse 32 D. Evans 100 S. Evdokimov 139 L. Fabbietti 96 M. Faggin 27 J. Faivre 73 F. Fan 6 W. Fan 74 A. Fantoni 48 M. Fasel 87 P. Fecchio 29 A. Feliciello 55 G. Feofilov 139 A. Fernández Téllez 44 M.B. Ferrer 32 A. Ferrero 127 A. Ferretti 24 V.J.G. Feuillard 95 J. Figiel 106 V. Filova 35 D. Finogeev 139 F.M. Fionda 51 G. Fiorenza 97 F. Flor 113 A.N. Flores 107 S. Foertsch 67 I. Fokin 95 S. Fokin 139 E. Fragiacomo 56 E. Frajna 135 U. Fuchs 32 N. Funicello 28 C. Furget 73 A. Furs 139 J.J. Gaardhøje 83 M. Gagliardi 24 A.M. Gago 101 A. Gal 126 C.D. Galvan 108 P. Ganoti 78 C. Garabatos 98 J.R.A. Garcia 44 E. Garcia-Solis 9 K. Garg 103 C. Gargiulo 32 A. Garibli 81 K. Garner 134 E.F. Gauger 107 A. Gautam 115 M.B. Gay Ducati 65 M. Germain 103 S.K. Ghosh 4 M. Giacalone 25 P. Gianotti 48 P. Giubellino 98 55 P. Giubilato 27 A.M.C. Glaenzer 127 P. Glässel 95 E. Glimos 119 D.J.Q. Goh 76 V. Gonzalez 133 L.H. González-Trueba 66 S. Gorbunov 38 M. Gorgon 2 L. Görlich 106 S. Gotovac 33 V. Grabski 66 L.K. Graczykowski 132 E. Grecka 86 L. Greiner 74 A. Grelli 58 C. Grigoras 32 V. Grigoriev 139 S. Grigoryan 140 1 F. Grosa 32 J.F. Grosse-Oetringhaus 32 R. Grosso 98 D. Grund 35 G.G. Guardiano 110 R. Guernane 73 M. Guilbaud 103 K. Gulbrandsen 83 T. Gunji 121 W. Guo 6 A. Gupta 91 R. Gupta 91 S.P. Guzman 44 L. Gyulai 135 M.K. Habib 98 C. Hadjidakis 72 H. Hamagaki 76 M. Hamid 6 Y. Han 137 R. Hannigan 107 M.R. Haque 132 A. Harlenderova 98 J.W. Harris 136 A. Harton 9 J.A. Hasenbichler 32 H. Hassan 87 D. Hatzifotiadou 50 P. Hauer 42 L.B. Havener 136 S.T. Heckel 96 E. Hellbär 98 H. Helstrup 34 T. Herman 35 G. Herrera Corral 8 F. Herrmann 134 K.F. Hetland 34 B. Heybeck 63 H. Hillemanns 32 C. Hills 116 B. Hippolyte 126 B. Hofman 58 B. Hohlweger 84 J. Honermann 134 G.H. Hong 137 D. Horak 35 A. Horzyk 2 R. Hosokawa 14 Y. Hou 6 P. Hristov 32 C. Hughes 119 P. Huhn 63 L.M. Huhta 114 C.V. Hulse 72 T.J. Humanic 88 H. Hushnud 99 A. Hutson 113 D. Hutter 38 J.P. Iddon 116 R. Ilkaev 139 H. Ilyas 13 M. Inaba 122 G.M. Innocenti 32 M. Ippolitov 139 A. Isakov 86 T. Isidori 115 M.S. Islam 99 M. Ivanov 98 V. Ivanov 139 V. Izucheev 139 M. Jablonski 2 B. Jacak 74 N. Jacazio 32 P.M. Jacobs 74 S. Jadlovska 105 J. Jadlovsky 105 L. Jaffe 38 C. Jahnke 110 M.A. Janik 132 T. Janson 69 M. Jercic 89 O. Jevons 100 A.A.P. Jimenez 64 F. Jonas 87 134 P.G. Jones 100 J.M. Jowett 32 98 J. Jung 63 M. Jung 63 A. Junique 32 A. Jusko 100 M.J. Kabus 32 132 J. Kaewjai 104 P. Kalinak 59 A.S. Kalteyer 98 A. Kalweit 32 V. Kaplin 139 A. Karasu Uysal 71 D. Karatovic 89 O. Karavichev 139 T. Karavicheva 139 P. Karczmarczyk 132 E. Karpechev 139 V. Kashyap 80 A. Kazantsev 139 U. Kebschull 69 R. Keidel 138 D.L.D. Keijdener 58 M. Keil 32 B. Ketzer 42 A.M. Khan 6 S. Khan 15 A. Khanzadeev 139 Y. Kharlov 139 A. Khatun 15 A. Khuntia 106 B. Kileng 34 B. Kim 16 C. Kim 16 D.J. Kim 114 E.J. Kim 68 J. Kim 137 J.S. Kim 40 J. Kim 95 J. Kim 68 M. Kim 95 S. Kim 17 T. Kim 137 S. Kirsch 63 I. Kisel 38 S. Kiselev 139 A. Kisiel 132 J.P. Kitowski 2 J.L. Klay 5 J. Klein 32 S. Klein 74 C. Klein-Bösing 134 M. Kleiner 63 T. Klemenz 96 A. Kluge 32 A.G. Knospe 113 C. Kobdaj 104 T. Kollegger 98 A. Kondratyev 140 N. Kondratyeva 139 E. Kondratyuk 139 J. Konig 63 S.A. Konigstorfer 96 P.J. Konopka 32 G. Kornakov 132 S.D. Koryciak 2 A. Kotliarov 86 O. Kovalenko 79 V. Kovalenko 139 M. Kowalski 106 I. Králik 59 A. Kravčáková 37 L. Kreis 98 M. Krivda 100 59 F. Krizek 86 K. Krizkova Gajdosova 35 M. Kroesen 95 M. Krüger 63 D.M. Krupova 35 E. Kryshen 139 M. Krzewicki 38 V. Kučera 32 C. Kuhn 126 P.G. Kuijer 84 T. Kumaoka 122 D. Kumar 131 L. Kumar 90 N. Kumar 90 S. Kundu 32 P. Kurashvili 79 A. Kurepin 139 A.B. Kurepin 139 S. Kushpil 86 J. Kvapil 100 M.J. Kweon 57 J.Y. Kwon 57 Y. Kwon 137 S.L. La Pointe 38 P. La Rocca 26 Y.S. Lai 74 A. Lakrathok 104 M. Lamanna 32 R. Langoy 118 P. Larionov 48 E. Laudi 32 L. Lautner 32 96 R. Lavicka 102 T. Lazareva 139 R. Lea 130 54 J. Lehrbach 38 R.C. Lemmon 85 I. León Monzón 108 M.M. Lesch 96 E.D. Lesser 18 M. Lettrich 96 P. Lévai 135 X. Li 10 X.L. Li 6 J. Lien 118 R. Lietava 100 B. Lim 16 S.H. Lim 16 V. Lindenstruth 38 A. Lindner 45 C. Lippmann 98 A. Liu 18 D.H. Liu 6 J. Liu 116 I.M. Lofnes 20 V. Loginov 139 C. Loizides 87 P. Loncar 33 J.A. Lopez 95 X. Lopez 124 E. López Torres 7 P. Lu 98 117 J.R. Luhder 134 M. Lunardon 27 G. Luparello 56 Y.G. Ma 39 A. Maevskaya 139 M. Mager 32 T. Mahmoud 42 A. Maire 126 M. Malaev 139 N.M. Malik 91 Q.W. Malik 19 S.K. Malik 91 L. Malinina 140 VII D. Mal'Kevich 139 D. Mallick 80 N. Mallick 47 G. Mandaglio 30 52 V. Manko 139 F. Manso 124 V. Manzari 49 Y. Mao 6 G.V. Margagliotti 23 A. Margotti 50 A. Marín 98 C. Markert 107 M. Marquard 63 N.A. Martin 95 P. Martinengo 32 J.L. Martinez 113 M.I. Martínez 44 G. Martínez García 103 S. Masciocchi 98 M. Masera 24 A. Masoni 51 L. Massacrier 72 A. Mastroserio 128 49 A.M. Mathis 96 O. Matonoha 75 P.F.T. Matuoka 109 A. Matyja 106 C. Mayer 106 A.L. Mazuecos 32 F. Mazzaschi 24 M. Mazzilli 32 J.E. Mdhluli 120 A.F. Mechler 63 Y. Melikyan 139 A. Menchaca-Rocha 66 E. Meninno 102 28 A.S. Menon 113 M. Meres 12 S. Mhlanga 112 67 Y. Miake 122 L. Micheletti 55 L.C. Migliorin 125 D.L. Mihaylov 96 K. Mikhaylov 140 139 A.N. Mishra 135 D. Miśkowiec 98 A. Modak 4 A.P. Mohanty 58 B. Mohanty 80 M. Mohisin Khan 15 V M.A. Molander 43 Z. Moravcova 83 C. Mordasini 96 D.A. Moreira De Godoy 134 I. Morozov 139 A. Morsch 32 T. Mrnjavac 32 V. Muccifora 48 E. Mudnic 33 S. Muhuri 131 J.D. Mulligan 74 A. Mulliri 22 M.G. Munhoz 109 R.H. Munzer 63 H. Murakami 121 S. Murray 112 L. Musa 32 J. Musinsky 59 J.W. Myrcha 132 B. Naik 120 R. Nair 79 B.K. Nandi 46 R. Nania 50 E. Nappi 49 A.F. Nassirpour 75 A. Nath 95 C. Nattrass 119 A. Neagu 19 A. Negru 123 L. Nellen 64 S.V. Nesbo 34 G. Neskovic 38 D. Nesterov 139 B.S. Nielsen 83 E.G. Nielsen 83 S. Nikolaev 139 S. Nikulin 139 V. Nikulin 139 F. Noferini 50 S. Noh 11 P. Nomokonov 140 J. Norman 116 N. Novitzky 122 P. Nowakowski 132 A. Nyanin 139 J. Nystrand 20 M. Ogino 76 A. Ohlson 75 V.A. Okorokov 139 J. Oleniacz 132 A.C. Oliveira Da Silva 119 M.H. Oliver 136 A. Onnerstad 114 C. Oppedisano 55 A. Ortiz Velasquez 64 A. Oskarsson 75 J. Otwinowski 106 M. Oya 93 K. Oyama 76 Y. Pachmayer 95 S. Padhan 46 D. Pagano 130 54 G. Paić 64 A. Palasciano 49 S. Panebianco 127 J. Park 57 J.E. Parkkila 32 114 S.P. Pathak 113 R.N. Patra 91 B. Paul 22 H. Pei 6 T. Peitzmann 58 X. Peng 6 L.G. Pereira 65 H. Pereira Da Costa 127 D. Peresunko 139 G.M. Perez 7 S. Perrin 127 Y. Pestov 139 V. Petráček 35 V. Petrov 139 M. Petrovici 45 R.P. Pezzi 103 65 S. Piano 56 M. Pikna 12 P. Pillot 103 O. Pinazza 50 32 L. Pinsky 113 C. Pinto 96 26 S. Pisano 48 M. Płoskoń 74 M. Planinic 89 F. Pliquett 63 M.G. Poghosyan 87 S. Politano 29 N. Poljak 89 A. Pop 45 S. Porteboeuf-Houssais 124 J. Porter 74 V. Pozdniakov 140 S.K. Prasad 4 S. Prasad 47 R. Preghenella 50 F. Prino 55 C.A. Pruneau 133 I. Pshenichnov 139 M. Puccio 32 S. Qiu 84 L. Quaglia 24 R.E. Quishpe 113 S. Ragoni 100 A. Rakotozafindrabe 127 L. Ramello 129 55 F. Rami 126 S.A.R. Ramirez 44 T.A. Rancien 73 R. Raniwala 92 S. Raniwala 92 S.S. Räsänen 43 R. Rath 47 I. Ravasenga 84 K.F. Read 87 119 A.R. Redelbach 38 K. Redlich 79 VI A. Rehman 20 P. Reichelt 63 F. Reidt 32 H.A. Reme-Ness 34 Z. Rescakova 37 K. Reygers 95 A. Riabov 139 V. Riabov 139 R. Ricci 28 T. Richert 75 M. Richter 19 W. Riegler 32 F. Riggi 26 C. Ristea 62 M. Rodríguez Cahuantzi 44 K. Røed 19 R. Rogalev 139 E. Rogochaya 140 T.S. Rogoschinski 63 D. Rohr 32 D. Röhrich 20 P.F. Rojas 44 S. Rojas Torres 35 P.S. Rokita 132 F. Ronchetti 48 A. Rosano 30 52 E.D. Rosas 64 A. Rossi 53 A. Roy 47 P. Roy 99 S. Roy 46 N. Rubini 25 O.V. Rueda 75 D. Ruggiano 132 R. Rui 23 B. Rumyantsev 140 P.G. Russek 2 R. Russo 84 A. Rustamov 81 E. Ryabinkin 139 Y. Ryabov 139 A. Rybicki 106 H. Rytkonen 114 W. Rzesa 132 O.A.M. Saarimaki 43 R. Sadek 103 S. Sadovsky 139 J. Saetre 20 K. Šafařík 35 S.K. Saha 131 S. Saha 80 B. Sahoo 46 P. Sahoo 46 R. Sahoo 47 S. Sahoo 60 D. Sahu 47 P.K. Sahu 60 J. Saini 131 K. Sajdakova 37 S. Sakai 122 M.P. Salvan 98 S. Sambyal 91 T.B. Saramela 109 D. Sarkar 133 N. Sarkar 131 P. Sarma 41 V. Sarritzu 22 V.M. Sarti 96 M.H.P. Sas 136 J. Schambach 87 H.S. Scheid 63 C. Schiaua 45 R. Schicker 95 A. Schmah 95 C. Schmidt 98 H.R. Schmidt 94 M.O. Schmidt 32 M. Schmidt 94 N.V. Schmidt 87 63 A.R. Schmier 119 R. Schotter 126 J. Schukraft 32 K. Schwarz 98 K. Schweda 98 G. Scioli 25 E. Scomparin 55 J.E. Seger 14 Y. Sekiguchi 121 D. Sekihata 121 I. Selyuzhenkov 98 139 S. Senyukov 126 J.J. Seo 57 D. Serebryakov 139 L. Šerkšnytė 96 A. Sevcenco 62 T.J. Shaba 67 A. Shabanov 139 A. Shabetai 103 R. Shahoyan 32 W. Shaikh 99 A. Shangaraev 139 A. Sharma 90 D. Sharma 46 H. Sharma 106 M. Sharma 91 N. Sharma 90 S. Sharma 91 U. Sharma 91 A. Shatat 72 O. Sheibani 113 K. Shigaki 93 M. Shimomura 77 S. Shirinkin 139 Q. Shou 39 Y. Sibiriak 139 S. Siddhanta 51 T. Siemiarczuk 79 T.F. Silva 109 D. Silvermyr 75 T. Simantathammakul 104 R. Simeonov 36 G. Simonetti 32 B. Singh 91 B. Singh 96 R. Singh 80 R. Singh 91 R. Singh 47 V.K. Singh 131 V. Singhal 131 T. Sinha 99 B. Sitar 12 M. Sitta 129 55 T.B. Skaali 19 G. Skorodumovs 95 M. Slupecki 43 N. Smirnov 136 R.J.M. Snellings 58 E.H. Solheim 19 C. Soncco 101 J. Song 113 A. Songmoolnak 104 F. Soramel 27 S. Sorensen 119 R. Spijkers 84 I. Sputowska 106 J. Staa 75 J. Stachel 95 I. Stan 62 P.J. Steffanic 119 S.F. Stiefelmaier 95 D. Stocco 103 I. Storehaug 19 M.M. Storetvedt 34 P. Stratmann 134 S. Strazzi 25 C.P. Stylianidis 84 A.A.P. Suaide 109 C. Suire 72 M. Sukhanov 139 M. Suljic 32 V. Sumberia 91 S. Sumowidagdo 82 S. Swain 60 A. Szabo 12 I. Szarka 12 U. Tabassam 13 S.F. Taghavi 96 G. Taillepied 98 124 J. Takahashi 110 G.J. Tambave 20 S. Tang 124 6 Z. Tang 117 J.D. Tapia Takaki 115 N. Tapus 123 L.A. Tarasovicova 134 M.G. Tarzila 45 A. Tauro 32 A. Telesca 32 L. Terlizzi 24 C. Terrevoli 113 G. Tersimonov 3 S. Thakur 131 D. Thomas 107 R. Tieulent 125 A. Tikhonov 139 A.R. Timmins 113 M. Tkacik 105 T. Tkacik 105 A. Toia 63 N. Topilskaya 139 M. Toppi 48 F. Torales-Acosta 18 T. Tork 72 A.G. Torres Ramos 31 A. Trifiró 30 52 A.S. Triolo 30 52 S. Tripathy 50 T. Tripathy 46 S. Trogolo 32 V. Trubnikov 3 W.H. Trzaska 114 T.P. Trzcinski 132 R. Turrisi 53 T.S. Tveter 19 K. Ullaland 20 B. Ulukutlu 96 A. Uras 125 M. Urioni 54 130 G.L. Usai 22 M. Vala 37 N. Valle 21 S. Vallero 55 L.V.R. van Doremalen 58 M. van Leeuwen 84 C.A. van Veen 95 R.J.G. van Weelden 84 P. Vande Vyvre 32 D. Varga 135 Z. Varga 135 M. Varga-Kofarago 135 M. Vasileiou 78 A. Vasiliev 139 O. Vázquez Doce 96 V. Vechernin 139 E. Vercellin 24 S. Vergara Limón 44 L. Vermunt 58 R. Vértesi 135 M. Verweij 58 L. Vickovic 33 Z. Vilakazi 120 O. Villalobos Baillie 100 G. Vino 49 A. Vinogradov 139 T. Virgili 28 V. Vislavicius 83 A. Vodopyanov 140 B. Volkel 32 M.A. Völkl 95 K. Voloshin 139 S.A. Voloshin 133 G. Volpe 31 B. von Haller 32 I. Vorobyev 96 N. Vozniuk 139 J. Vrláková 37 B. Wagner 20 C. Wang 39 D. Wang 39 M. Weber 102 A. Wegrzynek 32 F.T. Weiglhofer 38 S.C. Wenzel 32 J.P. Wessels 134 S.L. Weyhmiller 136 J. Wiechula 63 J. Wikne 19 G. Wilk 79 J. Wilkinson 98 G.A. Willems 134 B. Windelband 95 M. Winn 127 J.R. Wright 107 W. Wu 39 Y. Wu 117 R. Xu 6 A.K. Yadav 131 S. Yalcin 71 Y. Yamaguchi 93 K. Yamakawa 93 S. Yang 20 S. Yano 93 Z. Yin 6 I.-K. Yoo 16 J.H. Yoon 57 S. Yuan 20 A. Yuncu 95 V. Zaccolo 23 C. Zampolli 32 H.J.C. Zanoli 58 F. Zanone 95 N. Zardoshti 32 100 A. Zarochentsev 139 P. Závada 61 N. Zaviyalov 139 M. Zhalov 139 B. Zhang 6 S. Zhang 39 X. Zhang 6 Y. Zhang 117 M. Zhao 10 V. Zherebchevskii 139 Y. Zhi 10 N. Zhigareva 139 D. Zhou 6 Y. Zhou 83 J. Zhu 98 6 Y. Zhu 6 G. Zinovjev 3 I N. Zurlo 130 54 1 A.I. Alikhanyan National Science Laboratory (Yerevan Physics Institute) Foundation, Yerevan, Armenia A.I. Alikhanyan National Science Laboratory (Yerevan Physics Institute) Foundation Yerevan Armenia A.I. Alikhanyan National Science Laboratory (Yerevan Physics Institute) Foundation, Yerevan, Armenia 2 AGH University of Science and Technology, Cracow, Poland AGH University of Science and Technology Cracow Poland AGH University of Science and Technology, Cracow, Poland 3 Bogolyubov Institute for Theoretical Physics, National Academy of Sciences of Ukraine, Kiev, Ukraine Bogolyubov Institute for Theoretical Physics National Academy of Sciences of Ukraine Kiev Ukraine Bogolyubov Institute for Theoretical Physics, National Academy of Sciences of Ukraine, Kiev, Ukraine 4 Bose Institute, Department of Physics and Centre for Astroparticle Physics and Space Science (CAPSS), Kolkata, India Bose Institute Department of Physics Centre for Astroparticle Physics and Space Science (CAPSS) Kolkata India Bose Institute, Department of Physics and Centre for Astroparticle Physics and Space Science (CAPSS), Kolkata, India 5 California Polytechnic State University, San Luis Obispo, CA, United States California Polytechnic State University San Luis Obispo CA United States California Polytechnic State University, San Luis Obispo, California, United States 6 Central China Normal University, Wuhan, China Central China Normal University Wuhan China Central China Normal University, Wuhan, China 7 Centro de Aplicaciones Tecnológicas y Desarrollo Nuclear (CEADEN), Havana, Cuba Centro de Aplicaciones Tecnológicas y Desarrollo Nuclear (CEADEN) Havana Cuba Centro de Aplicaciones Tecnológicas y Desarrollo Nuclear (CEADEN), Havana, Cuba 8 Centro de Investigación y de Estudios Avanzados (CINVESTAV), Mexico City and Mérida, Mexico Centro de Investigación y de Estudios Avanzados (CINVESTAV) Mexico City and Mérida Mexico Centro de Investigación y de Estudios Avanzados (CINVESTAV), Mexico City and Mérida, Mexico 9 Chicago State University, Chicago, IL, United States Chicago State University Chicago IL United States Chicago State University, Chicago, Illinois, United States 10 China Institute of Atomic Energy, Beijing, China China Institute of Atomic Energy Beijing China China Institute of Atomic Energy, Beijing, China 11 Chungbuk National University, Cheongju, Republic of Korea Chungbuk National University Cheongju Republic of Korea Chungbuk National University, Cheongju, Republic of Korea 12 Comenius University Bratislava, Faculty of Mathematics, Physics and Informatics, Bratislava, Slovak Republic Comenius University Bratislava Faculty of Mathematics, Physics and Informatics Bratislava Slovak Republic Comenius University Bratislava, Faculty of Mathematics, Physics and Informatics, Bratislava, Slovak Republic 13 COMSATS University Islamabad, Islamabad, Pakistan COMSATS University Islamabad Islamabad Pakistan COMSATS University Islamabad, Islamabad, Pakistan 14 Creighton University, Omaha, NE, United States Creighton University Omaha NE United States Creighton University, Omaha, Nebraska, United States 15 Department of Physics, Aligarh Muslim University, Aligarh, India Department of Physics Aligarh Muslim University Aligarh India Department of Physics, Aligarh Muslim University, Aligarh, India 16 Department of Physics, Pusan National University, Pusan, Republic of Korea Department of Physics Pusan National University Pusan Republic of Korea Department of Physics, Pusan National University, Pusan, Republic of Korea 17 Department of Physics, Sejong University, Seoul, Republic of Korea Department of Physics Sejong University Seoul Republic of Korea Department of Physics, Sejong University, Seoul, Republic of Korea 18 Department of Physics, University of California, Berkeley, CA, United States Department of Physics University of California Berkeley CA United States Department of Physics, University of California, Berkeley, California, United States 19 Department of Physics, University of Oslo, Oslo, Norway Department of Physics University of Oslo Oslo Norway Department of Physics, University of Oslo, Oslo, Norway 20 Department of Physics and Technology, University of Bergen, Bergen, Norway Department of Physics and Technology University of Bergen Bergen Norway Department of Physics and Technology, University of Bergen, Bergen, Norway 21 Dipartimento di Fisica, Università di Pavia, Pavia, Italy Dipartimento di Fisica Università di Pavia Pavia Italy Dipartimento di Fisica, Università di Pavia, Pavia, Italy 22 Dipartimento di Fisica dell'Università and Sezione INFN, Cagliari, Italy Dipartimento di Fisica dell'Università Sezione INFN Cagliari Italy Dipartimento di Fisica dell'Università and Sezione INFN, Cagliari, Italy 23 Dipartimento di Fisica dell'Università and Sezione INFN, Trieste, Italy Dipartimento di Fisica dell'Università Sezione INFN Trieste Italy Dipartimento di Fisica dell'Università and Sezione INFN, Trieste, Italy 24 Dipartimento di Fisica dell'Università and Sezione INFN, Turin, Italy Dipartimento di Fisica dell'Università Sezione INFN Turin Italy Dipartimento di Fisica dell'Università and Sezione INFN, Turin, Italy 25 Dipartimento di Fisica e Astronomia dell'Università and Sezione INFN, Bologna, Italy Dipartimento di Fisica e Astronomia dell'Università Sezione INFN Bologna Italy Dipartimento di Fisica e Astronomia dell'Università and Sezione INFN, Bologna, Italy 26 Dipartimento di Fisica e Astronomia dell'Università and Sezione INFN, Catania, Italy Dipartimento di Fisica e Astronomia dell'Università Sezione INFN Catania Italy Dipartimento di Fisica e Astronomia dell'Università and Sezione INFN, Catania, Italy 27 Dipartimento di Fisica e Astronomia dell'Università and Sezione INFN, Padova, Italy Dipartimento di Fisica e Astronomia dell'Università Sezione INFN Padova Italy Dipartimento di Fisica e Astronomia dell'Università and Sezione INFN, Padova, Italy 28 Dipartimento di Fisica ‘E.R. Caianiello’ dell'Università and Gruppo Collegato INFN, Salerno, Italy Dipartimento di Fisica ‘E.R. Caianiello’ dell'Università Gruppo Collegato INFN Salerno Italy Dipartimento di Fisica ‘E.R. Caianiello’ dell'Università and Gruppo Collegato INFN, Salerno, Italy 29 Dipartimento DISAT del Politecnico and Sezione INFN, Turin, Italy Dipartimento DISAT del Politecnico Sezione INFN Turin Italy Dipartimento DISAT del Politecnico and Sezione INFN, Turin, Italy 30 Dipartimento di Scienze MIFT, Università di Messina, Messina, Italy Dipartimento di Scienze MIFT Università di Messina Messina Italy Dipartimento di Scienze MIFT, Università di Messina, Messina, Italy 31 Dipartimento Interateneo di Fisica ‘M. Merlin’ and Sezione INFN, Bari, Italy Dipartimento Interateneo di Fisica ‘M. Merlin’ Sezione INFN Bari Italy Dipartimento Interateneo di Fisica ‘M. Merlin’ and Sezione INFN, Bari, Italy 32 European Organization for Nuclear Research (CERN), Geneva, Switzerland European Organization for Nuclear Research (CERN) Geneva Switzerland European Organization for Nuclear Research (CERN), Geneva, Switzerland 33 Faculty of Electrical Engineering, Mechanical Engineering and Naval Architecture, University of Split, Split, Croatia Faculty of Electrical Engineering, Mechanical Engineering and Naval Architecture University of Split Split Croatia Faculty of Electrical Engineering, Mechanical Engineering and Naval Architecture, University of Split, Split, Croatia 34 Faculty of Engineering and Science, Western Norway University of Applied Sciences, Bergen, Norway Faculty of Engineering and Science Western Norway University of Applied Sciences Bergen Norway Faculty of Engineering and Science, Western Norway University of Applied Sciences, Bergen, Norway 35 Faculty of Nuclear Sciences and Physical Engineering, Czech Technical University in Prague, Prague, Czech Republic Faculty of Nuclear Sciences and Physical Engineering Czech Technical University in Prague Prague Czech Republic Faculty of Nuclear Sciences and Physical Engineering, Czech Technical University in Prague, Prague, Czech Republic 36 Faculty of Physics, Sofia University, Sofia, Bulgaria Faculty of Physics Sofia University Sofia Bulgaria Faculty of Physics, Sofia University, Sofia, Bulgaria 37 Faculty of Science, P.J. Šafárik University, Košice, Slovak Republic Faculty of Science P.J. Šafárik University Košice Slovak Republic Faculty of Science, P.J. Šafárik University, Košice, Slovak Republic 38 Frankfurt Institute for Advanced Studies, Johann Wolfgang Goethe-Universität Frankfurt, Frankfurt, Germany Frankfurt Institute for Advanced Studies Johann Wolfgang Goethe-Universität Frankfurt Frankfurt Germany Frankfurt Institute for Advanced Studies, Johann Wolfgang Goethe-Universität Frankfurt, Frankfurt, Germany 39 Fudan University, Shanghai, China Fudan University Shanghai China Fudan University, Shanghai, China 40 Gangneung-Wonju National University, Gangneung, Republic of Korea Gangneung-Wonju National University Gangneung Republic of Korea Gangneung-Wonju National University, Gangneung, Republic of Korea 41 Gauhati University, Department of Physics, Guwahati, India Gauhati University Department of Physics Guwahati India Gauhati University, Department of Physics, Guwahati, India 42 Helmholtz-Institut für Strahlen- und Kernphysik, Rheinische Friedrich-Wilhelms-Universität Bonn, Bonn, Germany Helmholtz-Institut für Strahlen- und Kernphysik Rheinische Friedrich-Wilhelms-Universität Bonn Bonn Germany Helmholtz-Institut für Strahlen- und Kernphysik, Rheinische Friedrich-Wilhelms-Universität Bonn, Bonn, Germany 43 Helsinki Institute of Physics (HIP), Helsinki, Finland Helsinki Institute of Physics (HIP) Helsinki Finland Helsinki Institute of Physics (HIP), Helsinki, Finland 44 High Energy Physics Group, Universidad Autónoma de Puebla, Puebla, Mexico High Energy Physics Group Universidad Autónoma de Puebla Puebla Mexico High Energy Physics Group, Universidad Autónoma de Puebla, Puebla, Mexico 45 Horia Hulubei National Institute of Physics and Nuclear Engineering, Bucharest, Romania Horia Hulubei National Institute of Physics and Nuclear Engineering Bucharest Romania Horia Hulubei National Institute of Physics and Nuclear Engineering, Bucharest, Romania 46 Indian Institute of Technology Bombay (IIT), Mumbai, India Indian Institute of Technology Bombay (IIT) Mumbai India Indian Institute of Technology Bombay (IIT), Mumbai, India 47 Indian Institute of Technology Indore, Indore, India Indian Institute of Technology Indore Indore India Indian Institute of Technology Indore, Indore, India 48 INFN, Laboratori Nazionali di Frascati, Frascati, Italy INFN, Laboratori Nazionali di Frascati Frascati Italy INFN, Laboratori Nazionali di Frascati, Frascati, Italy 49 INFN, Sezione di Bari, Bari, Italy INFN, Sezione di Bari Bari Italy INFN, Sezione di Bari, Bari, Italy 50 INFN, Sezione di Bologna, Bologna, Italy INFN, Sezione di Bologna Bologna Italy INFN, Sezione di Bologna, Bologna, Italy 51 INFN, Sezione di Cagliari, Cagliari, Italy INFN, Sezione di Cagliari Cagliari Italy INFN, Sezione di Cagliari, Cagliari, Italy 52 INFN, Sezione di Catania, Catania, Italy INFN, Sezione di Catania Catania Italy INFN, Sezione di Catania, Catania, Italy 53 INFN, Sezione di Padova, Padova, Italy INFN, Sezione di Padova Padova Italy INFN, Sezione di Padova, Padova, Italy 54 INFN, Sezione di Pavia, Pavia, Italy INFN, Sezione di Pavia Pavia Italy INFN, Sezione di Pavia, Pavia, Italy 55 INFN, Sezione di Torino, Turin, Italy INFN, Sezione di Torino Turin Italy INFN, Sezione di Torino, Turin, Italy 56 INFN, Sezione di Trieste, Trieste, Italy INFN, Sezione di Trieste Trieste Italy INFN, Sezione di Trieste, Trieste, Italy 57 Inha University, Incheon, Republic of Korea Inha University Incheon Republic of Korea Inha University, Incheon, Republic of Korea 58 Institute for Gravitational and Subatomic Physics (GRASP), Utrecht University/Nikhef, Utrecht, Netherlands Institute for Gravitational and Subatomic Physics (GRASP) Utrecht University/Nikhef Utrecht Netherlands Institute for Gravitational and Subatomic Physics (GRASP), Utrecht University/Nikhef, Utrecht, Netherlands 59 Institute of Experimental Physics, Slovak Academy of Sciences, Košice, Slovak Republic Institute of Experimental Physics Slovak Academy of Sciences Košice Slovak Republic Institute of Experimental Physics, Slovak Academy of Sciences, Košice, Slovak Republic 60 Institute of Physics, Homi Bhabha National Institute, Bhubaneswar, India Institute of Physics Homi Bhabha National Institute Bhubaneswar India Institute of Physics, Homi Bhabha National Institute, Bhubaneswar, India 61 Institute of Physics of the Czech Academy of Sciences, Prague, Czech Republic Institute of Physics of the Czech Academy of Sciences Prague Czech Republic Institute of Physics of the Czech Academy of Sciences, Prague, Czech Republic 62 Institute of Space Science (ISS), Bucharest, Romania Institute of Space Science (ISS) Bucharest Romania Institute of Space Science (ISS), Bucharest, Romania 63 Institut für Kernphysik, Johann Wolfgang Goethe-Universität Frankfurt, Frankfurt, Germany Institut für Kernphysik Johann Wolfgang Goethe-Universität Frankfurt Frankfurt Germany Institut für Kernphysik, Johann Wolfgang Goethe-Universität Frankfurt, Frankfurt, Germany 64 Instituto de Ciencias Nucleares, Universidad Nacional Autónoma de México, Mexico City, Mexico Instituto de Ciencias Nucleares Universidad Nacional Autónoma de México Mexico City Mexico Instituto de Ciencias Nucleares, Universidad Nacional Autónoma de México, Mexico City, Mexico 65 Instituto de Física, Universidade Federal do Rio Grande do Sul (UFRGS), Porto Alegre, Brazil Instituto de Física Universidade Federal do Rio Grande do Sul (UFRGS) Porto Alegre Brazil Instituto de Física, Universidade Federal do Rio Grande do Sul (UFRGS), Porto Alegre, Brazil 66 Instituto de Física, Universidad Nacional Autónoma de México, Mexico City, Mexico Instituto de Física Universidad Nacional Autónoma de México Mexico City Mexico Instituto de Física, Universidad Nacional Autónoma de México, Mexico City, Mexico 67 iThemba LABS, National Research Foundation, Somerset West, South Africa iThemba LABS National Research Foundation Somerset West South Africa iThemba LABS, National Research Foundation, Somerset West, South Africa 68 Jeonbuk National University, Jeonju, Republic of Korea Jeonbuk National University Jeonju Republic of Korea Jeonbuk National University, Jeonju, Republic of Korea 69 Johann-Wolfgang-Goethe Universität Frankfurt Institut für Informatik, Fachbereich Informatik und Mathematik, Frankfurt, Germany Johann-Wolfgang-Goethe Universität Frankfurt Institut für Informatik Fachbereich Informatik und Mathematik Frankfurt Germany Johann-Wolfgang-Goethe Universität Frankfurt Institut für Informatik, Fachbereich Informatik und Mathematik, Frankfurt, Germany 70 Korea Institute of Science and Technology Information, Daejeon, Republic of Korea Korea Institute of Science and Technology Information Daejeon Republic of Korea Korea Institute of Science and Technology Information, Daejeon, Republic of Korea 71 KTO Karatay University, Konya, Turkey KTO Karatay University Konya Turkey KTO Karatay University, Konya, Turkey 72 Laboratoire de Physique des 2 Infinis, Irène Joliot-Curie, Orsay, France Laboratoire de Physique des 2 Infinis, Irène Joliot-Curie Orsay France Laboratoire de Physique des 2 Infinis, Irène Joliot-Curie, Orsay, France 73 Laboratoire de Physique Subatomique et de Cosmologie, Université Grenoble-Alpes, CNRS-IN2P3, Grenoble, France Laboratoire de Physique Subatomique et de Cosmologie Université Grenoble-Alpes CNRS-IN2P3 Grenoble France Laboratoire de Physique Subatomique et de Cosmologie, Université Grenoble-Alpes, CNRS-IN2P3, Grenoble, France 74 Lawrence Berkeley National Laboratory, Berkeley, CA, United States Lawrence Berkeley National Laboratory Berkeley CA United States Lawrence Berkeley National Laboratory, Berkeley, California, United States 75 Lund University Department of Physics, Division of Particle Physics, Lund, Sweden Lund University Department of Physics Division of Particle Physics Lund Sweden Lund University Department of Physics, Division of Particle Physics, Lund, Sweden 76 Nagasaki Institute of Applied Science, Nagasaki, Japan Nagasaki Institute of Applied Science Nagasaki Japan Nagasaki Institute of Applied Science, Nagasaki, Japan 77 Nara Women's University (NWU), Nara, Japan Nara Women's University (NWU) Nara Japan Nara Women's University (NWU), Nara, Japan 78 National and Kapodistrian University of Athens, School of Science, Department of Physics , Athens, Greece National and Kapodistrian University of Athens School of Science Department of Physics Athens Greece National and Kapodistrian University of Athens, School of Science, Department of Physics , Athens, Greece 79 National Centre for Nuclear Research, Warsaw, Poland National Centre for Nuclear Research Warsaw Poland National Centre for Nuclear Research, Warsaw, Poland 80 National Institute of Science Education and Research, Homi Bhabha National Institute, Jatni, India National Institute of Science Education and Research Homi Bhabha National Institute Jatni India National Institute of Science Education and Research, Homi Bhabha National Institute, Jatni, India 81 National Nuclear Research Center, Baku, Azerbaijan National Nuclear Research Center Baku Azerbaijan National Nuclear Research Center, Baku, Azerbaijan 82 National Research and Innovation Agency - BRIN, Jakarta, Indonesia National Research and Innovation Agency - BRIN Jakarta Indonesia National Research and Innovation Agency - BRIN, Jakarta, Indonesia 83 Niels Bohr Institute, University of Copenhagen, Copenhagen, Denmark Niels Bohr Institute University of Copenhagen Copenhagen Denmark Niels Bohr Institute, University of Copenhagen, Copenhagen, Denmark 84 Nikhef, National institute for subatomic physics, Amsterdam, Netherlands Nikhef, National institute for subatomic physics Amsterdam Netherlands Nikhef, National institute for subatomic physics, Amsterdam, Netherlands 85 Nuclear Physics Group, STFC Daresbury Laboratory, Daresbury, United Kingdom Nuclear Physics Group STFC Daresbury Laboratory Daresbury United Kingdom Nuclear Physics Group, STFC Daresbury Laboratory, Daresbury, United Kingdom 86 Nuclear Physics Institute of the Czech Academy of Sciences, Husinec-Řež, Czech Republic Nuclear Physics Institute of the Czech Academy of Sciences Husinec-Řež Czech Republic Nuclear Physics Institute of the Czech Academy of Sciences, Husinec-Řež, Czech Republic 87 Oak Ridge National Laboratory, Oak Ridge, TN, United States Oak Ridge National Laboratory Oak Ridge TN United States Oak Ridge National Laboratory, Oak Ridge, Tennessee, United States 88 Ohio State University, Columbus, OH, United States Ohio State University Columbus OH United States Ohio State University, Columbus, Ohio, United States 89 Physics department, Faculty of science, University of Zagreb, Zagreb, Croatia Physics department Faculty of science, University of Zagreb Zagreb Croatia Physics department, Faculty of science, University of Zagreb, Zagreb, Croatia 90 Physics Department, Panjab University, Chandigarh, India Physics Department Panjab University Chandigarh India Physics Department, Panjab University, Chandigarh, India 91 Physics Department, University of Jammu, Jammu, India Physics Department University of Jammu Jammu India Physics Department, University of Jammu, Jammu, India 92 Physics Department, University of Rajasthan, Jaipur, India Physics Department University of Rajasthan Jaipur India Physics Department, University of Rajasthan, Jaipur, India 93 Physics Program and International Institute for Sustainability with Knotted Chiral Meta Matter (SKCM2), Hiroshima University, Hiroshima, Japan Physics Program and International Institute for Sustainability with Knotted Chiral Meta Matter (SKCM2) Hiroshima University Hiroshima Japan Physics Program and International Institute for Sustainability with Knotted Chiral Meta Matter (SKCM2), Hiroshima University, Hiroshima, Japan 94 Physikalisches Institut, Eberhard-Karls-Universität Tübingen, Tübingen, Germany Physikalisches Institut Eberhard-Karls-Universität Tübingen Tübingen Germany Physikalisches Institut, Eberhard-Karls-Universität Tübingen, Tübingen, Germany 95 Physikalisches Institut, Ruprecht-Karls-Universität Heidelberg, Heidelberg, Germany Physikalisches Institut Ruprecht-Karls-Universität Heidelberg Heidelberg Germany Physikalisches Institut, Ruprecht-Karls-Universität Heidelberg, Heidelberg, Germany 96 Physik Department, Technische Universität München, Munich, Germany Physik Department Technische Universität München Munich Germany Physik Department, Technische Universität München, Munich, Germany 97 Politecnico di Bari and Sezione INFN, Bari, Italy Politecnico di Bari Sezione INFN Bari Italy Politecnico di Bari and Sezione INFN, Bari, Italy 98 Research Division and ExtreMe Matter Institute EMMI, GSI Helmholtzzentrum für Schwerionenforschung GmbH, Darmstadt, Germany Research Division ExtreMe Matter Institute EMMI GSI Helmholtzzentrum für Schwerionenforschung GmbH Darmstadt Germany Research Division and ExtreMe Matter Institute EMMI, GSI Helmholtzzentrum für Schwerionenforschung GmbH, Darmstadt, Germany 99 Saha Institute of Nuclear Physics, Homi Bhabha National Institute, Kolkata, India Saha Institute of Nuclear Physics Homi Bhabha National Institute Kolkata India Saha Institute of Nuclear Physics, Homi Bhabha National Institute, Kolkata, India 100 School of Physics and Astronomy, University of Birmingham, Birmingham, United Kingdom School of Physics and Astronomy University of Birmingham Birmingham United Kingdom School of Physics and Astronomy, University of Birmingham, Birmingham, United Kingdom 101 Sección Física, Departamento de Ciencias, Pontificia Universidad Católica del Perú, Lima, Peru Sección Física Departamento de Ciencias Pontificia Universidad Católica del Perú Lima Peru Sección Física, Departamento de Ciencias, Pontificia Universidad Católica del Perú, Lima, Peru 102 Stefan Meyer Institut für Subatomare Physik (SMI), Vienna, Austria Stefan Meyer Institut für Subatomare Physik (SMI) Vienna Austria Stefan Meyer Institut für Subatomare Physik (SMI), Vienna, Austria 103 SUBATECH, IMT Atlantique, Nantes Université, CNRS-IN2P3, Nantes, France SUBATECH IMT Atlantique Nantes Université CNRS-IN2P3 Nantes France SUBATECH, IMT Atlantique, Nantes Université, CNRS-IN2P3, Nantes, France 104 Suranaree University of Technology, Nakhon Ratchasima, Thailand Suranaree University of Technology Nakhon Ratchasima Thailand Suranaree University of Technology, Nakhon Ratchasima, Thailand 105 Technical University of Košice, Košice, Slovak Republic Technical University of Košice Košice Slovak Republic Technical University of Košice, Košice, Slovak Republic 106 The Henryk Niewodniczanski Institute of Nuclear Physics, Polish Academy of Sciences, Cracow, Poland The Henryk Niewodniczanski Institute of Nuclear Physics Polish Academy of Sciences Cracow Poland The Henryk Niewodniczanski Institute of Nuclear Physics, Polish Academy of Sciences, Cracow, Poland 107 The University of Texas at Austin, Austin, TX, United States The University of Texas at Austin Austin TX United States The University of Texas at Austin, Austin, Texas, United States 108 Universidad Autónoma de Sinaloa, Culiacán, Mexico Universidad Autónoma de Sinaloa Culiacán Mexico Universidad Autónoma de Sinaloa, Culiacán, Mexico 109 Universidade de São Paulo (USP), São Paulo, Brazil Universidade de São Paulo (USP) São Paulo Brazil Universidade de São Paulo (USP), São Paulo, Brazil 110 Universidade Estadual de Campinas (UNICAMP), Campinas, Brazil Universidade Estadual de Campinas (UNICAMP) Campinas Brazil Universidade Estadual de Campinas (UNICAMP), Campinas, Brazil 111 Universidade Federal do ABC, Santo Andre, Brazil Universidade Federal do ABC Santo Andre Brazil Universidade Federal do ABC, Santo Andre, Brazil 112 University of Cape Town, Cape Town, South Africa University of Cape Town Cape Town South Africa University of Cape Town, Cape Town, South Africa 113 University of Houston, Houston, TX, United States University of Houston Houston TX United States University of Houston, Houston, Texas, United States 114 University of Jyväskylä, Jyväskylä, Finland University of Jyväskylä Jyväskylä Finland University of Jyväskylä, Jyväskylä, Finland 115 University of Kansas, Lawrence, KS, United States University of Kansas Lawrence KS United States University of Kansas, Lawrence, Kansas, United States 116 University of Liverpool, Liverpool, United Kingdom University of Liverpool Liverpool United Kingdom University of Liverpool, Liverpool, United Kingdom 117 University of Science and Technology of China, Hefei, China University of Science and Technology of China Hefei China University of Science and Technology of China, Hefei, China 118 University of South-Eastern Norway, Kongsberg, Norway University of South-Eastern Norway Kongsberg Norway University of South-Eastern Norway, Kongsberg, Norway 119 University of Tennessee, Knoxville, TN, United States University of Tennessee Knoxville TN United States University of Tennessee, Knoxville, Tennessee, United States 120 University of the Witwatersrand, Johannesburg, South Africa University of the Witwatersrand Johannesburg South Africa University of the Witwatersrand, Johannesburg, South Africa 121 University of Tokyo, Tokyo, Japan University of Tokyo Tokyo Japan University of Tokyo, Tokyo, Japan 122 University of Tsukuba, Tsukuba, Japan University of Tsukuba Tsukuba Japan University of Tsukuba, Tsukuba, Japan 123 University Politehnica of Bucharest, Bucharest, Romania University Politehnica of Bucharest Bucharest Romania University Politehnica of Bucharest, Bucharest, Romania 124 Université Clermont Auvergne, CNRS/IN2P3, LPC, Clermont-Ferrand, France Université Clermont Auvergne CNRS/IN2P3 LPC Clermont-Ferrand France Université Clermont Auvergne, CNRS/IN2P3, LPC, Clermont-Ferrand, France 125 Université de Lyon, CNRS/IN2P3, Institut de Physique des 2 Infinis de Lyon, Lyon, France Université de Lyon CNRS/IN2P3 Institut de Physique des 2 Infinis de Lyon Lyon France Université de Lyon, CNRS/IN2P3, Institut de Physique des 2 Infinis de Lyon, Lyon, France 126 Université de Strasbourg, CNRS, IPHC UMR 7178, F-67000 Strasbourg, France Université de Strasbourg CNRS IPHC UMR 7178 Strasbourg F-67000 France Université de Strasbourg, CNRS, IPHC UMR 7178, F-67000 Strasbourg, France, Strasbourg, France 127 Université Paris-Saclay Centre d'Etudes de Saclay (CEA), IRFU, Départment de Physique Nucléaire (DPhN), Saclay, France Université Paris-Saclay Centre d'Etudes de Saclay (CEA) IRFU Départment de Physique Nucléaire (DPhN) Saclay France Université Paris-Saclay Centre d'Etudes de Saclay (CEA), IRFU, Départment de Physique Nucléaire (DPhN), Saclay, France 128 Università degli Studi di Foggia, Foggia, Italy Università degli Studi di Foggia Foggia Italy Università degli Studi di Foggia, Foggia, Italy 129 Università del Piemonte Orientale, Vercelli, Italy Università del Piemonte Orientale Vercelli Italy Università del Piemonte Orientale, Vercelli, Italy 130 Università di Brescia, Brescia, Italy Università di Brescia Brescia Italy Università di Brescia, Brescia, Italy 131 Variable Energy Cyclotron Centre, Homi Bhabha National Institute, Kolkata, India Variable Energy Cyclotron Centre Homi Bhabha National Institute Kolkata India Variable Energy Cyclotron Centre, Homi Bhabha National Institute, Kolkata, India 132 Warsaw University of Technology, Warsaw, Poland Warsaw University of Technology Warsaw Poland Warsaw University of Technology, Warsaw, Poland 133 Wayne State University, Detroit, MI, United States Wayne State University Detroit MI United States Wayne State University, Detroit, Michigan, United States 134 Westfälische Wilhelms-Universität Münster, Institut für Kernphysik, Münster, Germany Westfälische Wilhelms-Universität Münster Institut für Kernphysik Münster Germany Westfälische Wilhelms-Universität Münster, Institut für Kernphysik, Münster, Germany 135 Wigner Research Centre for Physics, Budapest, Hungary Wigner Research Centre for Physics Budapest Hungary Wigner Research Centre for Physics, Budapest, Hungary 136 Yale University, New Haven, CT, United States Yale University New Haven CT United States Yale University, New Haven, Connecticut, United States 137 Yonsei University, Seoul, Republic of Korea Yonsei University Seoul Republic of Korea Yonsei University, Seoul, Republic of Korea 138 Zentrum für Technologie und Transfer (ZTT), Worms, Germany Zentrum für Technologie und Transfer (ZTT) Worms Germany Zentrum für Technologie und Transfer (ZTT), Worms, Germany 139 Affiliated with an institute covered by a cooperation agreement with CERN Affiliated with an institute covered by a cooperation agreement with CERN Affiliated with an institute covered by a cooperation agreement with CERN 140 Affiliated with an international laboratory covered by a cooperation agreement with CERN Affiliated with an international laboratory covered by a cooperation agreement with CERN Affiliated with an international laboratory covered by a cooperation agreement with CERN I Deceased. II Also at: Max-Planck-Institut für Physik, Munich, Germany. III Also at: Italian National Agency for New Technologies, Energy and Sustainable Economic Development (ENEA), Bologna, Italy. IV Also at: Dipartimento DET del Politecnico di Torino, Turin, Italy. V Also at: Department of Applied Physics, Aligarh Muslim University, Aligarh, India. VI Also at: Institute of Theoretical Physics, University of Wroclaw, Poland. VII Also at: An institution covered by a cooperation agreement with CERN. ⋆ E-mail address: alice-publications@cern.ch .Editor: M. Pierini Abstract This letter reports measurements which characterize the underlying event associated with hard scatterings at mid-pseudorapidity (| η | < 0.8 ) in pp, p–Pb and Pb–Pb collisions at centre-of-mass energy per nucleon pair, s NN = 5.02 TeV. The measurements are performed with ALICE at the LHC. Different multiplicity classes are defined based on the event activity measured at forward rapidities. The hard scatterings are identified by the leading particle defined as the charged particle with the largest transverse momentum (p T ) in the collision and having 8 < p T < 15 GeV/c . The p T spectra of associated particles (0.5 ≤ p T < 6 GeV/c ) are measured in different azimuthal regions defined with respect to the leading particle direction: toward, transverse, and away. The associated charged particle yields in the transverse region are subtracted from those of the away and toward regions. The remaining jet-like yields are reported as a function of the multiplicity measured in the transverse region. The measurements show a suppression of the jet-like yield in the away region and an enhancement of high-p T associated particles in the toward region in central Pb–Pb collisions, as compared to minimum-bias pp collisions. These observations are consistent with previous measurements that used two-particle correlations, and with an interpretation in terms of parton energy loss in a high-density quark gluon plasma. These yield modifications vanish in peripheral Pb–Pb collisions and are not observed in either high-multiplicity pp or p–Pb collisions. Data availability This manuscript has associated data in a HEPData repository at https://www.hepdata.net/ . 1 Introduction In proton-proton (pp) collisions, jets, originating from partonic scatterings with large momentum transfer, are accompanied by particles produced by initial- and final-state radiation (ISR and FSR, respectively), as well as, by a plethora of other mechanisms. These include proton break-up, and, in a scenario incorporating multi-parton interactions (MPI) [1,2] , several semi-hard parton-parton scatterings in a single pp collision. These jet-accompanying particles experimentally make up the underlying event (UE) and are commonly studied via azimuthal separations from the jets to minimise the influence of hard scatterings. The present study follows the strategy originally introduced by the CDF collaboration [3] . First, the leading charged particle in the event is found, i.e., the charged particle with the highest transverse momentum in the collision (p T trig ). Secondly, the associated particles (p T < p T trig ) are measured in three topological regions depending on their azimuthal angle relative to the leading particle, | Δ φ | = | φ assoc − φ trig | , see Fig. 1 . The toward region contains the primary jet within the acceptance of the detector, while the away region contains the back-scattered particles of the recoil jet [4] . In contrast, the transverse region is dominated by the underlying-event dynamics, but it also includes contributions from ISR and FSR [5] . The measurements performed at RHIC and LHC in small systems (pp, p–A, and d–A collisions) have shown for high particle multiplicities similar phenomena as were originally observed only in A–A collisions and have been attributed there to the formation of the strongly interacting quark gluon plasma [6,7] , namely, long range angular correlations and collectivity [8] . The origin of these effects in small systems is still an open question; on one hand, hydrodynamical calculations describe some aspects of the data [9] ; on the other hand, mechanisms like colour reconnection [10] , rope hadronisation [11] , and string shoving [12] can produce collective-like effects in Monte Carlo event generators such as PYTHIA 8 [13] . Thus, investigating pp collisions as a function of the charged particle multiplicity has become ever more pertinent [9,14–18] . The interpretation of the results from the analysis of high-multiplicity pp collisions is challenging due to the selection biases of the sample towards events in which partonic scatterings with large momentum transfer (hard scatterings) occurred. To mitigate this inherent bias, Martin et al. [19] suggested to use the charged-particle multiplicity in the transverse region (N ch T ) as a classifier of the activity in the collisions, since the correlation between N ch T and the hardest scattering in the collision is small. The ALICE collaboration has reported the first N ch T spectra measured in pp collisions at centre-of-mass energy, s = 13 TeV [20] . Event generators, such as PYTHIA 8 [13] and EPOS-LHC [21] , do not provide a good description of the measured distribution of the ratio N ch T /〈 N ch T 〉 , where 〈 N ch T 〉 is the event-averaged charged-particle multiplicity in the transverse region, underestimating in particular the number of collisions with large N ch T ( > 3 × 〈 N ch T 〉 ) . In the framework of MPI-based models, like those implemented in PYTHIA 8 and HERWIG 7 [22] , the probability for a hard scattering in the collision increases with decreasing impact parameterVIII VIII In event generators like PYTHIA 8 the impact parameter profile is described by an overlap matter distribution of the two incoming hadrons. between the colliding protons. Thus, requiring a high-p T particle (e.g., p T trig > 8 GeV/c ) in a given pp collision biases the selection of collisions towards those with a smaller impact parameter [23] , which in turn biases the selection towards pp collisions with more MPI [20] . This feature of the N ch T -based analysis is important for the isolation of potential MPI and colour reconnection effects, which according to PYTHIA 8, produce effects resembling collective behaviour [10] . By construction, MPI and colour reconnection effects are expected to be more relevant in the transverse region than in the away and toward regions [24] . It is worth mentioning that the MPI picture has been used to explain the p T spectra in p–Pb collisions and peripheral Pb–Pb collisions [25–27] . Studies, as a function of N ch T , are therefore important to the understanding of the effects observed in high-multiplicity pp collisions. Last but not least, measurements of UE observables are also important to tune event generators [28] that include hard partonic scatterings and MPI. This letter reports the inclusive charged-particle transverse momentum spectra in pp, p–Pb and Pb–Pb collisions at centre-of-mass energy per nucleon pair s NN = 5.02 TeV containing a high-p T leading particle within the kinematic intervals 8 ≤ p T trig < 15 GeV/c and | η | < 0.8 . This guarantees the selection of collisions in which the average activity in the transverse region is roughly flat as a function of p T trig [20] , and therefore, any additional selection on the charged particle multiplicity will only modulate the UE activity. The measurements are performed considering different event classes defined in terms of the multiplicity registered in the forward detectors. The p T spectra of associated charged particles (0.5 ≤ p T < 6 GeV/c and | η | < 0.8 ) are measured in the toward, away, and transverse regions as a function of the average charged particle multiplicity in the transverse region. To further investigate the possible modification of the particles produced in the hard scattering in pp, p–Pb, and Pb–Pb collisions, the p T distributions in the toward (d N ch t / d p T ) and away (d N ch a / d p T ) regions obtained after the subtraction of the p T spectra in the transverse region (d N ch T / d p T ) are also reported. The subtracted yields (d N ch st , sa / d p T ) are further normalised to those measured in minimum-bias (MB) pp collisions,(1) I X t , a ≡ ( d N ch t , a / d p T − d N ch T / d p T ) | X ( d N ch t , a / d p T − d N ch T / d p T ) | pp , MB = ( d N ch st , sa / d p T ) | X ( d N ch st , sa / d p T ) | pp , MB , where X indicates the collision system and the event multiplicity class. In this way, the hard process p T spectra in the toward and away regions are isolated, and thus allowing us to study possible modifications to the produced particles due to medium effects in high-multiplicity pp, p–Pb, and Pb–Pb collisions. In heavy-ion collisions, this ratio is sensitive to the same effects which were studied using the I AA quantity [29–31] , where jets produced in the early stage of the collision propagate through the hot and dense quark–gluon plasma. Their interaction with the coloured medium lead to parton-energy loss (jet quenching) [32] which, for example, results in the suppression of the charged-particle yield at high p T [33] , and the suppression of the high-p T yield in the away region [29,30] . It is worth mentioning that jet quenching effects have not been observed so far in small systems [33,34] . 2 Experiment and data analysis This analysis is based on the data recorded by the ALICE apparatus during the pp and Pb–Pb runs at s NN = 5.02 TeV in 2015, and the p–Pb run at s NN = 5.02 TeV in 2016. The present study uses the V0 detector, and the Silicon Pixel Detector (SPD) for triggering and background rejection. The V0 consists of two arrays of scintillating tiles placed on each side of the interaction point covering the full azimuthal acceptance and the pseudorapidity intervals of 2.8 < η < 5.1 (V0A) and − 3.7 < η < − 1.7 (V0C). The SPD is the innermost part of the Inner Tracking System (ITS) and it is the closest detector to the interaction point. It consists of two cylindrical silicon pixel layers at radial distances of 3.9 and 7.6 cm from the beam line and the pseudorapidity coverages of the two layers are | η | < 2 and | η | < 1.4 , respectively. The data were collected using a minimum-bias trigger, which required a signal in both V0A and V0C detectors. The offline event selection was optimised to reject beam-induced background in all collision systems by utilising the timing signals in the two V0 detectors. In Pb–Pb collisions, the beam-induced background is further suppressed by correlating the timing signals of the neutron zero degree calorimeters, which are positioned on both sides of the interaction point at 112.5 m distance along the beam axis [35] . The signals from the zero degree calorimeters are also used to suppress the contamination from electromagnetic interactions. This is performed by requesting the coincidence of the signals coming from both side zero degree calorimeters by which the background due to single nucleus electromagnetic dissociation processes is excluded. A criterion based on the offline reconstruction of multiple primary vertices in the SPD is applied to reduce the pileup caused by multiple interactions in the same bunch crossing [36] . The results presented in this letter are for minimum-bias triggered pp collisions having at least one charged particle in the pseudorapidity interval | η | < 1 (INEL>0). The INEL>0 event class corresponds to about 75% of the total inelastic cross section [37] . For pp and Pb–Pb collisions, the sample is subdivided into different multiplicity classes based on the total charge deposited in both V0 sub-detectors, which is termed as V0M amplitude [38] . For p–Pb collisions, the sample is subdivided based on the total charge deposited in V0A sub-detector (V0A amplitude) [39] , which is located in the Pb-going direction. The V0A estimator has been implemented in previous measurements that used p–Pb data (see e.g. [40] ). This allows for comparisons with other observables for similar V0A multiplicity classes. To ensure that a hard scattering took place in the collision, events are required to have a trigger particle within 8 ≤ p T trig < 15 GeV/c . In this p T trig interval, the momentum resolution effects are negligible on the extracted yields, and therefore, no p T trig resolution correction is applied. The total number of analysed collisions before the trigger particle selection are about 108 , 108 , and 107 for pp, p–Pb, and Pb–Pb collisions, respectively. The transverse momentum of particles is determined from measurements in the central barrel with the ITS and the Time Projection Chamber (TPC). The ITS is a tracking detector which consists of six cylindrical layers of silicon detectors. The TPC is a cylindrical drift detector which covers a radial distance of 85-247 cm from the beam axis and it has longitudinal dimension extending from about -250 cm to +250 cm around the nominal interaction point. Primary charged particles are measured in the pseudorapidity range of | η | < 0.8 and with p T > 0.5 GeV/c , where η is measured in the laboratory frame for the three collision systems. The configuration for p–Pb collisions with protons at 4 TeV energy colliding with Pb ions that have per-nucleon energies of (Z / A ) × 4 TeV ∼ 1.58 TeV results in a shift in the rapidity of the nucleon–nucleon centre-of-mass system by 0.465 in the direction of the proton beam (negative z-direction). Here Z and A are the atomic and mass numbers of the Pb ion, respectively. Therefore, the detector coverage | η | < 0.8 corresponds to roughly − 0.3 < | η cms | < 1.3 for p–Pb collisions. The particles with mean proper lifetime larger than 1 cm/c , which are either produced directly in the interaction or from decays of particles with mean proper lifetime smaller than 1 cm/c are termed as primary particles [41] . The track selection follows a procedure similar to the one described in Ref. [42] and only few specific details are reported here. Tracks (N tracks ) are required to have two hits in the ITS, out of which at least one should be in either of the two innermost layers. The geometrical track length L is calculated in the TPC readout plane, excluding the information from the pads at the sector boundaries (≈3 cm from the sector edges). The trajectory lengths built from radial segments, i.e. the crossed TPC pad rows, traversed in the TPC by a particle are required to be larger than 85% of the geometrical track length. The pad rows are made of at least 3 neighbouring individual observations (clusters), and their height varies from 7.5 mm to 15 mm [43] . The trajectory lengths built from clusters (one cluster per pad row) is required to be larger than 0.7 × L . The fraction of TPC clusters shared with another track is required to be lower than 0.4. The fit quality for the ITS and TPC track points must satisfy χ ITS 2 / N hits < 36 and χ TPC 2 / N clusters < 4 , respectively, where N hits and N clusters are the numbers of hits in the ITS and the number of clusters in the TPC, respectively. Only tracks with χ TPC − ITS 2 < 36 are included in the analysis, where χ TPC − ITS 2 is calculated comparing the track parameters from the combined ITS and TPC track reconstruction to that derived only from the TPC and constrained to the interaction point. The definition of χ TPC − ITS 2 can be found in Ref. [44] . To reduce the contamination from secondary particles, tracks are accepted if their distance-of-closest-approach (DCA) to the reconstructed primary interaction vertex satisfies in the longitudinal (d z ) and transverse (d xy ) directions the conditions d z < 2 cm and d xy < 0.018 cm + 0.035 (cm×GeV/c )/ p T . The measurement of the transverse momentum spectra of charged particles follows the standard procedure of the ALICE collaboration [42,45] . The raw yields are corrected for efficiency and contamination from secondary particles. The efficiency correction is calculated from Monte Carlo simulations with GEANT3 [46] transport code, which made use of PYTHIA 8 (Monash) [28] , EPOS-LHC [21] and HIJING [47] event generators for pp, p–Pb and Pb–Pb collisions, respectively and incorporated a detailed description of the detector material, geometry and response. Since the event generators do not reproduce the relative abundances of different particle species in the real data, the efficiency obtained from the simulations is re-weighted considering the particle composition from data as outlined in [42] . A multi-component template fit based on the DCA distributions from the simulation is used for the estimation of secondary contamination [42] . The p T spectra for the toward and away regions include contributions from the jet fragmentation, ISR, and FSR, as well as, the contribution from the underlying event. In order to increase the sensitivity to the hardest process of the event, the particle yields measured in the transverse region are subtracted from the corresponding yields in both the toward and away regions: d N ch t , a / d p T − d N ch T / d p T . This approach assumes that the background (UE, ISR, and FSR) in the toward and away regions is similar to the activity in the transverse region. However, one has to keep in mind that in Pb–Pb collisions two-particle correlations are affected by anisotropic transverse flow. In particular, the main contribution is due to the elliptic flow, v 2 , which is the second order coefficient in the Fourier expansion of the azimuthal distribution of the particle momenta [48] . This elliptic azimuthal anisotropy modulates the background according to:(2) B ( Δ φ ) = B 0 ( 1 + 2 V 2 cos ( 2 Δ φ ) ) , where V 2 is approximately given by the product of anisotropic flow coefficients for trigger and associated particles at their respective momenta i.e. V 2 ≈ v 2 trig v 2 assoc . The existing v 2 measurements over a broad transverse momentum range [49] suggest that the effect of the v 2 modulation of background should be more relevant in semi-central Pb–Pb collisions. The effect is expected to be important at low and intermediate transverse momenta and decreases for high transverse momentum particles [30] . In the high-p T region of interest for the jet quenching studies, namely p T > 4 GeV/c , the effect of the v 2 modulation is estimated to be small (about 5%) for Pb–Pb collisions. Given that the v 2 effect is larger in Pb–Pb collisions than in pp and p–Pb collisions, no correction for the v 2 modulation is applied for pp and p–Pb collisions since its effect is smaller than the other sources of systematic uncertainty. The results are shown as a function of the average number of charged particles in the transverse region 〈 N ch T 〉 . The values of 〈 N ch T 〉 are extracted in each multiplicity class from the N tracks distributions in the transverse region that are corrected for detector effects using a Bayesian unfolding [50] . The Bayesian unfolding requires the multiplicity response matrix, which is built from the correlation between the measured multiplicity and the multiplicity at generator level (without detector effects) in the transverse region. This has been obtained from MC simulations which include the propagation of particles through the detector using GEANT 3. As a crosscheck, the 〈 N ch T 〉 values are also calculated by integrating the transverse momentum distributions in the interval 0.5 ≤ p T < 8 GeV/c . The difference between the results from the two strategies is assigned as the systematic uncertainty on 〈 N ch T 〉 , where the effects related to the discrepancy between data and MC in the particle composition and secondary contamination are considered. This uncertainty amounts up to 3.5%, 4% and 6.5% for pp, p–Pb and Pb–Pb collisions, respectively. The systematic uncertainties related to the track selection criteria were studied by repeating the analysis varying one-by-one the track selection criteria [42,45] . In particular, the upper limits of the track fit quality parameters in the ITS (χ ITS 2 / N hits ) and in the TPC (χ TPC 2 / N clusters ) were varied in the ranges of 25–49 and 3–5, respectively. The maximum fraction of shared TPC clusters was varied between 0.2 to 1 and the maximum d z was varied between 1 and 5 cm [42] . We have also quantified the impact of not including the ITS hit requirement in the track selection. The systematic uncertainty on the primary particle composition was estimated using a procedure similar to the one described in [42] . To quantify the uncertainty due to the imperfect simulation of the detector response, the track matching between the TPC and the ITS information in the data and in the simulation were compared. To achieve this, the fraction of secondary particles was rescaled according to fits to the measured DCA distributions. After this rescaling, the agreement between data and model was found to be within 3% for all collision systems. This value was assigned as an additional systematic uncertainty [42] . The systematic uncertainty on the secondary particle contamination considers the imperfection of the method (multi-component template fit) used to extract the correction. The fit ranges were varied and the fit was repeated using templates with two (primaries, secondaries) or three (primaries, secondaries from material, secondaries from weak decays) components. The maximum spread among these variations was assigned as the systematic uncertainty on the secondary contamination. This contribution dominates at low p T . The density of materials used in simulations of the experimental setup was varied by ± 4.5% [35] , resulting in a negligible systematic uncertainty in the considered p T range of 0.5 to 6.0 GeV/c . For the estimation of total systematic uncertainty, all the above listed contributions were summed in quadrature. The systematic uncertainties are independent of the difference between the azimuthal angle of the associated particle and that of the trigger particle. The estimated systematic uncertainties on the p T spectra significantly depend on p T , while the dependence on the multiplicity classes is mild. The ranges of systematic uncertainties in the three considered collision systems are reported in Table 1 for the various sources described above. 3 Results and discussion The p T spectra measured in the transverse region for pp, p–Pb, and Pb–Pb collisions are shown in Fig. 2 (top panel). Results are presented for different multiplicity classes. The ratios between the spectra in the individual multiplicity classes and the MB (0−100%) one are shown in the bottom panel. In the p T range 0.5 − 6 GeV/c , the ratios for the highest multiplicity class (0−5%) are larger than unity and show an increasing trend with increasing p T at low p T (< 2 − 3 GeV/c ) followed at higher p T by a slow decrease. Instead, for the lowest multiplicity classes (40−60% and 60−90%) the ratios are lower than unity and follow an opposite trend with p T , decreasing at low p T and increasing for p T > 3 GeV/c . The behaviour of the ratios as a function of the event activity is reminiscent of analogous ratios as a function of the number of MPI in pp collisions simulated with PYTHIA 8, including colour reconnection [51] . In particular, at p T ≈ 2 − 3 GeV/c the p T spectrum of pp collisions with large MPI activity exhibits an enhancement with respect to the p T spectrum of MB pp collisions. The effect was not observed before in data because, in contrast to the present analysis, the jet contribution was included in the p T spectra [45] . The top (bottom) panel of Fig. 3 shows the charged particle yields for the toward (away) region after the subtraction of the yields measured in the transverse region in pp, p–Pb and Pb–Pb collisions. Results are compared with the p T spectra measured for MB pp collisions (0−100% V0M pp event class) quantified with the ratio I X t , a , as defined in Eq. (1) . At low transverse momenta, p T < 4 GeV/c , I X t , a is close to unity in pp and p–Pb collisions. In contrast, I X t , a in Pb–Pb collisions exhibits a strong multiplicity dependence over the whole measured p T interval. The I X t , a magnitude is larger for semi-peripheral Pb–Pb collisions, the maximum is observed for 20−40% Pb–Pb collisions, and is smaller for the most central and most peripheral classes. Given that the v 2 contribution is not subtracted from the jet-like yields reported in Fig. 3 , the centrality dependence of I X t , a follows the behaviour of v 2 as a function of collision centrality and particle p T in Pb–Pb collisions at LHC energies [52] . Fig. 4 shows the measured values of I X t , a in the transverse momentum interval 4 < p T < 6 GeV/c as a function of the average multiplicity in the transverse region for all the multiplicity classes considered in pp, p–Pb and Pb–Pb collisions. The figure shows that, within uncertainties, the I X t , a values are close to unity for all the multiplicity classes measured in pp and p–Pb collisions. This indicates that effects induced by possible energy loss in these systems are not observed within uncertainties. This result is consistent with previous studies of nuclear modification factor [33] and hadron-jet recoil measurements [34] . By contrast, for Pb–Pb collisions the I X t , a values are compatible to unity for peripheral collisions, and show a gradual enhancement (reduction) with the increase in multiplicity for the toward (away) region. The behaviour is the same for the I X t , a values measured either assuming a flat background or a v 2 -modulated background. The v 2 -modulated background was estimated following the approach depicted in Eq. (2) and using the v 2 data reported in [49] . This behaviour is qualitatively similar to the di-hadron correlation results reported by the STAR and ALICE collaborations [29,30] . In Pb–Pb collisions, I X t provides information about the fragmenting jet leaving the medium, while on the away side, I X a reflects the survival probability of the recoiling parton during passage through the medium. Thus a suppression of I X a would indicate that fewer partons survive the passage through the medium and is expected from the strong in-medium energy loss. On the other hand, the enhancement observed in the toward region is also subject to medium effects. The ratio is sensitive to a) a possible change of the fragmentation functions, b) a possible modification of the quark to gluon jet ratio in the final state due to different coupling with medium, and c) a possible bias on the parton spectrum due to trigger particle selection. Moreover, given that I X t , a is sensitive to the same effects as I AA , the interpretation of the results is similar to that reported in [30] . It is likely that all three effects play a role [30] . A detailed quantification of the contribution of each effect is beyond the scope of the present paper.In order to get further insight into the effect, the measured I X t , a values are compared in Fig. 5 with model predictions. Following the similar treatment of the experimental data, for the models, the total sample is subdivided into different V0M classes and the 〈 N ch T 〉 is calculated for each class. For high-multiplicity pp collisions, although I X t , a is close to unity, a small trend with multiplicity is visible, which is not seen at similar multiplicities (20−90% V0A) in p–Pb data. To understand the source of these slight deviations from unity, the data are compared with the predictions from the PYTHIA 8 (Monash tune [28] ) and EPOS-LHC [21] event generators. In PYTHIA, the hadronization of quarks is simulated using the Lund string fragmentation model [53] . Various PYTHIA tunes have been developed through extensive comparison of Monte Carlo distributions with the minimum-bias data from different experiments. The Monash tune of PYTHIA 8 is tuned to LHC data and uses an updated set of hadronization parameters compared to the previous tunes [28] . EPOS-LHC is built on the Parton-Based Gribov Regge Theory. Utilising the colour exchange mechanism of string excitation, the model is tuned to LHC data [21] . In this model, a part of the collision system which has high parton densities becomes a “core” region that evolves hydrodynamically as a quark–gluon plasma and it is surrounded by a more dilute “corona” for which fragmentation occurs in the vacuum. The upper panel of Fig. 5 shows I X t , a for different multiplicity classes. The observed deviations from unity are reproduced by PYTHIA 8 for both the toward and away regions. Given that PYTHIA 8 does not incorporate any jet quenching mechanism, the origin of the effect in high 〈 N ch T 〉 collisions is related to a remaining bias towards harder fragmentation and more activity from initial and final state radiation [54] . These effects enhance the high-p T yield in the toward region, and produce a broadening in the away region [55] . The EPOS-LHC results in the away region are similar to both data and PYTHIA 8. However, for I X t EPOS-LHC exhibits a trend with a maximum at intermediate multiplicity and a reduction toward low and high multiplicities, which is not consistent with the measurements. The middle and bottom panels of Fig. 5 show I X t , a measured for p–Pb and Pb–Pb collisions, respectively. The data are compared to PYTHIA 8/Angantyr [56] and EPOS-LHC predictions. The Angantyr model in PYTHIA 8 extrapolates the dynamics from pp collisions to p–Pb and Pb–Pb collisions, generalising the formalism adopted for pp collisions by including a description of the nucleon positions within the colliding nuclei and utilising the Glauber model to calculate the number of interacting nucleons and binary nucleon–nucleon collisions. PYTHIA 8/Angantyr, which does not include jet quenching effects, predicts I X t , a values consistent with unity for all the multiplicity classes in Pb–Pb collisions. Whereas for p–Pb collisions I X a is consistent with unity, and I X t is slightly below unity. In EPOS-LHC, a certain p T cutoff is defined in such a way that, above this cutoff, a particle loses part of its momentum in the core but survives as an independent particle produced by a flux tube. Soft particles, which are below the p T cutoff, get completely absorbed and form the core. This sort of energy loss mechanism implemented in EPOS-LHC depends on the system size [21,57,58] . Fig. 5 (middle) shows that for p–Pb collisions, EPOS-LHC does not describe either the magnitude or the trend of the multiplicity dependence of the measured ratio in the toward region, I X t . However, the model is in reasonable agreement with data in the away region. For Pb–Pb collisions, EPOS-LHC predicts a significant enhancement of I X t , a for low 〈 N ch T 〉 ranges and deviates significantly from the experimental results. In summary, while the data from Pb–Pb collisions are in qualitative agreement with expectations from parton energy loss due to the presence of a hot and dense medium, pp and p–Pb data do not show any hint of medium effects in the multiplicity range which is reported. 4 Summary The transverse momentum spectra (0.5 ≤ p T < 6 GeV/c ) of primary charged particles in three azimuthal regions (toward, away and transverse) defined with respect to the direction of the particle with the highest transverse momentum in the event (8 ≤ p T trig < 15 GeV/c ) are reported. The spectra are studied in intervals of the multiplicity measured at forward pseudorapidities for pp, p–Pb, and Pb–Pb collisions at s NN = 5.02 TeV. The p T spectra in the transverse region are subtracted from those of the away and toward regions. This is based on the assumption that the transverse side provides a good estimation of the underlying event contribution in both the toward and away regions. However, for the interpretation of the results one has to keep in mind that v 2 modulates the background and this effect is important for semi-central Pb–Pb collisions and for p T > 4 GeV/c the effect is less than 5% in central and peripheral Pb–Pb collisions. Ratios to MB pp (I X t , a ), i.e., the multiplicity dependent yields normalised to the yield measured in MB pp collisions, are reported. At low transverse momentum (p T < 2 GeV/c ), within 20%, the I X t , a values are multiplicity independent for both the toward and away regions in pp and p–Pb collisions. In contrast, in Pb–Pb collisions for both toward and away regions the I X t , a values exhibit a centrality dependence which is expected given the residual presence of elliptic flow. In the highest transverse momentum interval (4 < p T < 6 GeV/c ), the I X t , a values in pp collisions are closer to unity but they exhibit a small reduction (increase) towards high V0 activity in pp collisions. This trend is well reproduced by PYTHIA 8. In the model, it is due to a selection bias towards pp collisions with harder fragmentation and larger activity from initial and final state radiation. For p–Pb collisions, within uncertainties, the I X t , a values are consistent with unity and do not show a multiplicity dependence. PYTHIA 8/Angantyr fairly describes I X a , but it underestimates by about 10% the I X t values in the low multiplicity classes (40−90% V0A event class). For Pb–Pb collisions, the I X t , a values are close to unity for peripheral collisions, and show a gradual increase (reduction) in the toward (away) region with increasing multiplicity. A similar observable, I AA , based on the per-trigger yield of associated particles in di-hadron correlation has been studied for central and peripheral Pb–Pb collisions at s NN = 2.76 TeV. The behaviour of I X t , a exhibits the same features as I AA : in central collisions, on the away-side, a suppression is observed as expected from strong in-medium energy loss. In the toward region, an enhancement is observed. PYTHIA 8/Angantyr predicts I X t , a ≈ 1 for all multiplicity intervals, and it does not reproduce the observed away-side suppression or toward-side enhancement. Generally, EPOS-LHC does not describe the measured I X t , a ratios. In summary, within the multiplicity reach reported in this paper, no jet quenching effects are observed in pp and p–Pb collisions within uncertainties. Further studies are required to extend the present analysis to higher multiplicities, which are currently limited by the event selection based on the forward V0 detector. The analysis of future pp and p–Pb collisions with much larger integrated luminosity may remove this limitation. Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgements The ALICE Collaboration would like to thank all its engineers and technicians for their invaluable contributions to the construction of the experiment and the CERN accelerator teams for the outstanding performance of the LHC complex. The ALICE Collaboration gratefully acknowledges the resources and support provided by all Grid centres and the Worldwide LHC Computing Grid (WLCG) collaboration. The ALICE Collaboration acknowledges the following funding agencies for their support in building and running the ALICE detector: A.I. Alikhanyan National Science Laboratory (Yerevan Physics Institute) Foundation (ANSL) , State Committee of Science and World Federation of Scientists (WFS) , Armenia; Austrian Academy of Sciences , Austrian Science Fund (FWF): [M 2467-N36 ] and Nationalstiftung für Forschung, Technologie und Entwicklung , Austria; Ministry of Communications and High Technologies, National Nuclear Research Center , Azerbaijan; Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq ), Financiadora de Estudos e Projetos (Finep), Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP ) and Universidade Federal do Rio Grande do Sul (UFRGS ), Brazil; Bulgarian Ministry of Education and Science , within the National Roadmap for Research Infrastructures 2020–2027 (object CERN), Bulgaria; Ministry of Education of China (MOEC), Ministry of Science & Technology of China (MSTC) and National Natural Science Foundation of China (NSFC), China; Ministry of Science and Education and Croatian Science Foundation , Croatia; Centro de Aplicaciones Tecnológicas y Desarrollo Nuclear (CEADEN), Cubaenergía , Cuba; Ministry of Education, Youth and Sports of the Czech Republic , Czech Republic; The Danish Council for Independent Research | Natural Sciences , the Villum Fonden and Danish National Research Foundation (DNRF), Denmark; Helsinki Institute of Physics (HIP), Finland; Commissariat à l'Energie Atomique (CEA ) and Institut National de Physique Nucléaire et de Physique des Particules (IN2P3) and Centre National de la Recherche Scientifique (CNRS), France; Bundesministerium für Bildung und Forschung (BMBF ) and GSI Helmholtzzentrum für Schwerionenforschung GmbH , Germany; General Secretariat for Research and Technology , Ministry of Education, Research and Religions, Greece; National Research, Development and Innovation Office , Hungary; Department of Atomic Energy Government of India (DAE ), Department of Science and Technology, Government of India (DST ), University Grants Commission , Government of India (UGC) and Council of Scientific and Industrial Research (CSIR), India; National Research and Innovation Agency - BRIN , Indonesia; Istituto Nazionale di Fisica Nucleare (INFN ), Italy; Japanese Ministry of Education, Culture, Sports, Science and Technology (MEXT) and Japan Society for the Promotion of Science (JSPS) KAKENHI, Japan; Consejo Nacional de Ciencia (CONACYT ) y Tecnología, through Fondo de Cooperación Internacional en Ciencia y Tecnología (FONCICYT) and Dirección General de Asuntos del Personal Académico (DGAPA), Mexico; Nederlandse Organisatie voor Wetenschappelijk Onderzoek (NWO), Netherlands; The Research Council of Norway , Norway; Commission on Science and Technology for Sustainable Development in the South (COMSATS), Pakistan; Pontificia Universidad Católica del Perú , Peru; Ministry of Education and Science , National Science Centre and WUT ID-UB , Poland; Korea Institute of Science and Technology Information and National Research Foundation of Korea (NRF), Republic of Korea; Ministry of Education and Scientific Research , Institute of Atomic Physics , Ministry of Research and Innovation and Institute of Atomic Physics and University Politehnica of Bucharest , Romania; Ministry of Education, Science, Research and Sport of the Slovak Republic , Slovakia; National Research Foundation of South Africa , South Africa; Swedish Research Council (VR) and Knut & Alice Wallenberg Foundation (KAW), Sweden; European Organization for Nuclear Research , Switzerland; Suranaree University of Technology (SUT), National Science and Technology Development Agency (NSTDA), Thailand Science Research and Innovation (TSRI) and National Science, Research and Innovation Fund (NSRF), Thailand; Turkish Energy, Nuclear and Mineral Research Agency (TENMAK), Turkey; National Academy of Sciences of Ukraine , Ukraine; Science and Technology Facilities Council (STFC), United Kingdom; National Science Foundation of the United States of America (NSF ) and United States Department of Energy , Office of Nuclear Physics (DOE NP), United States of America. In addition, individual groups or members have received support from: Marie Skłodowska Curie, Strong 2020 - Horizon 2020 , European Research Council (grant nos. 824093 , 896850 , 950692 ), European Union ; Academy of Finland (Center of Excellence in Quark Matter) (grant nos. 346327 , 346328 ), Finland; Programa de Apoyos para la Superación del Personal Académico , UNAM , Mexico. References [1] T. Sjöstrand M. van Zijl A multiple interaction model for the event structure in hadron collisions Phys. Rev. D 36 1987 2019 T. Sjöstrand and M. van Zijl, “A Multiple Interaction Model for the Event Structure in Hadron Collisions”, Phys. Rev. D 36 (1987) 2019. [2] P. Bartalini J.R. Gaunt Multiple Parton Interactions at the LHC, vol. 29 2019 WSP P. Bartalini and J. R. Gaunt, eds., Multiple Parton Interactions at the LHC, vol. 29. WSP, 2019. [3] CDF Collaboration T. Affolder Charged jet evolution and the underlying event in p p ¯ collisions at 1.8 TeV Phys. Rev. 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-]>PLB 138109 138109 S0370-2693(23)00443-4 10.1016/j.physletb.2023.138109 The Author(s) Theory Fig. 1 The minimal bounce action (16) for β = 0,±6 and various values of ξ and ξ ˜ . The horizontal line in brown indicates the bounce action in the absence of gravity, S 0 = 8π 2 /(3|λ (μ )|). The metric and Palatini cases are represented by the black-dashed and gray-dot-dashed curves, respectively. Fig. 1 Electroweak vacuum decay in metric-affine gravity Ioannis D. Gialamas ⁎ ioannis.gialamas@kbfi.ee Hardi Veermäe hardi.veermae@cern.ch Laboratory of High Energy and Computational Physics, National Institute of Chemical Physics and Biophysics, Rävala pst. 10, 10143, Tallinn, Estonia Laboratory of High Energy and Computational Physics National Institute of Chemical Physics and Biophysics Rävala pst. 10 Tallinn 10143 Estonia Laboratory of High Energy and Computational Physics, National Institute of Chemical Physics and Biophysics, Rävala pst. 10, 10143, Tallinn, Estonia ⁎ Corresponding author. Editor: R. Gregory Abstract We investigate the stability of the electroweak vacuum in metric-affine gravity in which the Standard Model Higgs boson can be non-minimally coupled to both the Ricci scalar and the Holst invariant. We find that vacuum stability is improved in this framework across a wide range of model parameters. Data availability No data was used for the research described in the article. 1 Introduction It is well known that the potential of the Higgs boson in the Standard Model (SM) is deeper at high energies than in the electroweak vacuum permitting its decay through quantum tunneling [1–4] . Although this does not invalidate the SM, the electroweak vacuum is predicted to be metastable in the absence of contributions from UV physics [5–13] . Coleman and De Luccia [14] were the first to delve into the matter of gravitational effects on vacuum decay. Subsequently, multiple studies of gravitational corrections have been performed [15–23] , along with discussions about the impact of black holes on the amplification or reduction of the vacuum decay rate [24–40] . In this letter, we extend the calculation of gravitational corrections to vacuum decay in the context of metric-affine gravity. There, in contrast to general relativity, the connection is taken to be an independent variable without the usual symmetries of the Levi-Civita one. As a result, the Riemann tensor does not possess the symmetries it has in the metric case, and thus the gravitational action should be extended by including an additional scalar curvature invariant - the Holst invariant. This term commonly appears in Loop Quantum Gravity [41] and has been studied in various branches of high energy physics, such as black hole thermodynamics [42,43] . Lately, its significance in inflationary cosmology [44–49] , and high energy physics phenomenology [50–54] , has garnered a great deal of attention. 2 The gravitational action Metric-affine theories of gravity treat the metric tensor g μ ν and the connection Γ λ μ ν as independent variables. This should be contrasted with the usual metric gravity that uses the Levi-Civita connection { μ ν λ } , which is completely determined by the metric. It is useful to decompose the connection Γ λ μ ν as(1) Γ λ μ ν ≡ { μ ν λ } + C λ μ ν , where C λ μ ν is dubbed the distortion tensor. The Riemann tensor R μ ν ρ σ is constructed from the connection Γ in the usual way and one can form two scalars that are linear on it. These are the Ricci scalar and the Holst invariant [55,56] (2) R ≡ R μ ν μ ν , R ˜ ≡ 1 2 ϵ μ ν ρ σ R μ ν ρ σ , where g is the determinant of the metric tensor and ϵ μ ν ρ σ is the totally antisymmetric tensor. In metric gravity, the Holst invariant vanishes identically due to the symmetries of the Riemann tensor. The most general action linear in the Riemann tensor and containing terms of at most dimension 4 has the form1 1 Natural units with c = ħ = 1 are used throughout this paper. (3) S = ∫ d 4 x − g [ R 2 f ( h ) + R ˜ 2 f ˜ ( h ) − 1 2 ( ∂ h ) 2 − V ( h ) ] , where V ( h ) is the Higgs potential,(4) f ( h ) = 1 / κ + ξ h 2 , f ˜ ( h ) = β / κ + ξ ˜ h 2 , are non-minimal couplings, β , ξ and ξ ˜ are constant couplings, and κ = 1 / M Pl 2 . The function 1 / f ˜ ( h ) can be thought of as a field-dependent Barbero-Immirzi parameter [56,57] . We will first work out the formalism without assuming a specific functional form of f , f ˜ , and V and suppress their arguments for notational brevity. In addition to the Ricci and Holst terms, metric-affine gravity permits the construction of 20 additional scalars with mass dimension 2 from torsion T ρ μ ν ≡ 2 Γ ρ [ μ ν ] and non-metricity Q ρ μ ν ≡ ∇ ρ g μ ν [58,59] . Although such terms are not considered in this work, our perturbative results can be straightforwardly extended to include them as will be explained below. We will also neglect couplings between the connection and fermions as they will only generate Planck-suppressed four-fermion and higher-order scalar-fermion interactions [47,51,53,60] and do not affect the leading order corrections to vacuum stability. We remark that the action (3) also appears in Einstein-Cartan gravity. A crucial distinction with the current case is that the Einstein-Cartan connection is decomposed using the Levi-Civita connection and torsion and, unlike in metric-affine gravity, the non-metricity is taken to be zero. Nevertheless, as we will show below, the metric-affine framework contains both the metric and Palatini formulations as limiting cases and the non-metricity may be taken to zero without loss of generality. In order to study bounce solutions, we construct the Euclidean action (3) by analytically continuing the Lorentzian signature ( − , + , + , + ) to the Euclidean one ( + , + , + , + ) . Then, to bring the action to a more conventional form, we will first integrate out the connection. To this aim, we will express the Ricci scalar and the Holst invariant in terms of the metric Ricci scalar R [ g ] and the distortion tensor,(5a) R = R + D μ C μ ν ν − D ν C μ μ ν + C μ μ λ C λ ν ν − C μ ν λ C λ μ ν , (5b) R ˜ = ϵ μ ν ρ σ ( D μ C ρ ν σ + C ρ μ λ C λ ν σ ) , where D denotes the covariant derivative of the Levi-Civita connection. Substituting Eq. (5a) and (5b) into the action (3) , yields2 2 Although the Holst term picks up an imaginary unit when continuing to Euclidean space, analogously to the CP violating topological term in Yang-Mills theory (e.g., see Ref. [61] ), its effect is negated due to the dependence of ϵ μ ν ρ σ ϵ μ ν ρ σ = sign ( g ) 4 ! on the sign of the metric determinant. (6) S E = ∫ d 4 x g [ − R 2 f + 1 2 ( ∂ h ) 2 + V − f 2 ( D μ C μ ν ν − D ν C μ μ ν + C μ μ λ C λ ν ν − C μ ν λ C λ μ ν ) − i f ˜ 2 ϵ μ ν ρ σ ( D μ C ρ ν σ + C ρ μ λ C λ ν σ ) ] . The distortion tensor obeys an algebraic non-homogeneous linear equation of motion. Thus, in order to integrate it out in full generality, it is sufficient to find a particular solution to this equation [47] . Such a solution is given by [49] (7) C μ ν ρ = 1 2 ( g ν μ ∂ ρ X − g ν ρ ∂ μ X − i ϵ μ ν ρ σ ∂ σ Y ) , where(8) f = e X cos ( Y ) , f ˜ = e X sin ( Y ) . This solution is metric compatible, i.e., Q ρ μ ν = − 2 C ( ν | ρ | μ ) = 0 , and has torsion T ρ ν μ = 2 C ρ [ ν μ ] . So, the theory is dynamically equivalent to the Einstein-Cartan theory. On the other hand, the particular solution (7) is not the general one because of the projective symmetry of the action, C ρ ν μ → C ρ ν μ + g ρ μ A ν , which can be used to induce the non-metricity Q ρ μ ν = − 2 g μ ν A ρ . In particular, the Palatini limit with f ˜ = 0 is obtained by choosing A ν = ∂ ν X / 2 . Substituting (7) in the action (6) gives(9) S E = ∫ d 4 x g [ − R 2 f + 1 2 K ( ∂ h ) 2 + V ] , where the contribution of the independent connection is now fully captured by the kinetic function3 3 The primes denote differentiation with respect to the argument of the function, that is, depending on the context, with respect to h or r . (10) K = 1 + 3 2 f f ˜ ′ 2 − 2 f ′ f ˜ f ˜ ′ − f f ′ 2 f 2 + f ˜ 2 . Note that the action (9) does not depend on the sign of f ˜ . For specific combinations of the involved functions, the general metric-affine theory interpolates continuously between the metric (K = 1 ) and the Palatini (K = 1 − ( 3 / 2 ) f ′ 2 / f ) theories. In accordance with Ref. [44] , we find that these scenarios correspond to(11a) f ˜ = c f 2 − 1 4 c , (metric) (11b) f ˜ = c f , (Palatini) with c a constant. As an important case, the metric formulation can be obtained in the limit in which the constant part of f ˜ is large. More specifically, without loss of generality we can consider f ˜ ( h ) = β / κ + f ˜ 1 ( h ) , where f ˜ 1 , f are arbitrary functions of h . If | β | / κ ≫ f ˜ 1 , f , then(12) K = 1 − 3 κ β f ′ f ˜ ′ + O ( κ β ) 2 , and thus the metric-affine theory approaches the purely metric theory when | β | → ∞ . With f , f ˜ given by (4) , the Palatini formulation corresponds to β = ξ ˜ / ξ , of which β = ξ ˜ = 0 is only a special case. Consequently, as β ranges from − ∞ → ξ ˜ / ξ → ∞ , the metric formulation is continuously deformed to the Palatini one and back. 3 Corrections to vacuum decay in metric-affine gravity To compute the minimal bounce action, we will look for O ( 4 ) symmetric solutions [62] , with the line element d s 2 = d r 2 + ρ 2 d Ω 3 2 , where d Ω 3 2 denotes the line element of the unit 3-sphere and the Higgs field depends only on the radial coordinate r , i.e. h = h ( r ) . In this background, the metric Ricci scalar reads R = 6 ( 1 − ρ ρ ″ − ρ ′ 2 ) / ρ 2 and the Euclidean action can be recast as(13) S E = 2 π 2 ∫ d r ρ 3 [ 3 f ρ ρ ″ + ρ ′ 2 − 1 ρ 2 + 1 2 K h ′ 2 + V ] . The bounce solution is determined by the following equations of motion4 4 The ρ equations of motion follow from the rr component of the Einstein equations, but can also be derived by varying the action (13) . (14a) ρ ′ 2 = 1 + ρ 2 3 f ( 1 2 K h ′ 2 − V − 3 ρ ′ ρ f ′ h ′ ) , (14b) h ″ = − 3 ρ ′ ρ h ′ + 1 K ( − 1 2 K ′ h ′ 2 + V ′ − R 2 f ′ ) . To obtain the bounce action, we will adopt the perturbative method proposed in Ref. [15] and look for solutions as a series in κ , i.e. (15a) h ( r ) = h 0 ( r ) + κ h 1 ( r ) + O ( κ 2 ) , (15b) ρ ( r ) = r + κ ρ 1 ( r ) + κ 2 ρ 2 ( r ) + O ( κ 3 ) , This approach is suitable when the gravitational corrections are relatively small, and, as we will demonstrate, this technique is adequate for elucidating the differences that emerge due to the inclusion of the Holst invariant. In a similar vein, the bounce action (13) can be expanded as(16) S E = S 0 + κ S 1 + O ( κ 2 ) . The leading order solution5 5 See also [63] for exact solutions for vacuum decay in Higgs-like unbounded potentials. h 0 ( r ) of (14) is the so-called Fubini instanton [64,65] (17) h 0 ( r ) = 2 | λ | 2 μ 1 + μ 2 r 2 , with μ being an arbitrary scale of the bounce. It solves the equation of motion in the absence of gravity (ρ 0 = r ) and with V ( h ) = λ h 4 / 4 assuming that the Higgs quartic coupling λ is constant and negative. The leading order contribution to the action (13) is(18) S 0 = 2 π 2 ∫ d r r 3 ( h 0 ′ 2 2 + V ( h 0 ) ) = 8 π 2 3 | λ | , and gives the bounce action in the absence of gravity. To obtain the gravitationally corrected action one must account for the running of λ [17,19] . We will evaluate λ at the scale of the bounce μ and then minimize the action with respect to μ . The running of λ is computed at a 3-loop level [11] with the relevant parameters taken from [66] . Evaluating the gravitational correction S 1 relies on the specific form of f and f ˜ , for which we will assume the form (4) when needed. We will assume that the leading order gravitational corrections to the kinetic function (10) can be expressed as(19) K = 1 + κ K 1 h 2 + O ( κ 2 ) , where K 1 is a dimensionless constant. Indeed, with f and f ˜ given by (4) , we have that(20) K 1 ≡ − 6 ξ 2 + 2 β ξ ξ ˜ − ξ ˜ 2 1 + β 2 . Note that for large β we recover Eq. (12) . At order O ( κ ) , the equation of motion (14a) is(21) ρ 1 ′ = 1 6 r 2 ( 1 2 h 0 ′ 2 − V ( h 0 ) − 3 f ′ ( h 0 ) h 0 ′ ) , independently of the shape of f ˜ . For the Fubini bounce (17) , it is solved by(22) ρ 1 = 1 + 6 ξ 3 | λ | / μ 2 ( r μ 2 r 2 − 1 ( μ 2 r 2 + 1 ) 2 + μ − 1 arctan ( μ r ) ) . Knowing ρ 1 is sufficient to compute the O ( κ ) correction S 1 to the action. We checked explicitly that the dependence on h 1 can be eliminated by the h 0 equations of motion and partial integration. This is a general result, however, because h 0 minimizes the action in the absence of gravity and thus S 0 [ h 0 + κ h 1 ] = S 0 [ h 0 ] + O ( κ 2 ) [19] . In all, we obtain that(23) S 1 = 32 π 2 μ 2 45 λ 2 ( μ ) ( ( 1 + 6 ξ ) 2 + 6 K 1 ) = 32 π 2 μ 2 45 λ 2 ( μ ) ( ( 1 + 6 ξ ) 2 − 36 ξ 2 + 2 β ξ ξ ˜ − ξ ˜ 2 1 + β 2 ) , where K 1 encodes the modifications resulting from an independent connection, i.e. , setting K 1 = 0 recovers the metric case. The gravitational correction (23) can be negative for certain values of model parameters. If this happens, then the action cannot be minimized with respect to μ and the adopted perturbative approach is not applicable. However, by minimizing S 1 with respect to ξ , it is straightforward to show that S 1 is always positive when(24) β ξ ˜ ≥ 1 / 12 . Otherwise, the positivity of the gravitational correction S 1 can be achieved only in certain regions of the parameter space. Two special cases warrant being considered more closely:1. ξ ˜ = 0 : A minimally coupled Holst term f ˜ ( h ) = β / κ is probably the simplest scenario. The region allowed by the positivity of S 1 is(25) | ξ + 1 6 + 1 6 β 2 | ≥ 1 + β 2 6 β 2 . For β ≪ 1 , this gives ξ ≤ − 1 / ( 3 β 2 ) or ξ ≥ − 1 / 12 , while, when β ≫ 1 , only a narrow region around the conformal coupling ξ = − 1 / 6 is forbidden, that is, | ξ + 1 / 6 | ≥ 1 / ( 6 β ) .2. β = 0 : In this case, the contribution from the Holst term(26) S 1 = 32 π 2 μ 2 45 λ 2 ( μ ) ( 1 + 12 ξ + 36 ξ ˜ 2 ) , is always positive and will thus always improve the stability of the SM vacuum when compared to the Palatini case. This special case is depicted in the middle panel of Fig. 1 However, as in the Palatini limit, the positivity of S 1 implies a strict lower bound(27) ξ ≥ − 1 / 12 − 3 ξ ˜ 2 . The minimal bounce action is shown in Fig. 1 with the running of λ computed at the 3-loop level [11] . It shows that the metric limit β → ∞ is realized quite well already for | β | = 6 when ξ ˜ ≈ 0 . For ξ ˜ ≫ 1 , we see that the bounce action is typically enhanced when β and ξ have the opposite signs, thus improving the stability of the vacuum. Additionally, in comparison to the metric case, the stability is improved when | ξ | < | ξ ˜ | . In all depicted cases, the regions in which S 1 < 0 can be observed: When β = 6 , this region exists only for the ξ ˜ = 0 line and is contained in a narrow range around ξ = − 37 / 216 . In the β = − 6 case, a parameter region with S 1 < 0 can be observed for every ξ ˜ . The disallowed ξ range varies with ξ ˜ . Since the theory is independent of the sign of f ˜ , then the β = 6 panel covers the β = − 6 case with ξ ˜ ∈ [ − 100 , 0 ] and vice versa. Finally, it is important to point out that, as in the action (9) , the contributions from mass dimension 2 terms constructed from torsion and non-metricity that can appear in metric-affine gravity can be reduced to a non-canonical kinetic term in a metric theory [59] . This implies that our results can be straightforwardly extended to include corrections to vacuum stability from such terms by computing their contribution to the small κ expansion (19) . 4 Conclusions We analyzed the stability of the electroweak vacuum in metric-affine gravity, where the Higgs boson is expected to have an additional non-minimal coupling to the Holst invariant. This scenario can be reformulated in terms of an equivalent metric theory with a non-canonical kinetic term, where the gravitational corrections to the bounce action can be studied with established perturbative methods. Our results show that the stability of the electroweak vacuum in metric-affine gravity is improved across a wide range of model parameters. A non-minimally coupled Holst term provides a class of models that continuously connects metric and Palatini gravity. We find that the limiting case of Palatini gravity displays the mildest improvement to vacuum stability. Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgements We thank Tomi Koivisto for useful comments. This work was supported by the Estonian Research Council grants SJD18 and PSG869 . References [1] S.R. Coleman The fate of the false vacuum. 1. Semiclassical theory Phys. Rev. D 15 1977 2929 2936 Erratum: Phys. Rev. D 16 1977 1248 S. R. Coleman, The Fate of the False Vacuum. 1. Semiclassical Theory, Phys. Rev. D 15 (1977) 2929–2936.; Erratum: Phys.Rev.D 16, 1248 (1977). [2] P.B. Arnold Can the electroweak vacuum be unstable? Phys. Rev. D 40 1989 613 P. B. Arnold, Can the Electroweak Vacuum Be Unstable? Phys. Rev. D 40 (1989) 613. [3] M. Sher Electroweak Higgs potentials and vacuum stability Phys. Rep. 179 1989 273 418 M. Sher, Electroweak Higgs Potentials and Vacuum Stability, Phys. Rept. 179 (1989) 273–418. [4] P.B. 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+]>PLB 138109 138109 S0370-2693(23)00443-4 10.1016/j.physletb.2023.138109 The Author(s) Theory Fig. 1 The minimal bounce action (16) for β = 0,±6 and various values of ξ and ξ ˜ . The horizontal line in brown indicates the bounce action in the absence of gravity, S 0 = 8π 2 /(3|λ (μ )|). The metric and Palatini cases are represented by the black-dashed and gray-dot-dashed curves, respectively. Fig. 1 Electroweak vacuum decay in metric-affine gravity Ioannis D. Gialamas ⁎ ioannis.gialamas@kbfi.ee Hardi Veermäe hardi.veermae@cern.ch Laboratory of High Energy and Computational Physics, National Institute of Chemical Physics and Biophysics, Rävala pst. 10, 10143, Tallinn, Estonia Laboratory of High Energy and Computational Physics National Institute of Chemical Physics and Biophysics Rävala pst. 10 Tallinn 10143 Estonia Laboratory of High Energy and Computational Physics, National Institute of Chemical Physics and Biophysics, Rävala pst. 10, 10143, Tallinn, Estonia ⁎ Corresponding author. Editor: R. Gregory Abstract We investigate the stability of the electroweak vacuum in metric-affine gravity in which the Standard Model Higgs boson can be non-minimally coupled to both the Ricci scalar and the Holst invariant. We find that vacuum stability is improved in this framework across a wide range of model parameters. Data availability No data was used for the research described in the article. 1 Introduction It is well known that the potential of the Higgs boson in the Standard Model (SM) is deeper at high energies than in the electroweak vacuum permitting its decay through quantum tunneling [1–4] . Although this does not invalidate the SM, the electroweak vacuum is predicted to be metastable in the absence of contributions from UV physics [5–13] . Coleman and De Luccia [14] were the first to delve into the matter of gravitational effects on vacuum decay. Subsequently, multiple studies of gravitational corrections have been performed [15–23] , along with discussions about the impact of black holes on the amplification or reduction of the vacuum decay rate [24–40] . In this letter, we extend the calculation of gravitational corrections to vacuum decay in the context of metric-affine gravity. There, in contrast to general relativity, the connection is taken to be an independent variable without the usual symmetries of the Levi-Civita one. As a result, the Riemann tensor does not possess the symmetries it has in the metric case, and thus the gravitational action should be extended by including an additional scalar curvature invariant - the Holst invariant. This term commonly appears in Loop Quantum Gravity [41] and has been studied in various branches of high energy physics, such as black hole thermodynamics [42,43] . Lately, its significance in inflationary cosmology [44–49] , and high energy physics phenomenology [50–54] , has garnered a great deal of attention. 2 The gravitational action Metric-affine theories of gravity treat the metric tensor g μ ν and the connection Γ λ μ ν as independent variables. This should be contrasted with the usual metric gravity that uses the Levi-Civita connection { μ ν λ } , which is completely determined by the metric. It is useful to decompose the connection Γ λ μ ν as(1) Γ λ μ ν ≡ { μ ν λ } + C λ μ ν , where C λ μ ν is dubbed the distortion tensor. The Riemann tensor R μ ν ρ σ is constructed from the connection Γ in the usual way and one can form two scalars that are linear on it. These are the Ricci scalar and the Holst invariant [55,56] (2) R ≡ R μ ν μ ν , R ˜ ≡ 1 2 ϵ μ ν ρ σ R μ ν ρ σ , where g is the determinant of the metric tensor and ϵ μ ν ρ σ is the totally antisymmetric tensor. In metric gravity, the Holst invariant vanishes identically due to the symmetries of the Riemann tensor. The most general action linear in the Riemann tensor and containing terms of at most dimension 4 has the form1 1 Natural units with c = ħ = 1 are used throughout this paper. (3) S = ∫ d 4 x − g [ R 2 f ( h ) + R ˜ 2 f ˜ ( h ) − 1 2 ( ∂ h ) 2 − V ( h ) ] , where V ( h ) is the Higgs potential,(4) f ( h ) = 1 / κ + ξ h 2 , f ˜ ( h ) = β / κ + ξ ˜ h 2 , are non-minimal couplings, β , ξ and ξ ˜ are constant couplings, and κ = 1 / M Pl 2 . The function 1 / f ˜ ( h ) can be thought of as a field-dependent Barbero-Immirzi parameter [56,57] . We will first work out the formalism without assuming a specific functional form of f , f ˜ , and V and suppress their arguments for notational brevity. In addition to the Ricci and Holst terms, metric-affine gravity permits the construction of 20 additional scalars with mass dimension 2 from torsion T ρ μ ν ≡ 2 Γ ρ [ μ ν ] and non-metricity Q ρ μ ν ≡ ∇ ρ g μ ν [58,59] . Although such terms are not considered in this work, our perturbative results can be straightforwardly extended to include them as will be explained below. We will also neglect couplings between the connection and fermions as they will only generate Planck-suppressed four-fermion and higher-order scalar-fermion interactions [47,51,53,60] and do not affect the leading order corrections to vacuum stability. We remark that the action (3) also appears in Einstein-Cartan gravity. A crucial distinction with the current case is that the Einstein-Cartan connection is decomposed using the Levi-Civita connection and torsion and, unlike in metric-affine gravity, the non-metricity is taken to be zero. Nevertheless, as we will show below, the metric-affine framework contains both the metric and Palatini formulations as limiting cases and the non-metricity may be taken to zero without loss of generality. In order to study bounce solutions, we construct the Euclidean action (3) by analytically continuing the Lorentzian signature ( − , + , + , + ) to the Euclidean one ( + , + , + , + ) . Then, to bring the action to a more conventional form, we will first integrate out the connection. To this aim, we will express the Ricci scalar and the Holst invariant in terms of the metric Ricci scalar R [ g ] and the distortion tensor,(5a) R = R + D μ C μ ν ν − D ν C μ μ ν + C μ μ λ C λ ν ν − C μ ν λ C λ μ ν , (5b) R ˜ = ϵ μ ν ρ σ ( D μ C ρ ν σ + C ρ μ λ C λ ν σ ) , where D denotes the covariant derivative of the Levi-Civita connection. Substituting Eq. (5a) and (5b) into the action (3) , yields2 2 Although the Holst term picks up an imaginary unit when continuing to Euclidean space, analogously to the CP violating topological term in Yang-Mills theory (e.g., see Ref. [61] ), its effect is negated due to the dependence of ϵ μ ν ρ σ ϵ μ ν ρ σ = sign ( g ) 4 ! on the sign of the metric determinant. (6) S E = ∫ d 4 x g [ − R 2 f + 1 2 ( ∂ h ) 2 + V − f 2 ( D μ C μ ν ν − D ν C μ μ ν + C μ μ λ C λ ν ν − C μ ν λ C λ μ ν ) − i f ˜ 2 ϵ μ ν ρ σ ( D μ C ρ ν σ + C ρ μ λ C λ ν σ ) ] . The distortion tensor obeys an algebraic non-homogeneous linear equation of motion. Thus, in order to integrate it out in full generality, it is sufficient to find a particular solution to this equation [47] . Such a solution is given by [49] (7) C μ ν ρ = 1 2 ( g ν μ ∂ ρ X − g ν ρ ∂ μ X − i ϵ μ ν ρ σ ∂ σ Y ) , where(8) f = e X cos ( Y ) , f ˜ = e X sin ( Y ) . This solution is metric compatible, i.e., Q ρ μ ν = − 2 C ( ν | ρ | μ ) = 0 , and has torsion T ρ ν μ = 2 C ρ [ ν μ ] . So, the theory is dynamically equivalent to the Einstein-Cartan theory. On the other hand, the particular solution (7) is not the general one because of the projective symmetry of the action, C ρ ν μ → C ρ ν μ + g ρ μ A ν , which can be used to induce the non-metricity Q ρ μ ν = − 2 g μ ν A ρ . In particular, the Palatini limit with f ˜ = 0 is obtained by choosing A ν = ∂ ν X / 2 . Substituting (7) in the action (6) gives(9) S E = ∫ d 4 x g [ − R 2 f + 1 2 K ( ∂ h ) 2 + V ] , where the contribution of the independent connection is now fully captured by the kinetic function3 3 The primes denote differentiation with respect to the argument of the function, that is, depending on the context, with respect to h or r . (10) K = 1 + 3 2 f f ˜ ′ 2 − 2 f ′ f ˜ f ˜ ′ − f f ′ 2 f 2 + f ˜ 2 . Note that the action (9) does not depend on the sign of f ˜ . For specific combinations of the involved functions, the general metric-affine theory interpolates continuously between the metric (K = 1 ) and the Palatini (K = 1 − ( 3 / 2 ) f ′ 2 / f ) theories. In accordance with Ref. [44] , we find that these scenarios correspond to(11a) f ˜ = c f 2 − 1 4 c , (metric) (11b) f ˜ = c f , (Palatini) with c a constant. As an important case, the metric formulation can be obtained in the limit in which the constant part of f ˜ is large. More specifically, without loss of generality we can consider f ˜ ( h ) = β / κ + f ˜ 1 ( h ) , where f ˜ 1 , f are arbitrary functions of h . If | β | / κ ≫ f ˜ 1 , f , then(12) K = 1 − 3 κ β f ′ f ˜ ′ + O ( κ β ) 2 , and thus the metric-affine theory approaches the purely metric theory when | β | → ∞ . With f , f ˜ given by (4) , the Palatini formulation corresponds to β = ξ ˜ / ξ , of which β = ξ ˜ = 0 is only a special case. Consequently, as β ranges from − ∞ → ξ ˜ / ξ → ∞ , the metric formulation is continuously deformed to the Palatini one and back. 3 Corrections to vacuum decay in metric-affine gravity To compute the minimal bounce action, we will look for O ( 4 ) symmetric solutions [62] , with the line element d s 2 = d r 2 + ρ 2 d Ω 3 2 , where d Ω 3 2 denotes the line element of the unit 3-sphere and the Higgs field depends only on the radial coordinate r , i.e. h = h ( r ) . In this background, the metric Ricci scalar reads R = 6 ( 1 − ρ ρ ″ − ρ ′ 2 ) / ρ 2 and the Euclidean action can be recast as(13) S E = 2 π 2 ∫ d r ρ 3 [ 3 f ρ ρ ″ + ρ ′ 2 − 1 ρ 2 + 1 2 K h ′ 2 + V ] . The bounce solution is determined by the following equations of motion4 4 The ρ equations of motion follow from the rr component of the Einstein equations, but can also be derived by varying the action (13) . (14a) ρ ′ 2 = 1 + ρ 2 3 f ( 1 2 K h ′ 2 − V − 3 ρ ′ ρ f ′ h ′ ) , (14b) h ″ = − 3 ρ ′ ρ h ′ + 1 K ( − 1 2 K ′ h ′ 2 + V ′ − R 2 f ′ ) . To obtain the bounce action, we will adopt the perturbative method proposed in Ref. [15] and look for solutions as a series in κ , i.e. (15a) h ( r ) = h 0 ( r ) + κ h 1 ( r ) + O ( κ 2 ) , (15b) ρ ( r ) = r + κ ρ 1 ( r ) + κ 2 ρ 2 ( r ) + O ( κ 3 ) , This approach is suitable when the gravitational corrections are relatively small, and, as we will demonstrate, this technique is adequate for elucidating the differences that emerge due to the inclusion of the Holst invariant. In a similar vein, the bounce action (13) can be expanded as(16) S E = S 0 + κ S 1 + O ( κ 2 ) . The leading order solution5 5 See also [63] for exact solutions for vacuum decay in Higgs-like unbounded potentials. h 0 ( r ) of (14) is the so-called Fubini instanton [64,65] (17) h 0 ( r ) = 2 | λ | 2 μ 1 + μ 2 r 2 , with μ being an arbitrary scale of the bounce. It solves the equation of motion in the absence of gravity (ρ 0 = r ) and with V ( h ) = λ h 4 / 4 assuming that the Higgs quartic coupling λ is constant and negative. The leading order contribution to the action (13) is(18) S 0 = 2 π 2 ∫ d r r 3 ( h 0 ′ 2 2 + V ( h 0 ) ) = 8 π 2 3 | λ | , and gives the bounce action in the absence of gravity. To obtain the gravitationally corrected action one must account for the running of λ [17,19] . We will evaluate λ at the scale of the bounce μ and then minimize the action with respect to μ . The running of λ is computed at a 3-loop level [11] with the relevant parameters taken from [66] . Evaluating the gravitational correction S 1 relies on the specific form of f and f ˜ , for which we will assume the form (4) when needed. We will assume that the leading order gravitational corrections to the kinetic function (10) can be expressed as(19) K = 1 + κ K 1 h 2 + O ( κ 2 ) , where K 1 is a dimensionless constant. Indeed, with f and f ˜ given by (4) , we have that(20) K 1 ≡ − 6 ξ 2 + 2 β ξ ξ ˜ − ξ ˜ 2 1 + β 2 . Note that for large β we recover Eq. (12) . At order O ( κ ) , the equation of motion (14a) is(21) ρ 1 ′ = 1 6 r 2 ( 1 2 h 0 ′ 2 − V ( h 0 ) − 3 f ′ ( h 0 ) h 0 ′ ) , independently of the shape of f ˜ . For the Fubini bounce (17) , it is solved by(22) ρ 1 = 1 + 6 ξ 3 | λ | / μ 2 ( r μ 2 r 2 − 1 ( μ 2 r 2 + 1 ) 2 + μ − 1 arctan ( μ r ) ) . Knowing ρ 1 is sufficient to compute the O ( κ ) correction S 1 to the action. We checked explicitly that the dependence on h 1 can be eliminated by the h 0 equations of motion and partial integration. This is a general result, however, because h 0 minimizes the action in the absence of gravity and thus S 0 [ h 0 + κ h 1 ] = S 0 [ h 0 ] + O ( κ 2 ) [19] . In all, we obtain that(23) S 1 = 32 π 2 μ 2 45 λ 2 ( μ ) ( ( 1 + 6 ξ ) 2 + 6 K 1 ) = 32 π 2 μ 2 45 λ 2 ( μ ) ( ( 1 + 6 ξ ) 2 − 36 ξ 2 + 2 β ξ ξ ˜ − ξ ˜ 2 1 + β 2 ) , where K 1 encodes the modifications resulting from an independent connection, i.e. , setting K 1 = 0 recovers the metric case. The gravitational correction (23) can be negative for certain values of model parameters. If this happens, then the action cannot be minimized with respect to μ and the adopted perturbative approach is not applicable. However, by minimizing S 1 with respect to ξ , it is straightforward to show that S 1 is always positive when(24) β ξ ˜ ≥ 1 / 12 . Otherwise, the positivity of the gravitational correction S 1 can be achieved only in certain regions of the parameter space. Two special cases warrant being considered more closely:1. ξ ˜ = 0 : A minimally coupled Holst term f ˜ ( h ) = β / κ is probably the simplest scenario. The region allowed by the positivity of S 1 is(25) | ξ + 1 6 + 1 6 β 2 | ≥ 1 + β 2 6 β 2 . For β ≪ 1 , this gives ξ ≤ − 1 / ( 3 β 2 ) or ξ ≥ − 1 / 12 , while, when β ≫ 1 , only a narrow region around the conformal coupling ξ = − 1 / 6 is forbidden, that is, | ξ + 1 / 6 | ≥ 1 / ( 6 β ) .2. β = 0 : In this case, the contribution from the Holst term(26) S 1 = 32 π 2 μ 2 45 λ 2 ( μ ) ( 1 + 12 ξ + 36 ξ ˜ 2 ) , is always positive and will thus always improve the stability of the SM vacuum when compared to the Palatini case. This special case is depicted in the middle panel of Fig. 1 However, as in the Palatini limit, the positivity of S 1 implies a strict lower bound(27) ξ ≥ − 1 / 12 − 3 ξ ˜ 2 . The minimal bounce action is shown in Fig. 1 with the running of λ computed at the 3-loop level [11] . It shows that the metric limit β → ∞ is realized quite well already for | β | = 6 when ξ ˜ ≈ 0 . For ξ ˜ ≫ 1 , we see that the bounce action is typically enhanced when β and ξ have the opposite signs, thus improving the stability of the vacuum. Additionally, in comparison to the metric case, the stability is improved when | ξ | < | ξ ˜ | . In all depicted cases, the regions in which S 1 < 0 can be observed: When β = 6 , this region exists only for the ξ ˜ = 0 line and is contained in a narrow range around ξ = − 37 / 216 . In the β = − 6 case, a parameter region with S 1 < 0 can be observed for every ξ ˜ . The disallowed ξ range varies with ξ ˜ . Since the theory is independent of the sign of f ˜ , then the β = 6 panel covers the β = − 6 case with ξ ˜ ∈ [ − 100 , 0 ] and vice versa. Finally, it is important to point out that, as in the action (9) , the contributions from mass dimension 2 terms constructed from torsion and non-metricity that can appear in metric-affine gravity can be reduced to a non-canonical kinetic term in a metric theory [59] . This implies that our results can be straightforwardly extended to include corrections to vacuum stability from such terms by computing their contribution to the small κ expansion (19) . 4 Conclusions We analyzed the stability of the electroweak vacuum in metric-affine gravity, where the Higgs boson is expected to have an additional non-minimal coupling to the Holst invariant. This scenario can be reformulated in terms of an equivalent metric theory with a non-canonical kinetic term, where the gravitational corrections to the bounce action can be studied with established perturbative methods. Our results show that the stability of the electroweak vacuum in metric-affine gravity is improved across a wide range of model parameters. A non-minimally coupled Holst term provides a class of models that continuously connects metric and Palatini gravity. We find that the limiting case of Palatini gravity displays the mildest improvement to vacuum stability. Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgements We thank Tomi Koivisto for useful comments. This work was supported by the Estonian Research Council grants SJD18 and PSG869 . References [1] S.R. Coleman The fate of the false vacuum. 1. Semiclassical theory Phys. Rev. D 15 1977 2929 2936 Erratum: Phys. Rev. D 16 1977 1248 S. R. Coleman, The Fate of the False Vacuum. 1. Semiclassical Theory, Phys. Rev. D 15 (1977) 2929–2936.; Erratum: Phys.Rev.D 16, 1248 (1977). [2] P.B. Arnold Can the electroweak vacuum be unstable? Phys. Rev. D 40 1989 613 P. B. Arnold, Can the Electroweak Vacuum Be Unstable? Phys. Rev. D 40 (1989) 613. [3] M. Sher Electroweak Higgs potentials and vacuum stability Phys. Rep. 179 1989 273 418 M. Sher, Electroweak Higgs Potentials and Vacuum Stability, Phys. Rept. 179 (1989) 273–418. [4] P.B. 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-PLB 138173 138173 S0370-2693(23)00507-5 10.1016/j.physletb.2023.138173 The Author(s) Astrophysics & Cosmology Quantum gravity, the cosmological constant, and parity transformation Michael Bishop a mibishop@mail.fresnostate.edu Peter Martin b kotor2@mail.fresnostate.edu Douglas Singleton b c dougs@mail.fresnostate.edu a Mathematics Department, California State University Fresno, Fresno, CA 93740, USA Mathematics Department California State University Fresno Fresno CA 93740 USA Mathematics Department, California State University Fresno, Fresno, CA 93740 b Physics Department, California State University Fresno, Fresno, CA 93740, USA Physics Department California State University Fresno Fresno CA 93740 USA Physics Department, California State University Fresno, Fresno, CA 93740 c Kavli Institute for Theoretical Physics, University of California Santa Barbara, Santa Barbara, CA 93106, USA Kavli Institute for Theoretical Physics University of California Santa Barbara Santa Barbara CA 93106 USA Kavli Institute for Theoretical Physics, University of California Santa Barbara, Santa Barbara, CA 93106, USA Editor: R. Gregory Abstract One of the leading issues in quantum field theory and cosmology is the mismatch between the observed and calculated values for the cosmological constant in Einstein's field equations of up to 120 orders of magnitude. In this paper, we discuss new methods to potentially bridge this chasm using the generalized uncertainty principle (GUP). We find that if quantum gravity GUP models are the solution to this puzzle, then it may require the gravitationally modified position operator undergoes a parity transformation at high energies. Data availability No data was used for the research described in the article. 1 Cosmological constant puzzle The cosmological constant problem is the naive mismatch by ∼ 120 orders of magnitude between the observed and a simple theoretical value for the cosmological constant in Einstein's field equations; we will clarify in what sense this 120 order of magnitude mismatch is misleading/naive below. Quantum gravity has long been advertised as a potential solution to various puzzles like this cosmological constant problem. In this paper, we lay out the reasoning that led us to the conclusion that if GUP models are to resolve the cosmological constant problem, they would require the gravitationally modified position operator to undergo a parity transformation at high energy scales or short distance scales. GUP models modify the Heisenberg uncertainty principle to obtain a minimum length scale; that is, a positive lower bound on the uncertainty in position. GUP models are not elegant, top-down theories of quantum gravity like string theory [1] or loop quantum gravity [2] , but they have the advantage of being easy to work with and provide a phenomenological window into quantum gravity. We will begin by briefly reviewing the salient features of the cosmological constant problem; a more complete discussion can be found in the excellent review by Weinberg [3] . We will adopt the notation and units of [3] . The cosmological constant, λ , in Einstein's field equations (G μ ν = − 8 π G T μ ν + λ g μ ν ) is equivalent to space-time having a constant energy density ρ v a c = λ 8 π G , and is representative of the expansion of space. The approach for the calculation of λ in QFT is to add up all the energies of the zero modes (vacuum modes) of quantum fields. The vacuum modes of quantum fields are given by 1 2 ħ ω p = 1 2 E p = 1 2 p → 2 + m 2 . Integrating over all possible momenta up to some cut-off, p c , yields the vacuum energy density(1) ρ v a c = ∫ 1 2 p → 2 + m 2 d 3 p ( 2 π ) 3 = 1 2 ∫ 0 p c 4 π ( 2 π ) 3 d p p 2 p 2 + m 2 ≈ p c 4 16 π 2 . Here, p = | p → | is the magnitude of the momentum. We use p and | p → | interchangeably throughout this work. The integral in (1) is divergent, so it needs to be cut off at some momentum/energy scale, p c , which is usually taken to be the Planck scale, i.e. p c ∼ ( 8 π G ) − 1 / 2 . Inserting this value of p c into (1) gives ρ v a c ≈ 2 × 10 71 GeV4 . The observed vacuum energy density [4] is about ρ v a c o b s ≈ 10 − 47 GeV4 , which is a difference of 118 orders of magnitude - a terrible disagreement between theory and observation. As mentioned in the opening paragraph this approximately 120 orders of magnitude mismatch is somewhat naive. First, the hard cut-off of p c in (1) , leading to a quartic dependence on p c , violates relativistic invariance which requires ρ = − p i.e. energy density equals the negative pressure. This was pointed in [5,6] , and these two works also show that if one uses dimensional regularization of integrals like (1) , one can restore relativistic invariance, and the energy density is no longer quartically dependent on cut-off scale, but has a logarithmic dependence. Finally, in [6] it was shown that by using dimensional regularization of integrals like those in (1) and using known Standard Model fields (specifically the top quark, W ± , Z 0 bosons and Higgs boson with an estimated mass of 150 GeV) gave a disagreement between ρ v a c o b s ≈ 10 − 47 GeV4 and the theoretical value of “only” 56 orders of magnitude. This is still terrible, but essentially cuts in half the order of magnitude disagreement, from ∼ 120 to ∼ 56. This estimate of [6] is more realistic since the only scale probed experimentally is the electroweak scale of ∼ 100 GeV. Using the Planck scale gives an overestimate of the disagreement. Nevertheless using either the electroweak scale or the Planck scale gives a huge discrepancy between the theoretical and observed value of the vacuum energy density. One of the standard ideas for addressing the cosmological constant problem is via unbroken supersymmetry 1 1 In unbroken supersymmetry particles and their superpartner particles have the same mass. (SUSY) models [3,7] . SUSY models have equal numbers of bosonic and fermionic fields/degrees of freedom. Since bosonic and fermionic vacuum modes have opposite signs, their vacuum energies cancel one another exactly in unbroken SUSY and one would have a natural explanation for a cosmological constant that is exactly zero. However, one does not want an exactly zero cosmological constant and SUSY is broken at least up to some high energy scale greater than Λ S U S Y > 10 3 GeV. But if one used the lower limit of Λ S U S Y = 10 3 GeV as the cut-off p c , then the disagreement between theory and experiment is ∼ 60 orders of magnitude. Therefore, simple SUSY models do not provide an answer to the cosmological constant puzzle. However, despite SUSY (either broken or unbroken) not providing a solution to the cosmological constant puzzle, it nevertheless has the feature that one obtains a small/zero cosmological constant by a cancellation of large positive and negative contributions. In the GUP approach to the cosmological constant problem presented here, we find that we can similarly get a small cosmological constant by having a modified position operator which flips parity at some large energy/momentum scale. This changing of the parity of the modified position operator leads to a cancellation of large positive and large negative contributions to the cosmological constant. Such violations of parity in the gravitational interaction were studied theoretically in [8] and recently experimental bounds have been placed on such parity violations in gravity [9] . 2 Quantum gravity via generalized uncertainty principle In this section, we will lay out the GUP approach to quantum gravity [10–19] . GUP models are phenomenological methods to quantize gravity which modify the canonical position and momentum operators and by extension their commutator. This leads to a modified Heisenberg uncertainty principle which gives another avenue to analyze how quantum gravity works at short distances and high energies. Inspired by [16] , we will only modify the position operator and keep the standard momentum operator:(2) X ˆ i = i ħ f ( | p → | ) ∂ ∂ p i and p ˆ j = p j ; the capitalized X ˆ i indicates modification. These operators have the modified commutator(3) [ X ˆ i , p ˆ j ] = i ħ δ i j f ( | p → | ) . In [16] , f ( | p → | ) = 1 + β | p → | 2 where β is a phenomenological parameter which sets the scale for quantum gravity. Generally, β is taken to be the Planck scale β ∼ l P l 2 ħ 2 . The modified commutator in (3) implies an uncertainty relationship Δ X Δ p ∼ 1 + β ( Δ p ) 2 which leads to a minimum length of Δ X ˆ j = ħ β at Δ p ˆ j = 1 β . Additionally, in order for position and momentum operators to be symmetric, i.e. ( 〈 ψ | p ˆ i ) | ϕ 〉 = 〈 ψ | ( p ˆ i | ϕ 〉 ) and ( 〈 ψ | X ˆ i ) | ϕ 〉 = 〈 ψ | ( X ˆ i | ϕ 〉 ) , the scalar product of this model must have the form(4) 〈 ψ | ϕ 〉 = ∫ − ∞ ∞ d 3 p f ( | p → | ) ψ ⁎ ( p ) ϕ ( p ) . The modification of the scalar product as given by (4) is for three dimensions; even in n dimensions one still has the same modifying factor for the momentum integration, d n p f ( | p → | ) . 3 Vacuum energy calculations with GUP With these modified operators and scalar products, one can modify the vacuum energy integration in (1) following [20] . In [20] , the authors calculate how the GUP modifies Liouville's theorem and the phase space volume, d 3 x d 3 p . The modified phase space associated with the model from (2) and (3) is(5) d 3 x d 3 p ( f ( | p → | ) ) 3 . Upon integrating out the volume, ∫ d n x → V , and after quantization, the phase space volume is given by(6) V d 3 p ( 2 π ħ ) 3 ( f ( | p → | ) ) 3 → V d 3 p ( 2 π ) 3 ( f ( | p → | ) ) 3 . In the last step in (6) , we set ħ → 1 to match the units of reference [3] . Using the modified phase space volume in (6) for the calculation of ρ v a c in (1) gives(7) ρ v a c = ∫ 1 2 p → 2 + m 2 d 3 p ( 2 π ) 3 ( f ( | p → | ) ) 3 . This is beneficial because the factor of ( f ( | p → | ) ) 3 makes the integrand integrable and makes the vacuum energy density finite without using a ‘by hand’ cut-off. Substituting f ( | p → | ) = ( 1 + β | p → | 2 ) from [16,20] into (7) , one finds that the GUP vacuum energy density is finite with ρ v a c ∝ β − 2 . However, β is typically taken to be of the Planck scale and this leads to the GUP vacuum energy density from (7) yielding the same value as the by hand cut-off value obtained from (1) . Unfortunately, nothing has been gained by exchanging the by hand cut-off for a GUP-inspired functional cut-off, as already noted in [20] . Moreover, the inner product used for the vacuum energy density in (7) does not preserve the symmetry of the modified position operator. Equations (5) , (6) , and (7) imply that integration over momentum should come with the factor f ( | p → | ) ) − 3 . This violates the symmetry requirement for the position operator ( 〈 ψ | x ˆ i ) | ϕ 〉 = 〈 ψ | ( x ˆ i | ϕ 〉 ) . In order for the position operator to be symmetric, one needs the factor f ( | p → | ) ) − 1 as in equation (4) . To resolve this conflict between (4) and (6) , we need to reconsider the spatial/volume calculation. In the transition from (5) to (6) , it is assumed that the spatial volume does not change, i.e. the spatial volume is ∫ d n x = V ; but since this approach utilizes the GUP, one would imagine that the introduction of a minimal length should change the calculation of volumes. We pair a single factor of f ( | p → | ) to go with the d 3 p integration, which would be consistent with the symmetry of the position operator required by (4) . The remaining two factors are paired with the volume integration to account for the introduction of a minimal length. The phase space integration with the spatial volume in spherical coordinates becomes [21] (8) ( d 3 x ( f ( | p → | ) ) 2 ) ( d 3 p f ( | p → | ) ) = ( r 2 d r d Ω ( f ( | p → | ) ) 2 ) ( d 3 p f ( | p → | ) ) . The finite length factor in (8) is r 2 , so that the spatial integration that takes into account the GUP minimal length is r 2 f ( | p → | ) ) 2 . Now, the modified momentum integration in (8) agrees with the requirement of symmetry of the position and momentum operators as given in (4) . The corrected GUP-modified vacuum energy is then(9) ρ v a c = ∫ 1 2 p → 2 + m 2 d 3 p ( 2 π ) 3 f ( | p → | ) . Whether we use (1) , (7) , or (9) to calculated ρ v a c , the discrepancy between the calculated and observed vacuum energy density is still enormous. This discrepancy is due to the fact that f ( | p → | ) is always positive. One possible resolution is to allow f ( | p → | ) to become negative at large momentum. With this modification, the vacuum energy density integral has a negative contribution which can bring the calculated vacuum energy density closer to the observed value. Generally, GUP models do not consider an f ( | p → | ) which can be negative, because this results in a parity flip of the position operator, i.e. ( X ˆ ) → − ( X ˆ ) , at large momentum. Admittedly having a parity flip like this is very unusual and may lead to difficulties coming up with a good physical interpretation. However, the weak interaction is known to violate parity, and additionally there are works that have examined parity violation in gravity. For example, reference [8] examined parity violation in gravity, and recently reference [9] placed experimental bounds on parity violation and time-reversal symmetry violation in gravity using spin-gravity interactions. Further in [22] a model of parity violation is constructed with potential signatures appearing in the cosmic microwave background (CMB). As another example references [23,24] attempt to solve the cosmological constant problem in a loop quantum gravity model with degenerate geometry, with an implied parity violation. Thus while there are certainly questions as to what a parity flip like ( X ˆ ) → − ( X ˆ ) means physically, there is work examining parity violation in gravity both theoretically and experimentally. In the next section we investigate the possibility that quantum gravity may lead to a modified position operator which changes sign/violates parity at some high energy/momentum scale leading to a small cosmological constant consistent with observations. 4 GUP cosmological constant and parity transformation In order to recover the standard position operator at low energy scales, the function f ( | p → | ) which modifies the position operator must satisfy f ( | p → | ) ≈ 1 when | p → | ≪ p M , where p M designates the momentum where the parity flip occurs. As mentioned previously, one expects f ( | p → | ) ≈ 1 up to, for example, the electroweak scale with | p → | ≈ 100 GeV. At this point the calculated ρ v a c is already ∼56 orders of magnitude larger than the observed vacuum energy density. As discussed in the previous section, if f ( | p → | ) is positive definite, then the integration from the electroweak scale upward will only make further positive contributions to ρ v a c , increasing the discrepancy between the calculated and observed vacuum energy density. The only way we can see to counter this large reserve of positive vacuum energy density is to have the vacuum density integrand in (9) become negative as the energy/momentum scale increases. This is conceptually similar to unbroken supersymmetry, where the positive contribution of bosonic zero modes balances the negative contribution of fermionic zero modes. An example of a GUP model which satisfies the above requirement is(10) X ˆ i = i ħ [ 1 − ( | p → | p M ) 2 ] − 1 exp ( 2 | p → | 2 p N 2 ) ∂ p i ; P ˆ i = p i , where we have introduced a second momentum scale, p N . We will see the need for this later, but p N should be of the order of p M . If we do an expansion to second order in | p → | / p M and, | p → | / p N we find that (10) is equivalent to the GUP model of reference [16] with X ˆ i ≈ i ħ ( 1 + β | p → | 2 ) ∂ p i and P ˆ i = p i , and β depending on p M and p N . The specific form of the modified position operator in (10) is driven by two constraints: (i) we want the modified operators in (10) to give a minimum length; (ii) we want the position operator to flip sign at some large momentum scale so that the vacuum energy density integral will have positive (at low momentum) and negative (at high momentum) contributions. This flipping of the sign of the position operator can be seen as a form of parity violation; which proposes that the gravitational interaction may violate parity, as is also the case for the weak interaction. Taking the modified operators from (10) and using them to calculate the vacuum energy density (9) , taking into account that f ( | p → | ) = [ 1 − ( | p → | p M ) 2 ] − 1 exp ( 2 | p → | 2 p N 2 ) , we get(11) ρ v a c = 1 2 ∫ p → 2 + m 2 [ 1 − ( | p → | p M ) 2 ] exp ( − 2 | p → | 2 p N 2 ) d 3 p ( 2 π ) 3 ≈ 1 4 π 2 ∫ 0 ∞ p 3 [ 1 − ( p p M ) 2 ] exp ( − 2 p 2 p N 2 ) d p = p N 4 32 π 2 ( 1 − p N 2 p M 2 ) . From the first line to the second, we have written | p → | simply as p , we have done the integration over the solid angle giving 4π , and we have assumed that m 2 c 2 is small compared to both p M 2 and p N 2 allowing us to use p 2 + m 2 ≈ p . From the last expression in (11) , we can see a balancing between the positive contribution p N 4 coming from the integration over low p , and the negative contribution p N 6 p M 2 coming from the integration over high p . This is reminiscent of the balancing of positive and negative contributions to the vacuum energy density in unbroken supersymmetric models. One can use (11) to “solve” the cosmological constant problem. Setting the calculated vacuum energy density in (11) to the observed vacuum energy density, one can solve for p M and obtainp M = p N ( 1 − 32 π 2 ρ v a c o b s p N 4 ) − 1 / 2 . If p N is at the Planck scale this implies ρ v a c o b s p N 4 ∼ 10 − 118 , which in turn leads to p M ≈ p N , and p M and p N are both of the Planck scale. Furthermore p M and p N do not necessarily need to be at the Planck scale to resolve the cosmological constant problem – one just needs ρ v a c o b s ≪ p N 4 , which can be obtained even with p N and p M at a much lower scale than the Planck scale. One still has the fine tuning problem of why the two scales should deviate from the condition p M = p N by such an incredibly small amount compared to either p M or p N . Nonetheless, this example illustrates how a negative contribution to the vacuum energy density, from high momentum, helps the GUP approach to the cosmological constant problem. One could ask if this model would be able to connect the present small value of ρ v a c o b s with a much larger value of the vacuum energy density required for an inflationary epoch in the very early Universe. Looking at the last line of (11) one can have a large ρ v a c if, in the early Universe, the two momentum scales satisfy p M > p N , but not p M ≈ p N . Then from this initial state the momentum scales would need to evolve toward p M ≈ p N to give the small observed vacuum energy density of the present Universe. 5 Summary and conclusions In this paper we examined how GUP models might address the cosmological constant problem. For GUP models given by modified position and momentum operators of the form (2) X ˆ i = i ħ f ( | p → | ) ∂ ∂ p i and p ˆ j = p j , there were two variants of the associated modified vacuum energy density given in (7) and (9) . These two expressions differed by the number of factors of 1 f ( | p → | ) . We argued for the validity of the expression in (9) over (7) , since only the former expression satisfied the requirement of symmetry of the modified position and momentum operators discussed around equation (4) . However, regardless of whether one used (7) or (9) to calculate ρ v a c , one still has essentially the same problem as the by-hand cut-off of (1) : the vacuum energy density was proportional the momentum cut-off scale to the fourth power, ρ v a c ∼ p c 4 , which for p c near the Planck scale (or even the electroweak scale) made ρ v a c too large. With the GUP modified energy density of either (7) or (9) ρ v a c , while formally finite, would nevertheless be proportional to the inverse square of the functional cut-off parameter β i.e. ρ v a c ∼ β − 2 , and if the scale of β were taken as the Planck scale, then one finds the same problem as using a by-hand cut-off of the formally infinite integral in (1) - the vacuum energy density from GUP models will have the same, large, order-of-magnitude disagreement compared to the observed vacuum energy density. We propose that the modified position operator changes sign at some momentum scale, p M , as in (10) . Then because of the link between the function f ( | p → | ) from the modified position operator in (2) , and how it changes the vacuum energy density in (9) , one finds that the positive contribution to ρ v a c from the integration below p M is balanced by negative contribution from above p M This balancing of large positive and negative contributions to the vacuum energy density is similar to unbroken SUSY, where a large, positive bosonic contribution is balanced by a large, negative fermionic contribution. Here, however this balancing of positive and negative contributions comes from the parity flip - the change in sign of the modified position operator at some momentum scale. Declaration of Competing Interest The authors declare the following financial interests/personal relationships which may be considered as potential competing interests: Douglas Singleton reports financial support was provided by National Science Foundation . Michael Bishop reports financial support was provided by California State University, Fresno . Douglas Singleton reports financial support was provided by National Science Foundation . Acknowledgement DS is supported by a 2023-2024 KITP Fellows Award. This research was supported in part by the National Science Foundation under Grant No. NSF PHY-1748958 . The work of MB and DS were supported through a Fresno State 2023-2024 RSCA grant. References [1] J. Polchinski String Theory Vol. 1: An Introduction to the Bosonic String and String Theory Vol. 2: Superstring Theory and Beyond 1998 Cambridge University Press Cambridge, UK J. Polchinski, String Theory Vol. 1: An Introduction to the Bosonic String and String Theory Vol. 2: Superstring Theory and Beyond (Cambridge University Press, Cambridge, UK 1998). [2] C. Rovelli Loop quantum gravity Living Rev. Relativ. 11 2008 5 C. Rovelli, Loop Quantum Gravity, Living Reviews in Relativity 11, 5 (2008). [3] S. Weinberg Rev. Mod. Phys. 61 1989 1 S. 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Mangano R.B. Mann Phys. Rev. D 52 1995 1108 A. Kempf, G. Mangano and R. B. Mann, Phys. Rev. D 52, 1108 (1995). [17] F. Scardigli Phys. Lett. B 452 1999 39 F. Scardigli, Phys. Lett. B, 452, 39 (1999). [18] R.J. Adler D.I. Santiago Mod. Phys. Lett. A 14 1999 1371 R. J. Adler, D. I. Santiago, Mod. Phys. Lett. A 14, 1371 (1999). [19] R.J. Adler P. Chen D.I. Santiago Gen. Relativ. Gravit. 33 2001 2101 R. J. Adler, P. Chen, and D. I. Santiago, Gen. Relativ. Grav. 33, 2101 (2001). [20] L.N. Chang D. Minic N. Okamura T. Takeuchi Phys. Rev. D 65 2002 125028 L. N. Chang, D. Minic, N. Okamura, and T. Takeuchi, Phys. Rev. D 65, 125028 (2002). [21] M. Bishop J. Contreras P. Martin D. Singleton Front. Astron. Space Sci. 9 2022 978898 M. Bishop, J. Contreras, P. Martin, and D. Singleton, Front. Astron. Space Sci. 9, 978898 (2022). [22] A. Lue L. Wang M. Kamionkowski Phys. Rev. Lett. 83 1999 1506 A. Lue, L. Wang, and M. Kamionkowski, Phys. Rev. Letts. 83, 1506 (1999). [23] S.H.S. Alexander G. 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+PLB 138173 138173 S0370-2693(23)00507-5 10.1016/j.physletb.2023.138173 The Author(s) Astrophysics & Cosmology Quantum gravity, the cosmological constant, and parity transformation Michael Bishop a mibishop@mail.fresnostate.edu Peter Martin b kotor2@mail.fresnostate.edu Douglas Singleton b c dougs@mail.fresnostate.edu a Mathematics Department, California State University Fresno, Fresno, CA 93740, USA Mathematics Department California State University Fresno Fresno CA 93740 USA Mathematics Department, California State University Fresno, Fresno, CA 93740 b Physics Department, California State University Fresno, Fresno, CA 93740, USA Physics Department California State University Fresno Fresno CA 93740 USA Physics Department, California State University Fresno, Fresno, CA 93740 c Kavli Institute for Theoretical Physics, University of California Santa Barbara, Santa Barbara, CA 93106, USA Kavli Institute for Theoretical Physics University of California Santa Barbara Santa Barbara CA 93106 USA Kavli Institute for Theoretical Physics, University of California Santa Barbara, Santa Barbara, CA 93106, USA Editor: R. Gregory Abstract One of the leading issues in quantum field theory and cosmology is the mismatch between the observed and calculated values for the cosmological constant in Einstein's field equations of up to 120 orders of magnitude. In this paper, we discuss new methods to potentially bridge this chasm using the generalized uncertainty principle (GUP). We find that if quantum gravity GUP models are the solution to this puzzle, then it may require the gravitationally modified position operator undergoes a parity transformation at high energies. Data availability No data was used for the research described in the article. 1 Cosmological constant puzzle The cosmological constant problem is the naive mismatch by ∼ 120 orders of magnitude between the observed and a simple theoretical value for the cosmological constant in Einstein's field equations; we will clarify in what sense this 120 order of magnitude mismatch is misleading/naive below. Quantum gravity has long been advertised as a potential solution to various puzzles like this cosmological constant problem. In this paper, we lay out the reasoning that led us to the conclusion that if GUP models are to resolve the cosmological constant problem, they would require the gravitationally modified position operator to undergo a parity transformation at high energy scales or short distance scales. GUP models modify the Heisenberg uncertainty principle to obtain a minimum length scale; that is, a positive lower bound on the uncertainty in position. GUP models are not elegant, top-down theories of quantum gravity like string theory [1] or loop quantum gravity [2] , but they have the advantage of being easy to work with and provide a phenomenological window into quantum gravity. We will begin by briefly reviewing the salient features of the cosmological constant problem; a more complete discussion can be found in the excellent review by Weinberg [3] . We will adopt the notation and units of [3] . The cosmological constant, λ , in Einstein's field equations (G μ ν = − 8 π G T μ ν + λ g μ ν ) is equivalent to space-time having a constant energy density ρ v a c = λ 8 π G , and is representative of the expansion of space. The approach for the calculation of λ in QFT is to add up all the energies of the zero modes (vacuum modes) of quantum fields. The vacuum modes of quantum fields are given by 1 2 ħ ω p = 1 2 E p = 1 2 p → 2 + m 2 . Integrating over all possible momenta up to some cut-off, p c , yields the vacuum energy density(1) ρ v a c = ∫ 1 2 p → 2 + m 2 d 3 p ( 2 π ) 3 = 1 2 ∫ 0 p c 4 π ( 2 π ) 3 d p p 2 p 2 + m 2 ≈ p c 4 16 π 2 . Here, p = | p → | is the magnitude of the momentum. We use p and | p → | interchangeably throughout this work. The integral in (1) is divergent, so it needs to be cut off at some momentum/energy scale, p c , which is usually taken to be the Planck scale, i.e. p c ∼ ( 8 π G ) − 1 / 2 . Inserting this value of p c into (1) gives ρ v a c ≈ 2 × 10 71 GeV4 . The observed vacuum energy density [4] is about ρ v a c o b s ≈ 10 − 47 GeV4 , which is a difference of 118 orders of magnitude - a terrible disagreement between theory and observation. As mentioned in the opening paragraph this approximately 120 orders of magnitude mismatch is somewhat naive. First, the hard cut-off of p c in (1) , leading to a quartic dependence on p c , violates relativistic invariance which requires ρ = − p i.e. energy density equals the negative pressure. This was pointed in [5,6] , and these two works also show that if one uses dimensional regularization of integrals like (1) , one can restore relativistic invariance, and the energy density is no longer quartically dependent on cut-off scale, but has a logarithmic dependence. Finally, in [6] it was shown that by using dimensional regularization of integrals like those in (1) and using known Standard Model fields (specifically the top quark, W ± , Z 0 bosons and Higgs boson with an estimated mass of 150 GeV) gave a disagreement between ρ v a c o b s ≈ 10 − 47 GeV4 and the theoretical value of “only” 56 orders of magnitude. This is still terrible, but essentially cuts in half the order of magnitude disagreement, from ∼ 120 to ∼ 56. This estimate of [6] is more realistic since the only scale probed experimentally is the electroweak scale of ∼ 100 GeV. Using the Planck scale gives an overestimate of the disagreement. Nevertheless using either the electroweak scale or the Planck scale gives a huge discrepancy between the theoretical and observed value of the vacuum energy density. One of the standard ideas for addressing the cosmological constant problem is via unbroken supersymmetry 1 1 In unbroken supersymmetry particles and their superpartner particles have the same mass. (SUSY) models [3,7] . SUSY models have equal numbers of bosonic and fermionic fields/degrees of freedom. Since bosonic and fermionic vacuum modes have opposite signs, their vacuum energies cancel one another exactly in unbroken SUSY and one would have a natural explanation for a cosmological constant that is exactly zero. However, one does not want an exactly zero cosmological constant and SUSY is broken at least up to some high energy scale greater than Λ S U S Y > 10 3 GeV. But if one used the lower limit of Λ S U S Y = 10 3 GeV as the cut-off p c , then the disagreement between theory and experiment is ∼ 60 orders of magnitude. Therefore, simple SUSY models do not provide an answer to the cosmological constant puzzle. However, despite SUSY (either broken or unbroken) not providing a solution to the cosmological constant puzzle, it nevertheless has the feature that one obtains a small/zero cosmological constant by a cancellation of large positive and negative contributions. In the GUP approach to the cosmological constant problem presented here, we find that we can similarly get a small cosmological constant by having a modified position operator which flips parity at some large energy/momentum scale. This changing of the parity of the modified position operator leads to a cancellation of large positive and large negative contributions to the cosmological constant. Such violations of parity in the gravitational interaction were studied theoretically in [8] and recently experimental bounds have been placed on such parity violations in gravity [9] . 2 Quantum gravity via generalized uncertainty principle In this section, we will lay out the GUP approach to quantum gravity [10–19] . GUP models are phenomenological methods to quantize gravity which modify the canonical position and momentum operators and by extension their commutator. This leads to a modified Heisenberg uncertainty principle which gives another avenue to analyze how quantum gravity works at short distances and high energies. Inspired by [16] , we will only modify the position operator and keep the standard momentum operator:(2) X ˆ i = i ħ f ( | p → | ) ∂ ∂ p i and p ˆ j = p j ; the capitalized X ˆ i indicates modification. These operators have the modified commutator(3) [ X ˆ i , p ˆ j ] = i ħ δ i j f ( | p → | ) . In [16] , f ( | p → | ) = 1 + β | p → | 2 where β is a phenomenological parameter which sets the scale for quantum gravity. Generally, β is taken to be the Planck scale β ∼ l P l 2 ħ 2 . The modified commutator in (3) implies an uncertainty relationship Δ X Δ p ∼ 1 + β ( Δ p ) 2 which leads to a minimum length of Δ X ˆ j = ħ β at Δ p ˆ j = 1 β . Additionally, in order for position and momentum operators to be symmetric, i.e. ( 〈 ψ | p ˆ i ) | ϕ 〉 = 〈 ψ | ( p ˆ i | ϕ 〉 ) and ( 〈 ψ | X ˆ i ) | ϕ 〉 = 〈 ψ | ( X ˆ i | ϕ 〉 ) , the scalar product of this model must have the form(4) 〈 ψ | ϕ 〉 = ∫ − ∞ ∞ d 3 p f ( | p → | ) ψ ⁎ ( p ) ϕ ( p ) . The modification of the scalar product as given by (4) is for three dimensions; even in n dimensions one still has the same modifying factor for the momentum integration, d n p f ( | p → | ) . 3 Vacuum energy calculations with GUP With these modified operators and scalar products, one can modify the vacuum energy integration in (1) following [20] . In [20] , the authors calculate how the GUP modifies Liouville's theorem and the phase space volume, d 3 x d 3 p . The modified phase space associated with the model from (2) and (3) is(5) d 3 x d 3 p ( f ( | p → | ) ) 3 . Upon integrating out the volume, ∫ d n x → V , and after quantization, the phase space volume is given by(6) V d 3 p ( 2 π ħ ) 3 ( f ( | p → | ) ) 3 → V d 3 p ( 2 π ) 3 ( f ( | p → | ) ) 3 . In the last step in (6) , we set ħ → 1 to match the units of reference [3] . Using the modified phase space volume in (6) for the calculation of ρ v a c in (1) gives(7) ρ v a c = ∫ 1 2 p → 2 + m 2 d 3 p ( 2 π ) 3 ( f ( | p → | ) ) 3 . This is beneficial because the factor of ( f ( | p → | ) ) 3 makes the integrand integrable and makes the vacuum energy density finite without using a ‘by hand’ cut-off. Substituting f ( | p → | ) = ( 1 + β | p → | 2 ) from [16,20] into (7) , one finds that the GUP vacuum energy density is finite with ρ v a c ∝ β − 2 . However, β is typically taken to be of the Planck scale and this leads to the GUP vacuum energy density from (7) yielding the same value as the by hand cut-off value obtained from (1) . Unfortunately, nothing has been gained by exchanging the by hand cut-off for a GUP-inspired functional cut-off, as already noted in [20] . Moreover, the inner product used for the vacuum energy density in (7) does not preserve the symmetry of the modified position operator. Equations (5) , (6) , and (7) imply that integration over momentum should come with the factor f ( | p → | ) ) − 3 . This violates the symmetry requirement for the position operator ( 〈 ψ | x ˆ i ) | ϕ 〉 = 〈 ψ | ( x ˆ i | ϕ 〉 ) . In order for the position operator to be symmetric, one needs the factor f ( | p → | ) ) − 1 as in equation (4) . To resolve this conflict between (4) and (6) , we need to reconsider the spatial/volume calculation. In the transition from (5) to (6) , it is assumed that the spatial volume does not change, i.e. the spatial volume is ∫ d n x = V ; but since this approach utilizes the GUP, one would imagine that the introduction of a minimal length should change the calculation of volumes. We pair a single factor of f ( | p → | ) to go with the d 3 p integration, which would be consistent with the symmetry of the position operator required by (4) . The remaining two factors are paired with the volume integration to account for the introduction of a minimal length. The phase space integration with the spatial volume in spherical coordinates becomes [21] (8) ( d 3 x ( f ( | p → | ) ) 2 ) ( d 3 p f ( | p → | ) ) = ( r 2 d r d Ω ( f ( | p → | ) ) 2 ) ( d 3 p f ( | p → | ) ) . The finite length factor in (8) is r 2 , so that the spatial integration that takes into account the GUP minimal length is r 2 f ( | p → | ) ) 2 . Now, the modified momentum integration in (8) agrees with the requirement of symmetry of the position and momentum operators as given in (4) . The corrected GUP-modified vacuum energy is then(9) ρ v a c = ∫ 1 2 p → 2 + m 2 d 3 p ( 2 π ) 3 f ( | p → | ) . Whether we use (1) , (7) , or (9) to calculated ρ v a c , the discrepancy between the calculated and observed vacuum energy density is still enormous. This discrepancy is due to the fact that f ( | p → | ) is always positive. One possible resolution is to allow f ( | p → | ) to become negative at large momentum. With this modification, the vacuum energy density integral has a negative contribution which can bring the calculated vacuum energy density closer to the observed value. Generally, GUP models do not consider an f ( | p → | ) which can be negative, because this results in a parity flip of the position operator, i.e. ( X ˆ ) → − ( X ˆ ) , at large momentum. Admittedly having a parity flip like this is very unusual and may lead to difficulties coming up with a good physical interpretation. However, the weak interaction is known to violate parity, and additionally there are works that have examined parity violation in gravity. For example, reference [8] examined parity violation in gravity, and recently reference [9] placed experimental bounds on parity violation and time-reversal symmetry violation in gravity using spin-gravity interactions. Further in [22] a model of parity violation is constructed with potential signatures appearing in the cosmic microwave background (CMB). As another example references [23,24] attempt to solve the cosmological constant problem in a loop quantum gravity model with degenerate geometry, with an implied parity violation. Thus while there are certainly questions as to what a parity flip like ( X ˆ ) → − ( X ˆ ) means physically, there is work examining parity violation in gravity both theoretically and experimentally. In the next section we investigate the possibility that quantum gravity may lead to a modified position operator which changes sign/violates parity at some high energy/momentum scale leading to a small cosmological constant consistent with observations. 4 GUP cosmological constant and parity transformation In order to recover the standard position operator at low energy scales, the function f ( | p → | ) which modifies the position operator must satisfy f ( | p → | ) ≈ 1 when | p → | ≪ p M , where p M designates the momentum where the parity flip occurs. As mentioned previously, one expects f ( | p → | ) ≈ 1 up to, for example, the electroweak scale with | p → | ≈ 100 GeV. At this point the calculated ρ v a c is already ∼56 orders of magnitude larger than the observed vacuum energy density. As discussed in the previous section, if f ( | p → | ) is positive definite, then the integration from the electroweak scale upward will only make further positive contributions to ρ v a c , increasing the discrepancy between the calculated and observed vacuum energy density. The only way we can see to counter this large reserve of positive vacuum energy density is to have the vacuum density integrand in (9) become negative as the energy/momentum scale increases. This is conceptually similar to unbroken supersymmetry, where the positive contribution of bosonic zero modes balances the negative contribution of fermionic zero modes. An example of a GUP model which satisfies the above requirement is(10) X ˆ i = i ħ [ 1 − ( | p → | p M ) 2 ] − 1 exp ( 2 | p → | 2 p N 2 ) ∂ p i ; P ˆ i = p i , where we have introduced a second momentum scale, p N . We will see the need for this later, but p N should be of the order of p M . If we do an expansion to second order in | p → | / p M and, | p → | / p N we find that (10) is equivalent to the GUP model of reference [16] with X ˆ i ≈ i ħ ( 1 + β | p → | 2 ) ∂ p i and P ˆ i = p i , and β depending on p M and p N . The specific form of the modified position operator in (10) is driven by two constraints: (i) we want the modified operators in (10) to give a minimum length; (ii) we want the position operator to flip sign at some large momentum scale so that the vacuum energy density integral will have positive (at low momentum) and negative (at high momentum) contributions. This flipping of the sign of the position operator can be seen as a form of parity violation; which proposes that the gravitational interaction may violate parity, as is also the case for the weak interaction. Taking the modified operators from (10) and using them to calculate the vacuum energy density (9) , taking into account that f ( | p → | ) = [ 1 − ( | p → | p M ) 2 ] − 1 exp ( 2 | p → | 2 p N 2 ) , we get(11) ρ v a c = 1 2 ∫ p → 2 + m 2 [ 1 − ( | p → | p M ) 2 ] exp ( − 2 | p → | 2 p N 2 ) d 3 p ( 2 π ) 3 ≈ 1 4 π 2 ∫ 0 ∞ p 3 [ 1 − ( p p M ) 2 ] exp ( − 2 p 2 p N 2 ) d p = p N 4 32 π 2 ( 1 − p N 2 p M 2 ) . From the first line to the second, we have written | p → | simply as p , we have done the integration over the solid angle giving 4π , and we have assumed that m 2 c 2 is small compared to both p M 2 and p N 2 allowing us to use p 2 + m 2 ≈ p . From the last expression in (11) , we can see a balancing between the positive contribution p N 4 coming from the integration over low p , and the negative contribution p N 6 p M 2 coming from the integration over high p . This is reminiscent of the balancing of positive and negative contributions to the vacuum energy density in unbroken supersymmetric models. One can use (11) to “solve” the cosmological constant problem. Setting the calculated vacuum energy density in (11) to the observed vacuum energy density, one can solve for p M and obtainp M = p N ( 1 − 32 π 2 ρ v a c o b s p N 4 ) − 1 / 2 . If p N is at the Planck scale this implies ρ v a c o b s p N 4 ∼ 10 − 118 , which in turn leads to p M ≈ p N , and p M and p N are both of the Planck scale. Furthermore p M and p N do not necessarily need to be at the Planck scale to resolve the cosmological constant problem – one just needs ρ v a c o b s ≪ p N 4 , which can be obtained even with p N and p M at a much lower scale than the Planck scale. One still has the fine tuning problem of why the two scales should deviate from the condition p M = p N by such an incredibly small amount compared to either p M or p N . Nonetheless, this example illustrates how a negative contribution to the vacuum energy density, from high momentum, helps the GUP approach to the cosmological constant problem. One could ask if this model would be able to connect the present small value of ρ v a c o b s with a much larger value of the vacuum energy density required for an inflationary epoch in the very early Universe. Looking at the last line of (11) one can have a large ρ v a c if, in the early Universe, the two momentum scales satisfy p M > p N , but not p M ≈ p N . Then from this initial state the momentum scales would need to evolve toward p M ≈ p N to give the small observed vacuum energy density of the present Universe. 5 Summary and conclusions In this paper we examined how GUP models might address the cosmological constant problem. For GUP models given by modified position and momentum operators of the form (2) X ˆ i = i ħ f ( | p → | ) ∂ ∂ p i and p ˆ j = p j , there were two variants of the associated modified vacuum energy density given in (7) and (9) . These two expressions differed by the number of factors of 1 f ( | p → | ) . We argued for the validity of the expression in (9) over (7) , since only the former expression satisfied the requirement of symmetry of the modified position and momentum operators discussed around equation (4) . However, regardless of whether one used (7) or (9) to calculate ρ v a c , one still has essentially the same problem as the by-hand cut-off of (1) : the vacuum energy density was proportional the momentum cut-off scale to the fourth power, ρ v a c ∼ p c 4 , which for p c near the Planck scale (or even the electroweak scale) made ρ v a c too large. With the GUP modified energy density of either (7) or (9) ρ v a c , while formally finite, would nevertheless be proportional to the inverse square of the functional cut-off parameter β i.e. ρ v a c ∼ β − 2 , and if the scale of β were taken as the Planck scale, then one finds the same problem as using a by-hand cut-off of the formally infinite integral in (1) - the vacuum energy density from GUP models will have the same, large, order-of-magnitude disagreement compared to the observed vacuum energy density. We propose that the modified position operator changes sign at some momentum scale, p M , as in (10) . Then because of the link between the function f ( | p → | ) from the modified position operator in (2) , and how it changes the vacuum energy density in (9) , one finds that the positive contribution to ρ v a c from the integration below p M is balanced by negative contribution from above p M This balancing of large positive and negative contributions to the vacuum energy density is similar to unbroken SUSY, where a large, positive bosonic contribution is balanced by a large, negative fermionic contribution. Here, however this balancing of positive and negative contributions comes from the parity flip - the change in sign of the modified position operator at some momentum scale. Declaration of Competing Interest The authors declare the following financial interests/personal relationships which may be considered as potential competing interests: Douglas Singleton reports financial support was provided by National Science Foundation . Michael Bishop reports financial support was provided by California State University, Fresno . Douglas Singleton reports financial support was provided by National Science Foundation . Acknowledgement DS is supported by a 2023-2024 KITP Fellows Award. This research was supported in part by the National Science Foundation under Grant No. NSF PHY-1748958 . The work of MB and DS were supported through a Fresno State 2023-2024 RSCA grant. References [1] J. Polchinski String Theory Vol. 1: An Introduction to the Bosonic String and String Theory Vol. 2: Superstring Theory and Beyond 1998 Cambridge University Press Cambridge, UK J. Polchinski, String Theory Vol. 1: An Introduction to the Bosonic String and String Theory Vol. 2: Superstring Theory and Beyond (Cambridge University Press, Cambridge, UK 1998). [2] C. Rovelli Loop quantum gravity Living Rev. Relativ. 11 2008 5 C. Rovelli, Loop Quantum Gravity, Living Reviews in Relativity 11, 5 (2008). [3] S. Weinberg Rev. Mod. Phys. 61 1989 1 S. Weinberg, Rev. Mod. Phys. 61, 1 (1989). [4] P.A. Zyla Particle Data Group Prog. Theor. Exp. Phys. 2020 2020 083C01 P.A. Zyla et al. (Particle Data Group), Prog. Theor. Exp. Phys. 2020, 083C01 (2020). [5] E.Kh. Akhmedov Vacuum energy and relativistic invariance arXiv:hep-th/0204048 E. Kh. Akhmedov, “Vacuum energy and relativistic invariance”, arXiv:hep-th/0204048 [6] J.F. Koksma T. Prokopec The cosmological constant and Lorentz invariance of the vacuum state arXiv:1105.6296 [gr-qc] J. F. Koksma and T. Prokopec, “The Cosmological Constant and Lorentz Invariance of the Vacuum State”, e-Print: 1105.6296 [gr-qc] [7] M. Trodden S. Carroll TASI lectures: introduction to cosmology arXiv:astro-ph/0401547 M. Trodden and S. Carroll, “TASI Lectures: Introduction to Cosmology”, arXiv:astro-ph/0401547 [8] J. Leitner S. Okubo Phys. Rev. 136 1964 B1542 J. Leitner and S. Okubo, Phys. Rev. 136, B1542 (1964). [9] S.-B. Zhang Z.-L. Ba D.-H. Ning N.-F. Zhai Z.-T. Lu D. Sheng Phys. Rev. Lett. 130 2023 201401 S.-B. Zhang, Z.-L. Ba, D.-H. Ning, N.-F. Zhai, Z.-T. Lu, and D. Sheng, Phys. Rev. Letts. 130, 201401 (2023). [10] G. Veneziano Europhys. Lett. 2 1986 199 G. Veneziano, Europhys. Lett. 2 199 (1986). [11] D.J. Gross P.F. Mende Phys. Lett. B 197 1987 129 Nucl. Phys. B 303 1988 407 D.J. Gross and P.F. Mende, Phys. Lett. B 197, 129 (1987); Nucl. Phys. B 303, 407 (1988). [12] D. Amati M. Ciafaloni G. Veneziano Phys. Lett. B 197 1987 81 Int. J. Mod. Phys. A 03 1988 1615 Nucl. Phys. B 347 1990 550 D. Amati, M. Ciafaloni, and G. Veneziano, Phys. Lett. B 197, 81 (1987); Int. J. Mod. Phys. A, 03, 1615 (1988); Nucl. Phys. B 347, 550 (1990). [13] D. Amati M. Ciafaloni G. Veneziano Phys. Lett. B 216 1989 41 D. Amati, M. Ciafaloni, and G. Veneziano, Phys. Lett. B 216, 41 (1989). [14] M. Maggiore Phys. Lett. B 304 1993 65 M. Maggiore, Phys. Lett. B 304, 65 (1993). [15] L.J. Garay Int. J. Mod. Phys. A 10 1995 145 L. J. Garay, Int. J. Mod. Phys. A 10, 145 (1995). [16] A. Kempf G. Mangano R.B. Mann Phys. Rev. D 52 1995 1108 A. Kempf, G. Mangano and R. B. Mann, Phys. Rev. D 52, 1108 (1995). [17] F. Scardigli Phys. Lett. B 452 1999 39 F. Scardigli, Phys. Lett. B, 452, 39 (1999). [18] R.J. Adler D.I. Santiago Mod. Phys. Lett. A 14 1999 1371 R. J. Adler, D. I. Santiago, Mod. Phys. Lett. A 14, 1371 (1999). [19] R.J. Adler P. Chen D.I. Santiago Gen. Relativ. Gravit. 33 2001 2101 R. J. Adler, P. Chen, and D. I. Santiago, Gen. Relativ. Grav. 33, 2101 (2001). [20] L.N. Chang D. Minic N. Okamura T. Takeuchi Phys. Rev. D 65 2002 125028 L. N. Chang, D. Minic, N. Okamura, and T. Takeuchi, Phys. Rev. D 65, 125028 (2002). [21] M. Bishop J. Contreras P. Martin D. Singleton Front. Astron. Space Sci. 9 2022 978898 M. Bishop, J. Contreras, P. Martin, and D. Singleton, Front. Astron. Space Sci. 9, 978898 (2022). [22] A. Lue L. Wang M. Kamionkowski Phys. Rev. Lett. 83 1999 1506 A. Lue, L. Wang, and M. Kamionkowski, Phys. Rev. Letts. 83, 1506 (1999). [23] S.H.S. Alexander G. Calcagni Found. Phys. 38 2008 1148 S.H.S. Alexander and G. Calcagni, Found. Phys. 38, 1148 (2008). [24] S.H.S. Alexander G. Calcagni Phys. Lett. B 672 2009 386 S.H.S. Alexander and G. Calcagni, Phys. Letts. B 672, 386 (2009).
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-]>PLB 137730 137730 S0370-2693(23)00064-3 10.1016/j.physletb.2023.137730 European Center of Nuclear Research, ALICE experiment Experiments Fig. 1 Charged-particle pseudorapidity density in Pb Pb [2] and p Pb for the 5% most central collisions, and for pp collisions with INEL>0 trigger class. For symmetric collision systems (Pb Pb and pp) the data has been symmetrised around η = 0 and points for η > 3.5 have been reflected around η = 0. The boxes around the points and to the right reflect the uncorrelated and correlated, with respect to pseudorapidity, systematic uncertainty, respectively. The relative correlated, normalisation, uncertainties are evaluated at dN ch /dη |η =0 . The lines show fits of Eq. (1) (Pb Pb and pp) and Eq. (2) (p Pb) to the data (discussed in Section 4 ). Please note that the ordinate has been cut twice to accommodate for the very different ranges of the charged-particle pseudorapidity densities. Fig. 1 Fig. 2 Charged-particle pseudorapidity density in p Pb collisions at s NN = 5.02 TeV in seven centrality classes based on the V0A and V0C estimators. The lines are obtained using a fit of a scaled, normal distribution in rapidity Eq. (2) to the data (discussed in Section 4 ). Fig. 2 Fig. 3 Ratio r X of the charged-particle pseudorapidity density in Pb Pb (top) and p Pb (bottom) in different centrality classes to the charged-particle pseudorapidity density in pp in the INEL>0 event class. Note, for Pb Pb η lab is the same as the centre-of-mass pseudorapidity. Fig. 3 Fig. 4 The width (top) and effective p T /m (bottom) fit parameters as a function of the mean number of participants in pp, p Pb, and Pb Pb collisions at s NN = 5.02 TeV . Vertical uncertainties are the standard error on the best-fit parameter values, while horizontal uncertainties reflect the uncertainty on 〈N part 〉 from the Glauber calculations. Also shown are similar fit parameters from the same parameterisation of EPOS-LHC calculations as well as direct calculations of the standard deviation of the dN ch /dy distributions and the 〈p T 〉/〈m 〉 ratio from the EPOS-LHC calculations. Fig. 4 Fig. 5 The transverse area S T as calculated in a numerical Glauber model for two extreme cases: a) only the exclusive overlap of nucleons is considered (∩, open markers) and b) the inclusive area of participating nucleons contribute (∪, closed markers) in both p Pb and Pb Pb at s NN = 5.02 TeV . Fig. 5 Fig. 6 Estimate of the lower bound on the Bjorken transverse energy density in pp, p Pb, and Pb Pb collisions at s NN = 5.02 TeV , considering the exclusive (∩, open markers) and inclusive (∪, full markers) overlap area S T of the nucleons. The expression C N part p is fitted to case ∪, and we find C = (0.8 ± 0.3) GeV/(fm 2 c ) and p = 0.44 ± 0.08. Also shown is an estimate, via dE T /d y , of ε Bj from Pb Pb collisions at s NN = 2.76 TeV (stars with uncertainty band) [31] . Fig. 6 System-size dependence of the charged-particle pseudorapidity density at s NN = 5.02 TeV for pp, p Pb, and Pb Pb collisions ALICE Collaboration ⋆ S. Acharya 142 D. Adamová 96 142 Variable Energy Cyclotron Centre, Homi Bhabha National Institute, Kolkata, India Variable Energy Cyclotron Centre Homi Bhabha National Institute Kolkata India Variable Energy Cyclotron Centre, Homi Bhabha National Institute, Kolkata, India 96 Nuclear Physics Institute of the Czech Academy of Sciences, Řež u Prahy, Czech Republic Nuclear Physics Institute of the Czech Academy of Sciences Řež u Prahy Czech Republic Nuclear Physics Institute of the Czech Academy of Sciences, Řež u Prahy, Czech Republic ⋆ E-mail address: alice-publications@cern.ch .Editor: M. Doser Abstract We present the first systematic comparison of the charged-particle pseudorapidity densities for three widely different collision systems, pp, p Pb, and Pb Pb, at the top energy of the Large Hadron Collider (s NN = 5.02 TeV ) measured over a wide pseudorapidity range (− 3.5 < η < 5 ), the widest possible among the four experiments at that facility. The systematic uncertainties are minimised since the measurements are recorded by the same experimental apparatus (ALICE). The distributions for p Pb and Pb Pb collisions are determined as a function of the centrality of the collisions, while results from pp collisions are reported for inelastic events with at least one charged particle at midrapidity. The charged-particle pseudorapidity densities are, under simple and robust assumptions, transformed to charged-particle rapidity densities. This allows for the calculation and the presentation of the evolution of the width of the rapidity distributions and of a lower bound on the Bjorken energy density, as a function of the number of participants in all three collision systems. We find a decreasing width of the particle production, and roughly a smooth ten fold increase in the energy density, as the system size grows, which is consistent with a gradually higher dense phase of matter. 1 Introduction The number of charged particles produced in energetic nuclear collisions is an important indicator for the strong interaction processes that determine the particle production at the sub-nucleonic level. In particular, the production of charged particles is expected to reflect the number of quark and gluon collisions occurring during the initial stages of the reaction. The total number of particles produced also provides information on the energy transfer available from the initial colliding beams to particle production, as a consequence of nuclear stopping [1] . In order to help unravel this complex scenario it is important to compare the particle production amongst collision systems of different sizes over a wide kinematic range. We present the measured charged-particle pseudorapidity density, d N ch / d η , for pp, p Pb, and Pb Pb (previously published [2] ) collisions at the same collision energy of s NN = 5.02 TeV in the nucleon–nucleon centre-of-mass reference frame. This is, at present, the maximum available energy at CERN's Large Hadron Collider (LHC) for Pb Pb collisions. The measurements were carried out using ALICE at LHC (for earlier d N ch / d η results see for example Refs. [3–5] ). The three studied reactions have different characteristics probing widely different particle production yields and mechanisms. In Pb Pb collisions, the total particle yield for central collisions is of the order 10 4 [2] , and a strongly coupled plasma of quarks and gluons (sQGP) is formed [6–9] , whose collective and transport properties are currently under intense study. On the other hand, pp collisions represent the simplest possible nuclear collision system, where the average total particle production is much smaller (≈80, by integrating the measured distributions), and is to first approximation much less subject to collective effects [10] . The p Pb system is intermediate to the other reactions, corresponding to the situation where a single nucleon probes the nucleons in a narrow cylinder of the target nucleus. The extent to which p Pb is governed by the initial state cold nuclear matter of the lead ion or whether collective phenomena in the hot and dense medium play an important role is, at present, a matter under scrutiny by the community [10,11] . In this letter, we compare the three reactions and present the ratios of the charged-particle pseudorapidity density distributions (d N ch / d η ) of the more complex reactions to the pp distribution. Owing to ALICE's unique large acceptance in pseudorapidity, and using simple and robust assumptions, we transform the measured charged-particle pseudorapidity density distributions into charged-particle rapidity density distributions (d N ch / d y ). This allows us to calculate the width of the rapidity distributions as a function of the number of participating nucleons. The parameters of the transformation also allow us to estimate a lower bound on the energy density using the well-known formula from Bjorken [12] . An energy density exceeding the critical energy density of roughly 1 GeV / fm 3 [13] is a necessary condition for the formation of deconfined matter of quarks and gluons, and thus it is of the utmost interest to understand the development of these energy densities across different collision systems. 2 Experimental set-up, data sample, analysis method, systematic uncertainties A detailed description of the ALICE detector and its performance can be found elsewhere [14,15] . The present analysis uses the Silicon Pixel Detector (SPD) to determine the pseudorapidity densities in the range − 2 < η < 2 and the Forward Multiplicity Detector (FMD) in the ranges − 3.5 < η < − 1.8 and 1.8 < η < 5 . The V0, comprised of two plastic scintillator discs covering − 3.7 < η < − 1.7 (V0C) and 2.8 < η < 5.1 (V0A), and the ZDC, two zero-degree calorimeters located 112.5 m from the interaction point, measurements determine the collision centrality and are used for offline event selection [2] . The results presented are based on data from collisions at a centre-of-mass energy per nucleon pair of s NN = 5.02 TeV as collected by ALICE during LHC Run 1 (2013) for p Pb, and during Run 2 (2015) for pp and Pb Pb. The FMD suffered high levels of background noise during the 2016 p Pb campaign, due to the high collision rate, and this data is therefore not used for the present analysis. About 105 events with a minimum bias trigger requirement [2] were analysed in the centrality range from 0% to 90% and 0% to 100% of the visible cross section for Pb Pb and p Pb collisions, respectively. The minimum bias trigger for p Pb and Pb Pb collisions in ALICE was defined as a coincidence between the V0A and V0C sides of the V0 detector. The data from the p Pb collisions were taken in two beam configurations: one where the lead ion travelled toward positive pseudorapidity and one where it travelled toward negative pseudorapidity. The results from the latter collisions are mirrored around η = 0 . The centre-of-mass frame in p Pb collisions does not coincide with the laboratory frame, due to the single magnetic field in the LHC, and thus the rapidity of the centre-of-mass is y CM = ± 0.465 for the two directions, respectively, in the laboratory frame. For this reason, pseudorapidity, calculated with respect to the laboratory frame, is denoted η lab whenever p Pb results are presented. Likewise, for the pp collisions, about 105 events with coincidence between V0A and V0C and at least one charged particle in | η | < 1 were analysed. By requiring at least one charged particle at midrapidity, the so-called INEL>0 event class, the systematic uncertainty, related to the absolute normalisation to the full inelastic cross section, is reduced, while still sampling a large fraction (> 75 % ) of the hadronic cross section [16,17] . The standard ALICE event selection [18] and centrality estimator based on the V0 amplitude [19,20] are used in this analysis. The event selection consists of: a) exclusion of background events using the timing information from the ZDC (for Pb Pb and p Pb, e.g., beam–gas interactions) and V0 detectors, b) verification of the trigger conditions, and c) a reconstructed position of the collision (primary vertex). In Pb Pb collisions, centrality is obtained from the sum amplitude in both V0 detector arrays (V0M). For p Pb only the amplitude in the array on the lead-going side (V0A or V0C) is used. In Pb Pb collisions, the 10% most peripheral collisions have substantial contributions from electromagnetic processes and are therefore not included in the results presented here [19] . A primary charged particle is defined as a charged particle with a mean proper lifetime τ larger than 1 cm / c , which is either a) produced directly in the interaction, or b) from decays of particles with τ smaller than 1 cm / c [21] . All quantities reported here are for primary, charged particles, though “primary” is omitted in the following for brevity. The analysis method is identical to that of previous publications [2] : the measurement of the charged-particle pseudorapidity density at midrapidity is obtained from counting particle trajectories determined using the two layers of the SPD. The SPD has a lower transverse momentum acceptance of 50 MeV / c , and the yield is extrapolated down to p T = 0 MeV / c via simulations. In the forward regions, the measurement is provided by the analysis of the deposited energy signal in the FMD and a statistical method is employed to calculate the inclusive number of charged particles. A data-driven correction [22] , based on separate measurements exploiting displaced collision vertices, is applied to remove the background from secondary particles. Systematic uncertainty estimations for the midrapidity measurements are detailed elsewhere [2,16,20] , and are from background suppression, transverse momentum extrapolation, weak decays, and simulations. The estimates are obtained through variation of thresholds and simulation studies. For pp (p Pb), the total systematic uncertainty amounts to 1.5% (2.7%) over the whole pseudorapidity range; while for Pb Pb the total systematic uncertainty is 2.6% at η = 0 and 2.9% at | η | = 2 . The systematic uncertainty is mostly correlated over pseudorapidity for | η | < 2 , and largely independent of centrality. The uncertainty in the forward region, estimated via variations of thresholds and simulation studies, is the same for all collision systems and is uncorrelated across η , amounting to 6.9% for η > 3.5 and 6.4% elsewhere within the forward regions [22] . In the figures of this letter, uncorrelated, local in pseudorapidity, systematic uncertainties are indicated by open boxes on the data points, while correlated systematic uncertainties, those that affect the overall scale and typically from event classification and selection, are indicated by filled boxes to the right of the data. The systematic uncertainty on d N ch / d η , due to the centrality class definition in Pb Pb, is estimated to vary from 0.6% for the most central to 9.5% for the most peripheral class [23] . The 80% to 90% centrality class has residual contamination from electromagnetic processes as detailed elsewhere [19] , which gives rise to an additional 4% systematic uncertainty in the measurements. No overall systematic uncertainty has been estimated for p Pb collisions, as the centrality selection in that collision system is inherently difficult to map to the underlying dynamics of the collisions [20] . 3 Results Fig. 1 shows the measured pseudorapidity densities in pp, and in central p Pb, and the previously published results for Pb Pb [2] collisions at s NN = 5.02 TeV for primary particles.For the 5% most central Pb Pb collisions d N ch / d η ≈ 2000 at midrapidity (η = 0 ) [2] , while for p Pb collisions the distribution peaks at d N ch / d η lab ≈ 60 around η lab = 3 in the lead-going direction (η > 0 ). For pp collisions with the INEL>0 trigger condition discussed above, d N ch / d η = 5.7 ± 0.2 at midrapidity, consistent with previous results derived from p T spectra [24] . Fig. 2 shows, as a function of centrality, the measured charged-particle pseudorapidity densities for p Pb collisions at s NN = 5.02 TeV . The strategy of centrality selection for proton on nucleus reactions is explained elsewhere [20] . The ALICE Collaboration has previously presented d N ch / d η for Pb Pb collisions at this energy [2] .In Fig. 3 , the charged-particle pseudorapidity densities in p Pb and Pb Pb reactions are divided by the pp distributions corresponding to the INEL>0 trigger class. The ratio is r X = ( d N ch / d η | X ) / ( d N ch / d η | pp ) , where X labels p Pb and Pb Pb collisions, in centrality classes, as a function of pseudorapidity. In the ratios, systematic uncertainties, of common origin, are partially cancelled, and, as an estimate, the magnitude of the resulting systematic uncertainties are given only by the uncertainties in the d N ch / d η | X measurements, since the uncertainties are independent of the collision system. In p Pb collisions the rapidity of the centre-of-mass is non-zero, which is not taken into account in the ratios. Such a correction would require prior determination of the full Jacobian of the transformation from pseudorapidity to rapidity, which is not possible to perform reliably with the ALICE apparatus. The ratio of the p Pb relative to the pp distributions increases with pseudorapidity from the p-going to the Pb-going direction for central collisions, which Brodsky et al. and Adil et al. [25,26] suggest is a sign of scaling of the pp distribution with the increasing number of participants as the lead nucleus is probed by the incident proton, and thus independent proton–nucleon scatterings on the lead-ion side. A similar scaling, however, does not hold for the Pb Pb reaction. The ratios cannot be obtained by simple scaling of the elementary pp distributions. Instead, the ratio of the Pb Pb relative to the pp distributions exhibits an enhancement of particle production around midrapidity for the more central collisions which is indicative of the formation of the sQGP [7] . Likewise, r pPb increases for all but the two most peripheral centrality classes as η lab → 3 . In Pb Pb collisions it is seen that the various mechanisms behind the pseudorapidity distributions are more transversely directed than in pp collisions by the increase of r PbPb as | η | → 0 4 Rapidity and energy-density dependence on system size and discussion It has been shown that the charged-particle rapidity density (d N ch / d y ) in Pb Pb collisions, to a good accuracy, follows a normal distribution over the considered rapidity interval (| y | ≲ 5 ) [2,27] . Those results relied on calculating the average Jacobian d N ch / d y = 〈 J 〉 = 〈 β 〉 using the full p T spectra, at midrapidity, of charged pions and kaons as well as protons and antiprotons. Here, we use the approximationy ≈ η − 1 2 m 2 p T 2 cos ϑ , where ϑ is the polar angle of emission, and identify a = p T / m with an effective ratio of transverse momentum over mass. With this, the effective Jacobian can be written asJ ′ ( η , a ) = ( 1 + 1 a 2 1 cosh 2 η ) − 1 / 2 . We further make the ansatz that d N ch / d y is normal distributed for symmetric collision systems (pp and Pb Pb), so that d N ch / d η can be parameterised as(1) f ( η ; A , a , σ ) = J ′ ( η , a ) A 1 2 π σ exp ( − y 2 { η , a } 2 σ 2 ) , where A and σ are the total integral and width of the distribution, respectively, and y the rapidity in the centre-of-mass frame. Motivated by the observed approximate linearity of r pPb (see lower panel of Fig. 3 ), we replace A with ( α y + A ) for the asymmetric system (p Pb) and parameterise d N ch / d η lab as(2) g ( η ; A , a , α , σ ) = J ′ ( η , a ) ( α y { η , a } + A ) × 1 2 π σ exp ( − [ y { η , a } − y CM ] 2 2 σ 2 ) . The functions f and g defined in Eq. (1) and Eq. (2) , respectively, describe the measurements within the measured region with χ 2 per degrees of freedom (ν ) in the range of 0.1 to 0.5. The small χ 2 / ν values are a consequence of the relatively large uncorrelated systematic uncertainties on the measurements. That is, the charged-particle distributions for pp, p Pb, and Pb Pb collisions at s NN = 5.02 TeV follow a normal distribution in rapidity, with free parameters A , a , σ , and α in the asymmetric case. The top panel of Fig. 4 shows the best-fit parameter values of the normal width (σ d N ch / d y ) for all three collision systems as a function of the average number of participating nucleons (〈 N part 〉 ) calculated using a Glauber model [28] . The best-fit parameters are found taking statistical and uncorrelated systematic uncertainties into account. The result using the above procedure, for the most central Pb Pb collisions, is found to be compatible with previous results extracted by unfolding with the mean Jacobian estimated from transverse momentum spectra [2] . The open points (crosses) and dashed lines on the figure are from evaluations of Eq. (1) and Eq. (2) , and direct calculations of σ d N ch / d y , respectively, using model calculations with EPOS-LHC [29] . EPOS-LHC was chosen as it provides predictions for all three collision systems. The parameterisation, in terms of the two functions, of this model calculation generally reproduces the widths of the charged-particle rapidity densities, except in the asymmetric case where a direct evaluation of the standard deviation is less motivated. The general trend is that the widths decrease as 〈 N part 〉 increases, consistent with the behaviour of the r PbPb ratios. Notably, the width of the d N ch / d y distributions in p Pb and Pb Pb, for low number of participant nucleons in the collisions, approaches the width of the pp distribution, which, presumably, is dominated by kinematic and phase space constraints. The lower panel of Fig. 4 shows the dependence of a on the average number of participants. The right-hand ordinate is the same, but multiplied by the average mass 〈 m 〉 = ( 0.215 ± 0.001 ) GeV / c 2 estimated from measurements of identified particles in Pb Pb collisions at s NN = 2.76 TeV [30] . To better understand the parameter a , this parameter extracted from the EPOS-LHC calculations, using the above procedure, is also shown in the figure. The dotted lines show the average p T / m predicted by EPOS-LHC [29] . The EPOS-LHC calculations indicate that the extracted effective transverse momentum to mass ratio a is consistently smaller than the ratio of the average transverse momentum to the average mass. Thus a gives a lower bound on 〈 p T 〉 / 〈 m 〉 . We can estimate the energy density that is reached in the collisions as a function of the number of participants for the three systems. A conventional approach is to use the model originally proposed by Bjorken [12] in which the energy density (ε Bj ) depends on the rapidity density of particles and the volume of a longitudinal cylinder with cross sectional area determined by the overlap between the colliding partners and length determined by a characteristic particle formation timeε Bj = 1 c τ 1 S T 〈 d E T d y 〉 . Here, S T ≈ π R 2 ≈ π N part 2 / 3 is the transverse area spanned by the participating nucleons, d E T / d y is the transverse-energy rapidity density, and τ is the formation time. While a formation time of τ = 1 fm / c is often assumed, it is left as a free parameter here. With 〈 m T 〉 = 〈 m 〉 1 + ( 〈 p T 〉 / 〈 m 〉 ) 2 , the transverse-energy rapidity density can be approximated by〈 d E T d y 〉 ≈ 〈 m T 〉 1 f total d N ch d y = 〈 m 〉 1 + ( 〈 p T 〉 〈 m 〉 ) 2 1 f total d N ch d y , where f total = 0.55 ± 0.01 , the ratio of charged particles to all particles [31] , accounts for neutral particles not measured in the experiment, and is assumed the same for all collision systems. Substituting the derived d N ch / d y and the effective a = p T / m ≲ 〈 p T 〉 / 〈 m 〉 results in a lower bound estimate for the Bjorken energy density (ε LB )(3) ε Bj τ ≥ ε LB τ = 1 c 1 S T 〈 m 〉 1 + a 2 1 f total 1 + 1 a 2 1 cosh 2 η d N ch d η , where a and 〈 m 〉 are as in the top panel of Fig. 4 . The transverse area S T is estimated in a numerical Glauber model [32,33] as shown in Fig. 5 . We consider two extremes for the transverse area spanned by the participating nucleons: a) the exclusive (or direct) overlap between participating nucleons, ∩ and open markers in Fig. 5 , and b) the inclusive (or full) area of all participating nucleons, ∪ and full markers in Fig. 5 . Fig. 6 shows the lower-bound energy density estimate, ε LB τ ≤ ε Bj τ , as a function of the number of participants, which reaches values between 10 and 20 GeV / ( fm 2 c ) in the most central Pb Pb collisions. The uncertainties are from standard error propagation of Eq. (3) of uncertainties on the best-fit parameter values, the number of participants, mean mass, and f total . A rise from roughly 1 GeV / ( fm 2 c ) to over 10 GeV / ( fm 2 c ) is observed if the transverse area is assumed to be the inclusive area of participating nucleons. This trend is illustrated by a power-law (C N part p ) fit to the data in the figure, with the parameter values C = ( 0.8 ± 0.3 ) GeV / ( fm 2 c ) and p = 0.44 ± 0.08 . On the other hand, if the transverse area is assumed to be the smaller exclusive overlap area, we observe a substantially larger lower bound on the energy density, but a less dramatic increase with increasing number of participating nucleons. Also shown in the figure are estimates of the Bjorken energy density ε Bj τ for Pb Pb reactions at s NN = 2.76 TeV [31] . These results where obtained from measurements of the transverse energy in the collisions and using the inclusive estimate of the transverse area S T . The trend of the s NN = 5.02 TeV results is similar to these earlier results. Bearing in mind that for the largest LHC collision energy we show a lower bound estimate of the energy density in Fig. 6 , we find a likely overall increase in the energy density from s NN = 2.76 TeV to 5.02 TeV .5 Summary and conclusions We have measured the charged particle pseudorapidity density in pp, p Pb, and Pb Pb collisions at s NN = 5.02 TeV over the widest possible pseudorapidity range available at the LHC. The distributions where determined using the same experimental apparatus and methods, and systematic uncertainties have been minimised to within the capabilities of the set-up. While the particle production in central Pb Pb collisions clearly exhibits an enhancement as compared to pp collisions, particle production in p Pb collisions is consistent with dominantly incoherent nucleon–nucleon collisions. By transforming the measured pseudorapidity distributions to rapidity distributions we have obtained systematic trends for the width of the rapidity distributions and a lower bound on the energy density, which shows a clear scaling behaviour as a function of the average number of participant nucleons. The decreasing width of the deduced rapidity distributions with increasing participant number suggests that the kinematic spread of particles, including longitudinal degrees of freedom, is reduced due to interactions in the early stages of the collisions. This is also reflected in the accompanying growth of the energy density. Both observations are consistent with the gradual establishment of a high-density phase of matter with increasing size of the collision domain. Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgements The ALICE Collaboration would like to thank all its engineers and technicians for their invaluable contributions to the construction of the experiment and the CERN accelerator teams for the outstanding performance of the LHC complex. The ALICE Collaboration gratefully acknowledges the resources and support provided by all Grid centres and the Worldwide LHC Computing Grid (WLCG) collaboration. The ALICE Collaboration acknowledges the following funding agencies for their support in building and running the ALICE detector: A. I. Alikhanyan National Science Laboratory (Yerevan Physics Institute) Foundation (ANSL) , State Committee of Science and World Federation of Scientists (WFS), Armenia; Austrian Academy of Sciences , Austrian Science Fund (FWF): [M 2467-N36 ] and Nationalstiftung für Forschung, Technologie und Entwicklung , Austria; Ministry of Communications and High Technologies, National Nuclear Research Center , Azerbaijan; Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq ), Financiadora de Estudos e Projetos (Finep), Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP) and Universidade Federal do Rio Grande do Sul (UFRGS), Brazil; Ministry of Education of China (MOEC), Ministry of Science & Technology of China (MSTC) and National Natural Science Foundation of China (NSFC), China; Ministry of Science and Education and Croatian Science Foundation , Croatia; Centro de Aplicaciones Tecnológicas y Desarrollo Nuclear (CEADEN), Cubaenergía , Cuba; Ministry of Education, Youth and Sports of the Czech Republic , Czech Republic; The Danish Council for Independent Research | Natural Sciences , the Villum Fonden and Danish National Research Foundation (DNRF), Denmark; Helsinki Institute of Physics (HIP), Finland; Commissariat à l'Energie Atomique (CEA ) and Institut National de Physique Nucléaire et de Physique des Particules (IN2P3) and Centre National de la Recherche Scientifique (CNRS), France; Bundesministerium für Bildung und Forschung (BMBF) and GSI Helmholtzzentrum für Schwerionenforschung GmbH , Germany; General Secretariat for Research and Technology , Ministry of Education, Research and Religions, Greece; National Research, Development and Innovation Office , Hungary; Department of Atomic Energy Government of India (DAE ), Department of Science and Technology , Government of India (DST), University Grants Commission , Government of India (UGC) and Council of Scientific and Industrial Research (CSIR), India; Indonesian Institute of Sciences , Indonesia; Istituto Nazionale di Fisica Nucleare (INFN ), Italy; Japanese Ministry of Education, Culture, Sports, Science and Technology (MEXT) and Japan Society for the Promotion of Science (JSPS) KAKENHI, Japan; Consejo Nacional de Ciencia (CONACYT) y Tecnología, through Fondo de Cooperación Internacional en Ciencia y Tecnología (FONCICYT) and Dirección General de Asuntos del Personal Académico (DGAPA), Mexico; Nederlandse Organisatie voor Wetenschappelijk Onderzoek (NWO), Netherlands; The Research Council of Norway , Norway; Commission on Science and Technology for Sustainable Development in the South (COMSATS), Pakistan; Pontificia Universidad Católica del Perú , Peru; Ministry of Education and Science , National Science Centre and WUT ID-UB , Poland; Korea Institute of Science and Technology Information and National Research Foundation of Korea (NRF), Republic of Korea; Ministry of Education and Scientific Research , Institute of Atomic Physics , Ministry of Research and Innovation and Institute of Atomic Physics and University Politehnica of Bucharest , Romania; Joint Institute for Nuclear Research (JINR), Ministry of Education and Science of the Russian Federation , National Research Centre Kurchatov Institute , Russian Science Foundation and Russian Foundation for Basic Research , Russia; Ministry of Education, Science, Research and Sport of the Slovak Republic , Slovakia; National Research Foundation of South Africa, South Africa; Swedish Research Council (VR) and Knut and Alice Wallenberg Foundation (KAW), Sweden; European Organization for Nuclear Research , Switzerland; Suranaree University of Technology (SUT), National Science and Technology Development Agency (NSTDA), Thailand Science Research and Innovation (TSRI) and National Science, Research and Innovation Fund (NSRF), Thailand; Turkish Energy, Nuclear and Mineral Research Agency (TENMAK), Turkey; National Academy of Sciences of Ukraine , Ukraine; Science and Technology Facilities Council (STFC), United Kingdom; National Science Foundation of the United States of America (NSF ) and United States Department of Energy, Office of Nuclear Physics (DOE NP ), United States of America. 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+]>PLB 137730 137730 S0370-2693(23)00064-3 10.1016/j.physletb.2023.137730 European Center of Nuclear Research, ALICE experiment Experiments Fig. 1 Charged-particle pseudorapidity density in Pb Pb [2] and p Pb for the 5% most central collisions, and for pp collisions with INEL>0 trigger class. For symmetric collision systems (Pb Pb and pp) the data has been symmetrised around η = 0 and points for η > 3.5 have been reflected around η = 0. The boxes around the points and to the right reflect the uncorrelated and correlated, with respect to pseudorapidity, systematic uncertainty, respectively. The relative correlated, normalisation, uncertainties are evaluated at dN ch /dη |η =0 . The lines show fits of Eq. (1) (Pb Pb and pp) and Eq. (2) (p Pb) to the data (discussed in Section 4 ). Please note that the ordinate has been cut twice to accommodate for the very different ranges of the charged-particle pseudorapidity densities. Fig. 1 Fig. 2 Charged-particle pseudorapidity density in p Pb collisions at s NN = 5.02 TeV in seven centrality classes based on the V0A and V0C estimators. The lines are obtained using a fit of a scaled, normal distribution in rapidity Eq. (2) to the data (discussed in Section 4 ). Fig. 2 Fig. 3 Ratio r X of the charged-particle pseudorapidity density in Pb Pb (top) and p Pb (bottom) in different centrality classes to the charged-particle pseudorapidity density in pp in the INEL>0 event class. Note, for Pb Pb η lab is the same as the centre-of-mass pseudorapidity. Fig. 3 Fig. 4 The width (top) and effective p T /m (bottom) fit parameters as a function of the mean number of participants in pp, p Pb, and Pb Pb collisions at s NN = 5.02 TeV . Vertical uncertainties are the standard error on the best-fit parameter values, while horizontal uncertainties reflect the uncertainty on 〈N part 〉 from the Glauber calculations. Also shown are similar fit parameters from the same parameterisation of EPOS-LHC calculations as well as direct calculations of the standard deviation of the dN ch /dy distributions and the 〈p T 〉/〈m 〉 ratio from the EPOS-LHC calculations. Fig. 4 Fig. 5 The transverse area S T as calculated in a numerical Glauber model for two extreme cases: a) only the exclusive overlap of nucleons is considered (∩, open markers) and b) the inclusive area of participating nucleons contribute (∪, closed markers) in both p Pb and Pb Pb at s NN = 5.02 TeV . Fig. 5 Fig. 6 Estimate of the lower bound on the Bjorken transverse energy density in pp, p Pb, and Pb Pb collisions at s NN = 5.02 TeV , considering the exclusive (∩, open markers) and inclusive (∪, full markers) overlap area S T of the nucleons. The expression C N part p is fitted to case ∪, and we find C = (0.8 ± 0.3) GeV/(fm 2 c ) and p = 0.44 ± 0.08. Also shown is an estimate, via dE T /d y , of ε Bj from Pb Pb collisions at s NN = 2.76 TeV (stars with uncertainty band) [31] . Fig. 6 System-size dependence of the charged-particle pseudorapidity density at s NN = 5.02 TeV for pp, p Pb, and Pb Pb collisions ALICE Collaboration ⋆ S. Acharya 142 D. Adamová 96 142 Variable Energy Cyclotron Centre, Homi Bhabha National Institute, Kolkata, India Variable Energy Cyclotron Centre Homi Bhabha National Institute Kolkata India Variable Energy Cyclotron Centre, Homi Bhabha National Institute, Kolkata, India 96 Nuclear Physics Institute of the Czech Academy of Sciences, Řež u Prahy, Czech Republic Nuclear Physics Institute of the Czech Academy of Sciences Řež u Prahy Czech Republic Nuclear Physics Institute of the Czech Academy of Sciences, Řež u Prahy, Czech Republic ⋆ E-mail address: alice-publications@cern.ch .Editor: M. Doser Abstract We present the first systematic comparison of the charged-particle pseudorapidity densities for three widely different collision systems, pp, p Pb, and Pb Pb, at the top energy of the Large Hadron Collider (s NN = 5.02 TeV ) measured over a wide pseudorapidity range (− 3.5 < η < 5 ), the widest possible among the four experiments at that facility. The systematic uncertainties are minimised since the measurements are recorded by the same experimental apparatus (ALICE). The distributions for p Pb and Pb Pb collisions are determined as a function of the centrality of the collisions, while results from pp collisions are reported for inelastic events with at least one charged particle at midrapidity. The charged-particle pseudorapidity densities are, under simple and robust assumptions, transformed to charged-particle rapidity densities. This allows for the calculation and the presentation of the evolution of the width of the rapidity distributions and of a lower bound on the Bjorken energy density, as a function of the number of participants in all three collision systems. We find a decreasing width of the particle production, and roughly a smooth ten fold increase in the energy density, as the system size grows, which is consistent with a gradually higher dense phase of matter. 1 Introduction The number of charged particles produced in energetic nuclear collisions is an important indicator for the strong interaction processes that determine the particle production at the sub-nucleonic level. In particular, the production of charged particles is expected to reflect the number of quark and gluon collisions occurring during the initial stages of the reaction. The total number of particles produced also provides information on the energy transfer available from the initial colliding beams to particle production, as a consequence of nuclear stopping [1] . In order to help unravel this complex scenario it is important to compare the particle production amongst collision systems of different sizes over a wide kinematic range. We present the measured charged-particle pseudorapidity density, d N ch / d η , for pp, p Pb, and Pb Pb (previously published [2] ) collisions at the same collision energy of s NN = 5.02 TeV in the nucleon–nucleon centre-of-mass reference frame. This is, at present, the maximum available energy at CERN's Large Hadron Collider (LHC) for Pb Pb collisions. The measurements were carried out using ALICE at LHC (for earlier d N ch / d η results see for example Refs. [3–5] ). The three studied reactions have different characteristics probing widely different particle production yields and mechanisms. In Pb Pb collisions, the total particle yield for central collisions is of the order 10 4 [2] , and a strongly coupled plasma of quarks and gluons (sQGP) is formed [6–9] , whose collective and transport properties are currently under intense study. On the other hand, pp collisions represent the simplest possible nuclear collision system, where the average total particle production is much smaller (≈80, by integrating the measured distributions), and is to first approximation much less subject to collective effects [10] . The p Pb system is intermediate to the other reactions, corresponding to the situation where a single nucleon probes the nucleons in a narrow cylinder of the target nucleus. The extent to which p Pb is governed by the initial state cold nuclear matter of the lead ion or whether collective phenomena in the hot and dense medium play an important role is, at present, a matter under scrutiny by the community [10,11] . In this letter, we compare the three reactions and present the ratios of the charged-particle pseudorapidity density distributions (d N ch / d η ) of the more complex reactions to the pp distribution. Owing to ALICE's unique large acceptance in pseudorapidity, and using simple and robust assumptions, we transform the measured charged-particle pseudorapidity density distributions into charged-particle rapidity density distributions (d N ch / d y ). This allows us to calculate the width of the rapidity distributions as a function of the number of participating nucleons. The parameters of the transformation also allow us to estimate a lower bound on the energy density using the well-known formula from Bjorken [12] . An energy density exceeding the critical energy density of roughly 1 GeV / fm 3 [13] is a necessary condition for the formation of deconfined matter of quarks and gluons, and thus it is of the utmost interest to understand the development of these energy densities across different collision systems. 2 Experimental set-up, data sample, analysis method, systematic uncertainties A detailed description of the ALICE detector and its performance can be found elsewhere [14,15] . The present analysis uses the Silicon Pixel Detector (SPD) to determine the pseudorapidity densities in the range − 2 < η < 2 and the Forward Multiplicity Detector (FMD) in the ranges − 3.5 < η < − 1.8 and 1.8 < η < 5 . The V0, comprised of two plastic scintillator discs covering − 3.7 < η < − 1.7 (V0C) and 2.8 < η < 5.1 (V0A), and the ZDC, two zero-degree calorimeters located 112.5 m from the interaction point, measurements determine the collision centrality and are used for offline event selection [2] . The results presented are based on data from collisions at a centre-of-mass energy per nucleon pair of s NN = 5.02 TeV as collected by ALICE during LHC Run 1 (2013) for p Pb, and during Run 2 (2015) for pp and Pb Pb. The FMD suffered high levels of background noise during the 2016 p Pb campaign, due to the high collision rate, and this data is therefore not used for the present analysis. About 105 events with a minimum bias trigger requirement [2] were analysed in the centrality range from 0% to 90% and 0% to 100% of the visible cross section for Pb Pb and p Pb collisions, respectively. The minimum bias trigger for p Pb and Pb Pb collisions in ALICE was defined as a coincidence between the V0A and V0C sides of the V0 detector. The data from the p Pb collisions were taken in two beam configurations: one where the lead ion travelled toward positive pseudorapidity and one where it travelled toward negative pseudorapidity. The results from the latter collisions are mirrored around η = 0 . The centre-of-mass frame in p Pb collisions does not coincide with the laboratory frame, due to the single magnetic field in the LHC, and thus the rapidity of the centre-of-mass is y CM = ± 0.465 for the two directions, respectively, in the laboratory frame. For this reason, pseudorapidity, calculated with respect to the laboratory frame, is denoted η lab whenever p Pb results are presented. Likewise, for the pp collisions, about 105 events with coincidence between V0A and V0C and at least one charged particle in | η | < 1 were analysed. By requiring at least one charged particle at midrapidity, the so-called INEL>0 event class, the systematic uncertainty, related to the absolute normalisation to the full inelastic cross section, is reduced, while still sampling a large fraction (> 75 % ) of the hadronic cross section [16,17] . The standard ALICE event selection [18] and centrality estimator based on the V0 amplitude [19,20] are used in this analysis. The event selection consists of: a) exclusion of background events using the timing information from the ZDC (for Pb Pb and p Pb, e.g., beam–gas interactions) and V0 detectors, b) verification of the trigger conditions, and c) a reconstructed position of the collision (primary vertex). In Pb Pb collisions, centrality is obtained from the sum amplitude in both V0 detector arrays (V0M). For p Pb only the amplitude in the array on the lead-going side (V0A or V0C) is used. In Pb Pb collisions, the 10% most peripheral collisions have substantial contributions from electromagnetic processes and are therefore not included in the results presented here [19] . A primary charged particle is defined as a charged particle with a mean proper lifetime τ larger than 1 cm / c , which is either a) produced directly in the interaction, or b) from decays of particles with τ smaller than 1 cm / c [21] . All quantities reported here are for primary, charged particles, though “primary” is omitted in the following for brevity. The analysis method is identical to that of previous publications [2] : the measurement of the charged-particle pseudorapidity density at midrapidity is obtained from counting particle trajectories determined using the two layers of the SPD. The SPD has a lower transverse momentum acceptance of 50 MeV / c , and the yield is extrapolated down to p T = 0 MeV / c via simulations. In the forward regions, the measurement is provided by the analysis of the deposited energy signal in the FMD and a statistical method is employed to calculate the inclusive number of charged particles. A data-driven correction [22] , based on separate measurements exploiting displaced collision vertices, is applied to remove the background from secondary particles. Systematic uncertainty estimations for the midrapidity measurements are detailed elsewhere [2,16,20] , and are from background suppression, transverse momentum extrapolation, weak decays, and simulations. The estimates are obtained through variation of thresholds and simulation studies. For pp (p Pb), the total systematic uncertainty amounts to 1.5% (2.7%) over the whole pseudorapidity range; while for Pb Pb the total systematic uncertainty is 2.6% at η = 0 and 2.9% at | η | = 2 . The systematic uncertainty is mostly correlated over pseudorapidity for | η | < 2 , and largely independent of centrality. The uncertainty in the forward region, estimated via variations of thresholds and simulation studies, is the same for all collision systems and is uncorrelated across η , amounting to 6.9% for η > 3.5 and 6.4% elsewhere within the forward regions [22] . In the figures of this letter, uncorrelated, local in pseudorapidity, systematic uncertainties are indicated by open boxes on the data points, while correlated systematic uncertainties, those that affect the overall scale and typically from event classification and selection, are indicated by filled boxes to the right of the data. The systematic uncertainty on d N ch / d η , due to the centrality class definition in Pb Pb, is estimated to vary from 0.6% for the most central to 9.5% for the most peripheral class [23] . The 80% to 90% centrality class has residual contamination from electromagnetic processes as detailed elsewhere [19] , which gives rise to an additional 4% systematic uncertainty in the measurements. No overall systematic uncertainty has been estimated for p Pb collisions, as the centrality selection in that collision system is inherently difficult to map to the underlying dynamics of the collisions [20] . 3 Results Fig. 1 shows the measured pseudorapidity densities in pp, and in central p Pb, and the previously published results for Pb Pb [2] collisions at s NN = 5.02 TeV for primary particles.For the 5% most central Pb Pb collisions d N ch / d η ≈ 2000 at midrapidity (η = 0 ) [2] , while for p Pb collisions the distribution peaks at d N ch / d η lab ≈ 60 around η lab = 3 in the lead-going direction (η > 0 ). For pp collisions with the INEL>0 trigger condition discussed above, d N ch / d η = 5.7 ± 0.2 at midrapidity, consistent with previous results derived from p T spectra [24] . Fig. 2 shows, as a function of centrality, the measured charged-particle pseudorapidity densities for p Pb collisions at s NN = 5.02 TeV . The strategy of centrality selection for proton on nucleus reactions is explained elsewhere [20] . The ALICE Collaboration has previously presented d N ch / d η for Pb Pb collisions at this energy [2] .In Fig. 3 , the charged-particle pseudorapidity densities in p Pb and Pb Pb reactions are divided by the pp distributions corresponding to the INEL>0 trigger class. The ratio is r X = ( d N ch / d η | X ) / ( d N ch / d η | pp ) , where X labels p Pb and Pb Pb collisions, in centrality classes, as a function of pseudorapidity. In the ratios, systematic uncertainties, of common origin, are partially cancelled, and, as an estimate, the magnitude of the resulting systematic uncertainties are given only by the uncertainties in the d N ch / d η | X measurements, since the uncertainties are independent of the collision system. In p Pb collisions the rapidity of the centre-of-mass is non-zero, which is not taken into account in the ratios. Such a correction would require prior determination of the full Jacobian of the transformation from pseudorapidity to rapidity, which is not possible to perform reliably with the ALICE apparatus. The ratio of the p Pb relative to the pp distributions increases with pseudorapidity from the p-going to the Pb-going direction for central collisions, which Brodsky et al. and Adil et al. [25,26] suggest is a sign of scaling of the pp distribution with the increasing number of participants as the lead nucleus is probed by the incident proton, and thus independent proton–nucleon scatterings on the lead-ion side. A similar scaling, however, does not hold for the Pb Pb reaction. The ratios cannot be obtained by simple scaling of the elementary pp distributions. Instead, the ratio of the Pb Pb relative to the pp distributions exhibits an enhancement of particle production around midrapidity for the more central collisions which is indicative of the formation of the sQGP [7] . Likewise, r pPb increases for all but the two most peripheral centrality classes as η lab → 3 . In Pb Pb collisions it is seen that the various mechanisms behind the pseudorapidity distributions are more transversely directed than in pp collisions by the increase of r PbPb as | η | → 0 4 Rapidity and energy-density dependence on system size and discussion It has been shown that the charged-particle rapidity density (d N ch / d y ) in Pb Pb collisions, to a good accuracy, follows a normal distribution over the considered rapidity interval (| y | ≲ 5 ) [2,27] . Those results relied on calculating the average Jacobian d N ch / d y = 〈 J 〉 = 〈 β 〉 using the full p T spectra, at midrapidity, of charged pions and kaons as well as protons and antiprotons. Here, we use the approximationy ≈ η − 1 2 m 2 p T 2 cos ϑ , where ϑ is the polar angle of emission, and identify a = p T / m with an effective ratio of transverse momentum over mass. With this, the effective Jacobian can be written asJ ′ ( η , a ) = ( 1 + 1 a 2 1 cosh 2 η ) − 1 / 2 . We further make the ansatz that d N ch / d y is normal distributed for symmetric collision systems (pp and Pb Pb), so that d N ch / d η can be parameterised as(1) f ( η ; A , a , σ ) = J ′ ( η , a ) A 1 2 π σ exp ( − y 2 { η , a } 2 σ 2 ) , where A and σ are the total integral and width of the distribution, respectively, and y the rapidity in the centre-of-mass frame. Motivated by the observed approximate linearity of r pPb (see lower panel of Fig. 3 ), we replace A with ( α y + A ) for the asymmetric system (p Pb) and parameterise d N ch / d η lab as(2) g ( η ; A , a , α , σ ) = J ′ ( η , a ) ( α y { η , a } + A ) × 1 2 π σ exp ( − [ y { η , a } − y CM ] 2 2 σ 2 ) . The functions f and g defined in Eq. (1) and Eq. (2) , respectively, describe the measurements within the measured region with χ 2 per degrees of freedom (ν ) in the range of 0.1 to 0.5. The small χ 2 / ν values are a consequence of the relatively large uncorrelated systematic uncertainties on the measurements. That is, the charged-particle distributions for pp, p Pb, and Pb Pb collisions at s NN = 5.02 TeV follow a normal distribution in rapidity, with free parameters A , a , σ , and α in the asymmetric case. The top panel of Fig. 4 shows the best-fit parameter values of the normal width (σ d N ch / d y ) for all three collision systems as a function of the average number of participating nucleons (〈 N part 〉 ) calculated using a Glauber model [28] . The best-fit parameters are found taking statistical and uncorrelated systematic uncertainties into account. The result using the above procedure, for the most central Pb Pb collisions, is found to be compatible with previous results extracted by unfolding with the mean Jacobian estimated from transverse momentum spectra [2] . The open points (crosses) and dashed lines on the figure are from evaluations of Eq. (1) and Eq. (2) , and direct calculations of σ d N ch / d y , respectively, using model calculations with EPOS-LHC [29] . EPOS-LHC was chosen as it provides predictions for all three collision systems. The parameterisation, in terms of the two functions, of this model calculation generally reproduces the widths of the charged-particle rapidity densities, except in the asymmetric case where a direct evaluation of the standard deviation is less motivated. The general trend is that the widths decrease as 〈 N part 〉 increases, consistent with the behaviour of the r PbPb ratios. Notably, the width of the d N ch / d y distributions in p Pb and Pb Pb, for low number of participant nucleons in the collisions, approaches the width of the pp distribution, which, presumably, is dominated by kinematic and phase space constraints. The lower panel of Fig. 4 shows the dependence of a on the average number of participants. The right-hand ordinate is the same, but multiplied by the average mass 〈 m 〉 = ( 0.215 ± 0.001 ) GeV / c 2 estimated from measurements of identified particles in Pb Pb collisions at s NN = 2.76 TeV [30] . To better understand the parameter a , this parameter extracted from the EPOS-LHC calculations, using the above procedure, is also shown in the figure. The dotted lines show the average p T / m predicted by EPOS-LHC [29] . The EPOS-LHC calculations indicate that the extracted effective transverse momentum to mass ratio a is consistently smaller than the ratio of the average transverse momentum to the average mass. Thus a gives a lower bound on 〈 p T 〉 / 〈 m 〉 . We can estimate the energy density that is reached in the collisions as a function of the number of participants for the three systems. A conventional approach is to use the model originally proposed by Bjorken [12] in which the energy density (ε Bj ) depends on the rapidity density of particles and the volume of a longitudinal cylinder with cross sectional area determined by the overlap between the colliding partners and length determined by a characteristic particle formation timeε Bj = 1 c τ 1 S T 〈 d E T d y 〉 . Here, S T ≈ π R 2 ≈ π N part 2 / 3 is the transverse area spanned by the participating nucleons, d E T / d y is the transverse-energy rapidity density, and τ is the formation time. While a formation time of τ = 1 fm / c is often assumed, it is left as a free parameter here. With 〈 m T 〉 = 〈 m 〉 1 + ( 〈 p T 〉 / 〈 m 〉 ) 2 , the transverse-energy rapidity density can be approximated by〈 d E T d y 〉 ≈ 〈 m T 〉 1 f total d N ch d y = 〈 m 〉 1 + ( 〈 p T 〉 〈 m 〉 ) 2 1 f total d N ch d y , where f total = 0.55 ± 0.01 , the ratio of charged particles to all particles [31] , accounts for neutral particles not measured in the experiment, and is assumed the same for all collision systems. Substituting the derived d N ch / d y and the effective a = p T / m ≲ 〈 p T 〉 / 〈 m 〉 results in a lower bound estimate for the Bjorken energy density (ε LB )(3) ε Bj τ ≥ ε LB τ = 1 c 1 S T 〈 m 〉 1 + a 2 1 f total 1 + 1 a 2 1 cosh 2 η d N ch d η , where a and 〈 m 〉 are as in the top panel of Fig. 4 . The transverse area S T is estimated in a numerical Glauber model [32,33] as shown in Fig. 5 . We consider two extremes for the transverse area spanned by the participating nucleons: a) the exclusive (or direct) overlap between participating nucleons, ∩ and open markers in Fig. 5 , and b) the inclusive (or full) area of all participating nucleons, ∪ and full markers in Fig. 5 . Fig. 6 shows the lower-bound energy density estimate, ε LB τ ≤ ε Bj τ , as a function of the number of participants, which reaches values between 10 and 20 GeV / ( fm 2 c ) in the most central Pb Pb collisions. The uncertainties are from standard error propagation of Eq. (3) of uncertainties on the best-fit parameter values, the number of participants, mean mass, and f total . A rise from roughly 1 GeV / ( fm 2 c ) to over 10 GeV / ( fm 2 c ) is observed if the transverse area is assumed to be the inclusive area of participating nucleons. This trend is illustrated by a power-law (C N part p ) fit to the data in the figure, with the parameter values C = ( 0.8 ± 0.3 ) GeV / ( fm 2 c ) and p = 0.44 ± 0.08 . On the other hand, if the transverse area is assumed to be the smaller exclusive overlap area, we observe a substantially larger lower bound on the energy density, but a less dramatic increase with increasing number of participating nucleons. Also shown in the figure are estimates of the Bjorken energy density ε Bj τ for Pb Pb reactions at s NN = 2.76 TeV [31] . These results where obtained from measurements of the transverse energy in the collisions and using the inclusive estimate of the transverse area S T . The trend of the s NN = 5.02 TeV results is similar to these earlier results. Bearing in mind that for the largest LHC collision energy we show a lower bound estimate of the energy density in Fig. 6 , we find a likely overall increase in the energy density from s NN = 2.76 TeV to 5.02 TeV .5 Summary and conclusions We have measured the charged particle pseudorapidity density in pp, p Pb, and Pb Pb collisions at s NN = 5.02 TeV over the widest possible pseudorapidity range available at the LHC. The distributions where determined using the same experimental apparatus and methods, and systematic uncertainties have been minimised to within the capabilities of the set-up. While the particle production in central Pb Pb collisions clearly exhibits an enhancement as compared to pp collisions, particle production in p Pb collisions is consistent with dominantly incoherent nucleon–nucleon collisions. By transforming the measured pseudorapidity distributions to rapidity distributions we have obtained systematic trends for the width of the rapidity distributions and a lower bound on the energy density, which shows a clear scaling behaviour as a function of the average number of participant nucleons. The decreasing width of the deduced rapidity distributions with increasing participant number suggests that the kinematic spread of particles, including longitudinal degrees of freedom, is reduced due to interactions in the early stages of the collisions. This is also reflected in the accompanying growth of the energy density. Both observations are consistent with the gradual establishment of a high-density phase of matter with increasing size of the collision domain. Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgements The ALICE Collaboration would like to thank all its engineers and technicians for their invaluable contributions to the construction of the experiment and the CERN accelerator teams for the outstanding performance of the LHC complex. The ALICE Collaboration gratefully acknowledges the resources and support provided by all Grid centres and the Worldwide LHC Computing Grid (WLCG) collaboration. The ALICE Collaboration acknowledges the following funding agencies for their support in building and running the ALICE detector: A. I. Alikhanyan National Science Laboratory (Yerevan Physics Institute) Foundation (ANSL) , State Committee of Science and World Federation of Scientists (WFS), Armenia; Austrian Academy of Sciences , Austrian Science Fund (FWF): [M 2467-N36 ] and Nationalstiftung für Forschung, Technologie und Entwicklung , Austria; Ministry of Communications and High Technologies, National Nuclear Research Center , Azerbaijan; Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq ), Financiadora de Estudos e Projetos (Finep), Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP) and Universidade Federal do Rio Grande do Sul (UFRGS), Brazil; Ministry of Education of China (MOEC), Ministry of Science & Technology of China (MSTC) and National Natural Science Foundation of China (NSFC), China; Ministry of Science and Education and Croatian Science Foundation , Croatia; Centro de Aplicaciones Tecnológicas y Desarrollo Nuclear (CEADEN), Cubaenergía , Cuba; Ministry of Education, Youth and Sports of the Czech Republic , Czech Republic; The Danish Council for Independent Research | Natural Sciences , the Villum Fonden and Danish National Research Foundation (DNRF), Denmark; Helsinki Institute of Physics (HIP), Finland; Commissariat à l'Energie Atomique (CEA ) and Institut National de Physique Nucléaire et de Physique des Particules (IN2P3) and Centre National de la Recherche Scientifique (CNRS), France; Bundesministerium für Bildung und Forschung (BMBF) and GSI Helmholtzzentrum für Schwerionenforschung GmbH , Germany; General Secretariat for Research and Technology , Ministry of Education, Research and Religions, Greece; National Research, Development and Innovation Office , Hungary; Department of Atomic Energy Government of India (DAE ), Department of Science and Technology , Government of India (DST), University Grants Commission , Government of India (UGC) and Council of Scientific and Industrial Research (CSIR), India; Indonesian Institute of Sciences , Indonesia; Istituto Nazionale di Fisica Nucleare (INFN ), Italy; Japanese Ministry of Education, Culture, Sports, Science and Technology (MEXT) and Japan Society for the Promotion of Science (JSPS) KAKENHI, Japan; Consejo Nacional de Ciencia (CONACYT) y Tecnología, through Fondo de Cooperación Internacional en Ciencia y Tecnología (FONCICYT) and Dirección General de Asuntos del Personal Académico (DGAPA), Mexico; Nederlandse Organisatie voor Wetenschappelijk Onderzoek (NWO), Netherlands; The Research Council of Norway , Norway; Commission on Science and Technology for Sustainable Development in the South (COMSATS), Pakistan; Pontificia Universidad Católica del Perú , Peru; Ministry of Education and Science , National Science Centre and WUT ID-UB , Poland; Korea Institute of Science and Technology Information and National Research Foundation of Korea (NRF), Republic of Korea; Ministry of Education and Scientific Research , Institute of Atomic Physics , Ministry of Research and Innovation and Institute of Atomic Physics and University Politehnica of Bucharest , Romania; Joint Institute for Nuclear Research (JINR), Ministry of Education and Science of the Russian Federation , National Research Centre Kurchatov Institute , Russian Science Foundation and Russian Foundation for Basic Research , Russia; Ministry of Education, Science, Research and Sport of the Slovak Republic , Slovakia; National Research Foundation of South Africa, South Africa; Swedish Research Council (VR) and Knut and Alice Wallenberg Foundation (KAW), Sweden; European Organization for Nuclear Research , Switzerland; Suranaree University of Technology (SUT), National Science and Technology Development Agency (NSTDA), Thailand Science Research and Innovation (TSRI) and National Science, Research and Innovation Fund (NSRF), Thailand; Turkish Energy, Nuclear and Mineral Research Agency (TENMAK), Turkey; National Academy of Sciences of Ukraine , Ukraine; Science and Technology Facilities Council (STFC), United Kingdom; National Science Foundation of the United States of America (NSF ) and United States Department of Energy, Office of Nuclear Physics (DOE NP ), United States of America. 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-]>PLB 137649 137649 S0370-2693(22)00783-3 10.1016/j.physletb.2022.137649 The Author(s) Experiments https://www.hepdata.net/ Fig. 1 Illustration of toward, away and transverse regions with respect to the leading particle in a collision. Fig. 1 Fig. 2 Top panels: transverse momentum spectra of charged particles in the transverse region for different multiplicity classes in pp (left), p–Pb (middle) and Pb–Pb (right) collisions at s NN = 5.02 TeV. The p T spectra are measured at mid pseudorapidity (|η | < 0.8). Lower panels: Ratio of p T spectra in different multiplicity classes to the p T spectrum in the 0−100% multiplicity class for the corresponding collision systems. The statistical and systematic uncertainties are shown by bars and boxes, respectively. Fig. 2 Fig. 3 Transverse momentum spectra of charged particles in Toward-Transverse, d N ch st / d p T (top plot) and Away-Transverse, d N ch sa / d p T (bottom plot) regions for different multiplicity classes in pp (left), p–Pb (middle) and Pb–Pb (right) collisions at s NN = 5.02 TeV. The p T spectra are measured at mid pseudorapidity (|η | < 0.8). The lower panels of both plots show the ratio to minimum bias pp collisions. The statistical and systematic uncertainties are shown by bars and boxes, respectively. Fig. 3 Fig. 4 The I X t (left) and I X a (right) as a function of 〈 N ch T 〉 in 4 <p T < 6 GeV/c for different multiplicity classes in pp, p–Pb and Pb–Pb collisions at s NN = 5.02 TeV. Pb–Pb results are shown assuming a flat background (filled markers), and assuming a v 2 -modulated background (empty markers). The statistical and systematic uncertainties are shown by bars and boxes, respectively. Fig. 4 Fig. 5 Comparison of the measured the I X t (left) and I X a (right) in 4 <p T < 6 GeV/c with model predictions. The results are shown as a function of 〈 N ch T 〉 for different multiplicity classes in pp (top panel), p–Pb (middle panel) and Pb–Pb (bottom panel) collisions at s NN = 5.02 TeV. The red and magenta lines show the PYTHIA 8 (Monash) [28] and PYTHIA 8/Angantyr [28] predictions, respectively. The blue lines show the EPOS-LHC [21] results. The statistical and systematic uncertainties are shown by bars and boxes, respectively. Fig. 5 Table 1 Contributions to the relative (%) systematic uncertainty on the p T spectra of primary charged particles in pp, p–Pb, and Pb–Pb collisions at s NN = 5.02 TeV. Just for illustration, the range in the table corresponds to the lowest and highest relative systematic uncertainty in the considered p T range. The individual contributions are summed in quadrature to obtain the total uncertainty. Table 1 Source of uncertainty pp p–Pb Pb–Pb |
Track selection 2.1–8.2 2.4–5.8 3.0–9.9 |
Particle composition 0.3–1.8 0.5–1.9 0.3–2.4 |
Secondary particles 0.0–0.4 0.0–2.4 0.0–1.9 |
Matching efficiency 2.0–4.2 0.7–3.7 0.6–3.7 |
Total 3.2–8.8 3.6–6.3 3.5–10.0 |
Total (N ch -dependent) 2.0–4.5 1.7–4.0 1.1–3.7 |
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Inaba 122 G.M. Innocenti 32 M. Ippolitov 139 A. Isakov 86 T. Isidori 115 M.S. Islam 99 M. Ivanov 98 V. Ivanov 139 V. Izucheev 139 M. Jablonski 2 B. Jacak 74 N. Jacazio 32 P.M. Jacobs 74 S. Jadlovska 105 J. Jadlovsky 105 L. Jaffe 38 C. Jahnke 110 M.A. Janik 132 T. Janson 69 M. Jercic 89 O. Jevons 100 A.A.P. Jimenez 64 F. Jonas 87 134 P.G. Jones 100 J.M. Jowett 32 98 J. Jung 63 M. Jung 63 A. Junique 32 A. Jusko 100 M.J. Kabus 32 132 J. Kaewjai 104 P. Kalinak 59 A.S. Kalteyer 98 A. Kalweit 32 V. Kaplin 139 A. Karasu Uysal 71 D. Karatovic 89 O. Karavichev 139 T. Karavicheva 139 P. Karczmarczyk 132 E. Karpechev 139 V. Kashyap 80 A. Kazantsev 139 U. Kebschull 69 R. Keidel 138 D.L.D. Keijdener 58 M. Keil 32 B. Ketzer 42 A.M. Khan 6 S. Khan 15 A. Khanzadeev 139 Y. Kharlov 139 A. Khatun 15 A. Khuntia 106 B. Kileng 34 B. Kim 16 C. Kim 16 D.J. Kim 114 E.J. Kim 68 J. Kim 137 J.S. Kim 40 J. Kim 95 J. Kim 68 M. Kim 95 S. Kim 17 T. Kim 137 S. Kirsch 63 I. Kisel 38 S. Kiselev 139 A. Kisiel 132 J.P. 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Lesser 18 M. Lettrich 96 P. Lévai 135 X. Li 10 X.L. Li 6 J. Lien 118 R. Lietava 100 B. Lim 16 S.H. Lim 16 V. Lindenstruth 38 A. Lindner 45 C. Lippmann 98 A. Liu 18 D.H. Liu 6 J. Liu 116 I.M. Lofnes 20 V. Loginov 139 C. Loizides 87 P. Loncar 33 J.A. Lopez 95 X. Lopez 124 E. López Torres 7 P. Lu 98 117 J.R. Luhder 134 M. Lunardon 27 G. Luparello 56 Y.G. Ma 39 A. Maevskaya 139 M. Mager 32 T. Mahmoud 42 A. Maire 126 M. Malaev 139 N.M. Malik 91 Q.W. Malik 19 S.K. Malik 91 L. Malinina 140 VII D. Mal'Kevich 139 D. Mallick 80 N. Mallick 47 G. Mandaglio 30 52 V. Manko 139 F. Manso 124 V. Manzari 49 Y. Mao 6 G.V. Margagliotti 23 A. Margotti 50 A. Marín 98 C. Markert 107 M. Marquard 63 N.A. Martin 95 P. Martinengo 32 J.L. Martinez 113 M.I. Martínez 44 G. Martínez García 103 S. Masciocchi 98 M. Masera 24 A. Masoni 51 L. Massacrier 72 A. Mastroserio 128 49 A.M. Mathis 96 O. Matonoha 75 P.F.T. Matuoka 109 A. Matyja 106 C. Mayer 106 A.L. Mazuecos 32 F. Mazzaschi 24 M. Mazzilli 32 J.E. Mdhluli 120 A.F. Mechler 63 Y. Melikyan 139 A. Menchaca-Rocha 66 E. Meninno 102 28 A.S. Menon 113 M. Meres 12 S. Mhlanga 112 67 Y. Miake 122 L. Micheletti 55 L.C. Migliorin 125 D.L. Mihaylov 96 K. Mikhaylov 140 139 A.N. Mishra 135 D. Miśkowiec 98 A. Modak 4 A.P. Mohanty 58 B. Mohanty 80 M. Mohisin Khan 15 V M.A. Molander 43 Z. Moravcova 83 C. Mordasini 96 D.A. Moreira De Godoy 134 I. Morozov 139 A. Morsch 32 T. Mrnjavac 32 V. Muccifora 48 E. Mudnic 33 S. Muhuri 131 J.D. Mulligan 74 A. Mulliri 22 M.G. Munhoz 109 R.H. Munzer 63 H. Murakami 121 S. Murray 112 L. Musa 32 J. Musinsky 59 J.W. Myrcha 132 B. Naik 120 R. Nair 79 B.K. Nandi 46 R. Nania 50 E. Nappi 49 A.F. Nassirpour 75 A. Nath 95 C. Nattrass 119 A. Neagu 19 A. Negru 123 L. Nellen 64 S.V. Nesbo 34 G. Neskovic 38 D. Nesterov 139 B.S. Nielsen 83 E.G. Nielsen 83 S. Nikolaev 139 S. Nikulin 139 V. Nikulin 139 F. Noferini 50 S. Noh 11 P. Nomokonov 140 J. Norman 116 N. Novitzky 122 P. Nowakowski 132 A. Nyanin 139 J. Nystrand 20 M. Ogino 76 A. Ohlson 75 V.A. Okorokov 139 J. Oleniacz 132 A.C. Oliveira Da Silva 119 M.H. Oliver 136 A. Onnerstad 114 C. Oppedisano 55 A. Ortiz Velasquez 64 A. Oskarsson 75 J. Otwinowski 106 M. Oya 93 K. Oyama 76 Y. Pachmayer 95 S. Padhan 46 D. Pagano 130 54 G. Paić 64 A. Palasciano 49 S. Panebianco 127 J. Park 57 J.E. Parkkila 32 114 S.P. Pathak 113 R.N. Patra 91 B. Paul 22 H. Pei 6 T. Peitzmann 58 X. Peng 6 L.G. Pereira 65 H. Pereira Da Costa 127 D. Peresunko 139 G.M. Perez 7 S. Perrin 127 Y. Pestov 139 V. Petráček 35 V. Petrov 139 M. Petrovici 45 R.P. Pezzi 103 65 S. Piano 56 M. Pikna 12 P. Pillot 103 O. Pinazza 50 32 L. Pinsky 113 C. Pinto 96 26 S. Pisano 48 M. Płoskoń 74 M. Planinic 89 F. Pliquett 63 M.G. Poghosyan 87 S. Politano 29 N. Poljak 89 A. Pop 45 S. Porteboeuf-Houssais 124 J. Porter 74 V. Pozdniakov 140 S.K. Prasad 4 S. Prasad 47 R. Preghenella 50 F. Prino 55 C.A. Pruneau 133 I. Pshenichnov 139 M. Puccio 32 S. Qiu 84 L. Quaglia 24 R.E. Quishpe 113 S. Ragoni 100 A. Rakotozafindrabe 127 L. Ramello 129 55 F. Rami 126 S.A.R. Ramirez 44 T.A. Rancien 73 R. Raniwala 92 S. Raniwala 92 S.S. Räsänen 43 R. Rath 47 I. Ravasenga 84 K.F. Read 87 119 A.R. Redelbach 38 K. Redlich 79 VI A. Rehman 20 P. Reichelt 63 F. Reidt 32 H.A. Reme-Ness 34 Z. Rescakova 37 K. Reygers 95 A. Riabov 139 V. Riabov 139 R. Ricci 28 T. Richert 75 M. Richter 19 W. Riegler 32 F. Riggi 26 C. Ristea 62 M. Rodríguez Cahuantzi 44 K. Røed 19 R. Rogalev 139 E. Rogochaya 140 T.S. Rogoschinski 63 D. Rohr 32 D. Röhrich 20 P.F. Rojas 44 S. Rojas Torres 35 P.S. Rokita 132 F. Ronchetti 48 A. Rosano 30 52 E.D. Rosas 64 A. Rossi 53 A. Roy 47 P. Roy 99 S. Roy 46 N. Rubini 25 O.V. Rueda 75 D. Ruggiano 132 R. Rui 23 B. Rumyantsev 140 P.G. Russek 2 R. Russo 84 A. Rustamov 81 E. Ryabinkin 139 Y. Ryabov 139 A. Rybicki 106 H. Rytkonen 114 W. Rzesa 132 O.A.M. Saarimaki 43 R. Sadek 103 S. Sadovsky 139 J. Saetre 20 K. Šafařík 35 S.K. Saha 131 S. Saha 80 B. Sahoo 46 P. Sahoo 46 R. Sahoo 47 S. Sahoo 60 D. Sahu 47 P.K. Sahu 60 J. Saini 131 K. Sajdakova 37 S. Sakai 122 M.P. Salvan 98 S. Sambyal 91 T.B. Saramela 109 D. Sarkar 133 N. Sarkar 131 P. Sarma 41 V. Sarritzu 22 V.M. Sarti 96 M.H.P. Sas 136 J. Schambach 87 H.S. Scheid 63 C. Schiaua 45 R. Schicker 95 A. Schmah 95 C. Schmidt 98 H.R. Schmidt 94 M.O. Schmidt 32 M. Schmidt 94 N.V. Schmidt 87 63 A.R. Schmier 119 R. Schotter 126 J. Schukraft 32 K. Schwarz 98 K. Schweda 98 G. Scioli 25 E. Scomparin 55 J.E. Seger 14 Y. Sekiguchi 121 D. Sekihata 121 I. Selyuzhenkov 98 139 S. Senyukov 126 J.J. Seo 57 D. Serebryakov 139 L. Šerkšnytė 96 A. Sevcenco 62 T.J. Shaba 67 A. Shabanov 139 A. Shabetai 103 R. Shahoyan 32 W. Shaikh 99 A. Shangaraev 139 A. Sharma 90 D. Sharma 46 H. Sharma 106 M. Sharma 91 N. Sharma 90 S. Sharma 91 U. Sharma 91 A. Shatat 72 O. Sheibani 113 K. Shigaki 93 M. Shimomura 77 S. Shirinkin 139 Q. Shou 39 Y. Sibiriak 139 S. Siddhanta 51 T. Siemiarczuk 79 T.F. Silva 109 D. Silvermyr 75 T. Simantathammakul 104 R. Simeonov 36 G. Simonetti 32 B. Singh 91 B. Singh 96 R. Singh 80 R. Singh 91 R. Singh 47 V.K. Singh 131 V. Singhal 131 T. Sinha 99 B. Sitar 12 M. Sitta 129 55 T.B. Skaali 19 G. Skorodumovs 95 M. Slupecki 43 N. Smirnov 136 R.J.M. Snellings 58 E.H. Solheim 19 C. Soncco 101 J. Song 113 A. Songmoolnak 104 F. Soramel 27 S. Sorensen 119 R. Spijkers 84 I. Sputowska 106 J. Staa 75 J. Stachel 95 I. Stan 62 P.J. Steffanic 119 S.F. Stiefelmaier 95 D. Stocco 103 I. Storehaug 19 M.M. Storetvedt 34 P. Stratmann 134 S. Strazzi 25 C.P. Stylianidis 84 A.A.P. Suaide 109 C. Suire 72 M. Sukhanov 139 M. Suljic 32 V. Sumberia 91 S. Sumowidagdo 82 S. Swain 60 A. Szabo 12 I. Szarka 12 U. Tabassam 13 S.F. Taghavi 96 G. Taillepied 98 124 J. Takahashi 110 G.J. Tambave 20 S. Tang 124 6 Z. Tang 117 J.D. Tapia Takaki 115 N. Tapus 123 L.A. Tarasovicova 134 M.G. Tarzila 45 A. Tauro 32 A. Telesca 32 L. Terlizzi 24 C. Terrevoli 113 G. Tersimonov 3 S. Thakur 131 D. Thomas 107 R. Tieulent 125 A. Tikhonov 139 A.R. Timmins 113 M. Tkacik 105 T. Tkacik 105 A. Toia 63 N. Topilskaya 139 M. Toppi 48 F. Torales-Acosta 18 T. Tork 72 A.G. Torres Ramos 31 A. Trifiró 30 52 A.S. Triolo 30 52 S. Tripathy 50 T. Tripathy 46 S. Trogolo 32 V. Trubnikov 3 W.H. Trzaska 114 T.P. Trzcinski 132 R. Turrisi 53 T.S. Tveter 19 K. Ullaland 20 B. Ulukutlu 96 A. Uras 125 M. Urioni 54 130 G.L. Usai 22 M. Vala 37 N. Valle 21 S. Vallero 55 L.V.R. van Doremalen 58 M. van Leeuwen 84 C.A. van Veen 95 R.J.G. van Weelden 84 P. Vande Vyvre 32 D. Varga 135 Z. Varga 135 M. Varga-Kofarago 135 M. Vasileiou 78 A. Vasiliev 139 O. Vázquez Doce 96 V. Vechernin 139 E. Vercellin 24 S. Vergara Limón 44 L. Vermunt 58 R. Vértesi 135 M. Verweij 58 L. Vickovic 33 Z. Vilakazi 120 O. Villalobos Baillie 100 G. Vino 49 A. Vinogradov 139 T. Virgili 28 V. Vislavicius 83 A. Vodopyanov 140 B. Volkel 32 M.A. Völkl 95 K. Voloshin 139 S.A. Voloshin 133 G. Volpe 31 B. von Haller 32 I. Vorobyev 96 N. Vozniuk 139 J. Vrláková 37 B. Wagner 20 C. Wang 39 D. Wang 39 M. Weber 102 A. Wegrzynek 32 F.T. Weiglhofer 38 S.C. Wenzel 32 J.P. Wessels 134 S.L. Weyhmiller 136 J. Wiechula 63 J. Wikne 19 G. Wilk 79 J. Wilkinson 98 G.A. Willems 134 B. Windelband 95 M. Winn 127 J.R. Wright 107 W. Wu 39 Y. Wu 117 R. Xu 6 A.K. Yadav 131 S. Yalcin 71 Y. Yamaguchi 93 K. Yamakawa 93 S. Yang 20 S. Yano 93 Z. Yin 6 I.-K. Yoo 16 J.H. Yoon 57 S. Yuan 20 A. Yuncu 95 V. Zaccolo 23 C. Zampolli 32 H.J.C. Zanoli 58 F. Zanone 95 N. Zardoshti 32 100 A. Zarochentsev 139 P. Závada 61 N. Zaviyalov 139 M. Zhalov 139 B. Zhang 6 S. Zhang 39 X. Zhang 6 Y. Zhang 117 M. Zhao 10 V. Zherebchevskii 139 Y. Zhi 10 N. Zhigareva 139 D. Zhou 6 Y. Zhou 83 J. Zhu 98 6 Y. Zhu 6 G. Zinovjev 3 I N. Zurlo 130 54 1 A.I. Alikhanyan National Science Laboratory (Yerevan Physics Institute) Foundation, Yerevan, Armenia A.I. Alikhanyan National Science Laboratory (Yerevan Physics Institute) Foundation Yerevan Armenia A.I. Alikhanyan National Science Laboratory (Yerevan Physics Institute) Foundation, Yerevan, Armenia 2 AGH University of Science and Technology, Cracow, Poland AGH University of Science and Technology Cracow Poland AGH University of Science and Technology, Cracow, Poland 3 Bogolyubov Institute for Theoretical Physics, National Academy of Sciences of Ukraine, Kiev, Ukraine Bogolyubov Institute for Theoretical Physics National Academy of Sciences of Ukraine Kiev Ukraine Bogolyubov Institute for Theoretical Physics, National Academy of Sciences of Ukraine, Kiev, Ukraine 4 Bose Institute, Department of Physics and Centre for Astroparticle Physics and Space Science (CAPSS), Kolkata, India Bose Institute Department of Physics Centre for Astroparticle Physics and Space Science (CAPSS) Kolkata India Bose Institute, Department of Physics and Centre for Astroparticle Physics and Space Science (CAPSS), Kolkata, India 5 California Polytechnic State University, San Luis Obispo, CA, United States California Polytechnic State University San Luis Obispo CA United States California Polytechnic State University, San Luis Obispo, California, United States 6 Central China Normal University, Wuhan, China Central China Normal University Wuhan China Central China Normal University, Wuhan, China 7 Centro de Aplicaciones Tecnológicas y Desarrollo Nuclear (CEADEN), Havana, Cuba Centro de Aplicaciones Tecnológicas y Desarrollo Nuclear (CEADEN) Havana Cuba Centro de Aplicaciones Tecnológicas y Desarrollo Nuclear (CEADEN), Havana, Cuba 8 Centro de Investigación y de Estudios Avanzados (CINVESTAV), Mexico City and Mérida, Mexico Centro de Investigación y de Estudios Avanzados (CINVESTAV) Mexico City and Mérida Mexico Centro de Investigación y de Estudios Avanzados (CINVESTAV), Mexico City and Mérida, Mexico 9 Chicago State University, Chicago, IL, United States Chicago State University Chicago IL United States Chicago State University, Chicago, Illinois, United States 10 China Institute of Atomic Energy, Beijing, China China Institute of Atomic Energy Beijing China China Institute of Atomic Energy, Beijing, China 11 Chungbuk National University, Cheongju, Republic of Korea Chungbuk National University Cheongju Republic of Korea Chungbuk National University, Cheongju, Republic of Korea 12 Comenius University Bratislava, Faculty of Mathematics, Physics and Informatics, Bratislava, Slovak Republic Comenius University Bratislava Faculty of Mathematics, Physics and Informatics Bratislava Slovak Republic Comenius University Bratislava, Faculty of Mathematics, Physics and Informatics, Bratislava, Slovak Republic 13 COMSATS University Islamabad, Islamabad, Pakistan COMSATS University Islamabad Islamabad Pakistan COMSATS University Islamabad, Islamabad, Pakistan 14 Creighton University, Omaha, NE, United States Creighton University Omaha NE United States Creighton University, Omaha, Nebraska, United States 15 Department of Physics, Aligarh Muslim University, Aligarh, India Department of Physics Aligarh Muslim University Aligarh India Department of Physics, Aligarh Muslim University, Aligarh, India 16 Department of Physics, Pusan National University, Pusan, Republic of Korea Department of Physics Pusan National University Pusan Republic of Korea Department of Physics, Pusan National University, Pusan, Republic of Korea 17 Department of Physics, Sejong University, Seoul, Republic of Korea Department of Physics Sejong University Seoul Republic of Korea Department of Physics, Sejong University, Seoul, Republic of Korea 18 Department of Physics, University of California, Berkeley, CA, United States Department of Physics University of California Berkeley CA United States Department of Physics, University of California, Berkeley, California, United States 19 Department of Physics, University of Oslo, Oslo, Norway Department of Physics University of Oslo Oslo Norway Department of Physics, University of Oslo, Oslo, Norway 20 Department of Physics and Technology, University of Bergen, Bergen, Norway Department of Physics and Technology University of Bergen Bergen Norway Department of Physics and Technology, University of Bergen, Bergen, Norway 21 Dipartimento di Fisica, Università di Pavia, Pavia, Italy Dipartimento di Fisica Università di Pavia Pavia Italy Dipartimento di Fisica, Università di Pavia, Pavia, Italy 22 Dipartimento di Fisica dell'Università and Sezione INFN, Cagliari, Italy Dipartimento di Fisica dell'Università Sezione INFN Cagliari Italy Dipartimento di Fisica dell'Università and Sezione INFN, Cagliari, Italy 23 Dipartimento di Fisica dell'Università and Sezione INFN, Trieste, Italy Dipartimento di Fisica dell'Università Sezione INFN Trieste Italy Dipartimento di Fisica dell'Università and Sezione INFN, Trieste, Italy 24 Dipartimento di Fisica dell'Università and Sezione INFN, Turin, Italy Dipartimento di Fisica dell'Università Sezione INFN Turin Italy Dipartimento di Fisica dell'Università and Sezione INFN, Turin, Italy 25 Dipartimento di Fisica e Astronomia dell'Università and Sezione INFN, Bologna, Italy Dipartimento di Fisica e Astronomia dell'Università Sezione INFN Bologna Italy Dipartimento di Fisica e Astronomia dell'Università and Sezione INFN, Bologna, Italy 26 Dipartimento di Fisica e Astronomia dell'Università and Sezione INFN, Catania, Italy Dipartimento di Fisica e Astronomia dell'Università Sezione INFN Catania Italy Dipartimento di Fisica e Astronomia dell'Università and Sezione INFN, Catania, Italy 27 Dipartimento di Fisica e Astronomia dell'Università and Sezione INFN, Padova, Italy Dipartimento di Fisica e Astronomia dell'Università Sezione INFN Padova Italy Dipartimento di Fisica e Astronomia dell'Università and Sezione INFN, Padova, Italy 28 Dipartimento di Fisica ‘E.R. Caianiello’ dell'Università and Gruppo Collegato INFN, Salerno, Italy Dipartimento di Fisica ‘E.R. Caianiello’ dell'Università Gruppo Collegato INFN Salerno Italy Dipartimento di Fisica ‘E.R. Caianiello’ dell'Università and Gruppo Collegato INFN, Salerno, Italy 29 Dipartimento DISAT del Politecnico and Sezione INFN, Turin, Italy Dipartimento DISAT del Politecnico Sezione INFN Turin Italy Dipartimento DISAT del Politecnico and Sezione INFN, Turin, Italy 30 Dipartimento di Scienze MIFT, Università di Messina, Messina, Italy Dipartimento di Scienze MIFT Università di Messina Messina Italy Dipartimento di Scienze MIFT, Università di Messina, Messina, Italy 31 Dipartimento Interateneo di Fisica ‘M. Merlin’ and Sezione INFN, Bari, Italy Dipartimento Interateneo di Fisica ‘M. Merlin’ Sezione INFN Bari Italy Dipartimento Interateneo di Fisica ‘M. Merlin’ and Sezione INFN, Bari, Italy 32 European Organization for Nuclear Research (CERN), Geneva, Switzerland European Organization for Nuclear Research (CERN) Geneva Switzerland European Organization for Nuclear Research (CERN), Geneva, Switzerland 33 Faculty of Electrical Engineering, Mechanical Engineering and Naval Architecture, University of Split, Split, Croatia Faculty of Electrical Engineering, Mechanical Engineering and Naval Architecture University of Split Split Croatia Faculty of Electrical Engineering, Mechanical Engineering and Naval Architecture, University of Split, Split, Croatia 34 Faculty of Engineering and Science, Western Norway University of Applied Sciences, Bergen, Norway Faculty of Engineering and Science Western Norway University of Applied Sciences Bergen Norway Faculty of Engineering and Science, Western Norway University of Applied Sciences, Bergen, Norway 35 Faculty of Nuclear Sciences and Physical Engineering, Czech Technical University in Prague, Prague, Czech Republic Faculty of Nuclear Sciences and Physical Engineering Czech Technical University in Prague Prague Czech Republic Faculty of Nuclear Sciences and Physical Engineering, Czech Technical University in Prague, Prague, Czech Republic 36 Faculty of Physics, Sofia University, Sofia, Bulgaria Faculty of Physics Sofia University Sofia Bulgaria Faculty of Physics, Sofia University, Sofia, Bulgaria 37 Faculty of Science, P.J. Šafárik University, Košice, Slovak Republic Faculty of Science P.J. Šafárik University Košice Slovak Republic Faculty of Science, P.J. Šafárik University, Košice, Slovak Republic 38 Frankfurt Institute for Advanced Studies, Johann Wolfgang Goethe-Universität Frankfurt, Frankfurt, Germany Frankfurt Institute for Advanced Studies Johann Wolfgang Goethe-Universität Frankfurt Frankfurt Germany Frankfurt Institute for Advanced Studies, Johann Wolfgang Goethe-Universität Frankfurt, Frankfurt, Germany 39 Fudan University, Shanghai, China Fudan University Shanghai China Fudan University, Shanghai, China 40 Gangneung-Wonju National University, Gangneung, Republic of Korea Gangneung-Wonju National University Gangneung Republic of Korea Gangneung-Wonju National University, Gangneung, Republic of Korea 41 Gauhati University, Department of Physics, Guwahati, India Gauhati University Department of Physics Guwahati India Gauhati University, Department of Physics, Guwahati, India 42 Helmholtz-Institut für Strahlen- und Kernphysik, Rheinische Friedrich-Wilhelms-Universität Bonn, Bonn, Germany Helmholtz-Institut für Strahlen- und Kernphysik Rheinische Friedrich-Wilhelms-Universität Bonn Bonn Germany Helmholtz-Institut für Strahlen- und Kernphysik, Rheinische Friedrich-Wilhelms-Universität Bonn, Bonn, Germany 43 Helsinki Institute of Physics (HIP), Helsinki, Finland Helsinki Institute of Physics (HIP) Helsinki Finland Helsinki Institute of Physics (HIP), Helsinki, Finland 44 High Energy Physics Group, Universidad Autónoma de Puebla, Puebla, Mexico High Energy Physics Group Universidad Autónoma de Puebla Puebla Mexico High Energy Physics Group, Universidad Autónoma de Puebla, Puebla, Mexico 45 Horia Hulubei National Institute of Physics and Nuclear Engineering, Bucharest, Romania Horia Hulubei National Institute of Physics and Nuclear Engineering Bucharest Romania Horia Hulubei National Institute of Physics and Nuclear Engineering, Bucharest, Romania 46 Indian Institute of Technology Bombay (IIT), Mumbai, India Indian Institute of Technology Bombay (IIT) Mumbai India Indian Institute of Technology Bombay (IIT), Mumbai, India 47 Indian Institute of Technology Indore, Indore, India Indian Institute of Technology Indore Indore India Indian Institute of Technology Indore, Indore, India 48 INFN, Laboratori Nazionali di Frascati, Frascati, Italy INFN, Laboratori Nazionali di Frascati Frascati Italy INFN, Laboratori Nazionali di Frascati, Frascati, Italy 49 INFN, Sezione di Bari, Bari, Italy INFN, Sezione di Bari Bari Italy INFN, Sezione di Bari, Bari, Italy 50 INFN, Sezione di Bologna, Bologna, Italy INFN, Sezione di Bologna Bologna Italy INFN, Sezione di Bologna, Bologna, Italy 51 INFN, Sezione di Cagliari, Cagliari, Italy INFN, Sezione di Cagliari Cagliari Italy INFN, Sezione di Cagliari, Cagliari, Italy 52 INFN, Sezione di Catania, Catania, Italy INFN, Sezione di Catania Catania Italy INFN, Sezione di Catania, Catania, Italy 53 INFN, Sezione di Padova, Padova, Italy INFN, Sezione di Padova Padova Italy INFN, Sezione di Padova, Padova, Italy 54 INFN, Sezione di Pavia, Pavia, Italy INFN, Sezione di Pavia Pavia Italy INFN, Sezione di Pavia, Pavia, Italy 55 INFN, Sezione di Torino, Turin, Italy INFN, Sezione di Torino Turin Italy INFN, Sezione di Torino, Turin, Italy 56 INFN, Sezione di Trieste, Trieste, Italy INFN, Sezione di Trieste Trieste Italy INFN, Sezione di Trieste, Trieste, Italy 57 Inha University, Incheon, Republic of Korea Inha University Incheon Republic of Korea Inha University, Incheon, Republic of Korea 58 Institute for Gravitational and Subatomic Physics (GRASP), Utrecht University/Nikhef, Utrecht, Netherlands Institute for Gravitational and Subatomic Physics (GRASP) Utrecht University/Nikhef Utrecht Netherlands Institute for Gravitational and Subatomic Physics (GRASP), Utrecht University/Nikhef, Utrecht, Netherlands 59 Institute of Experimental Physics, Slovak Academy of Sciences, Košice, Slovak Republic Institute of Experimental Physics Slovak Academy of Sciences Košice Slovak Republic Institute of Experimental Physics, Slovak Academy of Sciences, Košice, Slovak Republic 60 Institute of Physics, Homi Bhabha National Institute, Bhubaneswar, India Institute of Physics Homi Bhabha National Institute Bhubaneswar India Institute of Physics, Homi Bhabha National Institute, Bhubaneswar, India 61 Institute of Physics of the Czech Academy of Sciences, Prague, Czech Republic Institute of Physics of the Czech Academy of Sciences Prague Czech Republic Institute of Physics of the Czech Academy of Sciences, Prague, Czech Republic 62 Institute of Space Science (ISS), Bucharest, Romania Institute of Space Science (ISS) Bucharest Romania Institute of Space Science (ISS), Bucharest, Romania 63 Institut für Kernphysik, Johann Wolfgang Goethe-Universität Frankfurt, Frankfurt, Germany Institut für Kernphysik Johann Wolfgang Goethe-Universität Frankfurt Frankfurt Germany Institut für Kernphysik, Johann Wolfgang Goethe-Universität Frankfurt, Frankfurt, Germany 64 Instituto de Ciencias Nucleares, Universidad Nacional Autónoma de México, Mexico City, Mexico Instituto de Ciencias Nucleares Universidad Nacional Autónoma de México Mexico City Mexico Instituto de Ciencias Nucleares, Universidad Nacional Autónoma de México, Mexico City, Mexico 65 Instituto de Física, Universidade Federal do Rio Grande do Sul (UFRGS), Porto Alegre, Brazil Instituto de Física Universidade Federal do Rio Grande do Sul (UFRGS) Porto Alegre Brazil Instituto de Física, Universidade Federal do Rio Grande do Sul (UFRGS), Porto Alegre, Brazil 66 Instituto de Física, Universidad Nacional Autónoma de México, Mexico City, Mexico Instituto de Física Universidad Nacional Autónoma de México Mexico City Mexico Instituto de Física, Universidad Nacional Autónoma de México, Mexico City, Mexico 67 iThemba LABS, National Research Foundation, Somerset West, South Africa iThemba LABS National Research Foundation Somerset West South Africa iThemba LABS, National Research Foundation, Somerset West, South Africa 68 Jeonbuk National University, Jeonju, Republic of Korea Jeonbuk National University Jeonju Republic of Korea Jeonbuk National University, Jeonju, Republic of Korea 69 Johann-Wolfgang-Goethe Universität Frankfurt Institut für Informatik, Fachbereich Informatik und Mathematik, Frankfurt, Germany Johann-Wolfgang-Goethe Universität Frankfurt Institut für Informatik Fachbereich Informatik und Mathematik Frankfurt Germany Johann-Wolfgang-Goethe Universität Frankfurt Institut für Informatik, Fachbereich Informatik und Mathematik, Frankfurt, Germany 70 Korea Institute of Science and Technology Information, Daejeon, Republic of Korea Korea Institute of Science and Technology Information Daejeon Republic of Korea Korea Institute of Science and Technology Information, Daejeon, Republic of Korea 71 KTO Karatay University, Konya, Turkey KTO Karatay University Konya Turkey KTO Karatay University, Konya, Turkey 72 Laboratoire de Physique des 2 Infinis, Irène Joliot-Curie, Orsay, France Laboratoire de Physique des 2 Infinis, Irène Joliot-Curie Orsay France Laboratoire de Physique des 2 Infinis, Irène Joliot-Curie, Orsay, France 73 Laboratoire de Physique Subatomique et de Cosmologie, Université Grenoble-Alpes, CNRS-IN2P3, Grenoble, France Laboratoire de Physique Subatomique et de Cosmologie Université Grenoble-Alpes CNRS-IN2P3 Grenoble France Laboratoire de Physique Subatomique et de Cosmologie, Université Grenoble-Alpes, CNRS-IN2P3, Grenoble, France 74 Lawrence Berkeley National Laboratory, Berkeley, CA, United States Lawrence Berkeley National Laboratory Berkeley CA United States Lawrence Berkeley National Laboratory, Berkeley, California, United States 75 Lund University Department of Physics, Division of Particle Physics, Lund, Sweden Lund University Department of Physics Division of Particle Physics Lund Sweden Lund University Department of Physics, Division of Particle Physics, Lund, Sweden 76 Nagasaki Institute of Applied Science, Nagasaki, Japan Nagasaki Institute of Applied Science Nagasaki Japan Nagasaki Institute of Applied Science, Nagasaki, Japan 77 Nara Women's University (NWU), Nara, Japan Nara Women's University (NWU) Nara Japan Nara Women's University (NWU), Nara, Japan 78 National and Kapodistrian University of Athens, School of Science, Department of Physics , Athens, Greece National and Kapodistrian University of Athens School of Science Department of Physics Athens Greece National and Kapodistrian University of Athens, School of Science, Department of Physics , Athens, Greece 79 National Centre for Nuclear Research, Warsaw, Poland National Centre for Nuclear Research Warsaw Poland National Centre for Nuclear Research, Warsaw, Poland 80 National Institute of Science Education and Research, Homi Bhabha National Institute, Jatni, India National Institute of Science Education and Research Homi Bhabha National Institute Jatni India National Institute of Science Education and Research, Homi Bhabha National Institute, Jatni, India 81 National Nuclear Research Center, Baku, Azerbaijan National Nuclear Research Center Baku Azerbaijan National Nuclear Research Center, Baku, Azerbaijan 82 National Research and Innovation Agency - BRIN, Jakarta, Indonesia National Research and Innovation Agency - BRIN Jakarta Indonesia National Research and Innovation Agency - BRIN, Jakarta, Indonesia 83 Niels Bohr Institute, University of Copenhagen, Copenhagen, Denmark Niels Bohr Institute University of Copenhagen Copenhagen Denmark Niels Bohr Institute, University of Copenhagen, Copenhagen, Denmark 84 Nikhef, National institute for subatomic physics, Amsterdam, Netherlands Nikhef, National institute for subatomic physics Amsterdam Netherlands Nikhef, National institute for subatomic physics, Amsterdam, Netherlands 85 Nuclear Physics Group, STFC Daresbury Laboratory, Daresbury, United Kingdom Nuclear Physics Group STFC Daresbury Laboratory Daresbury United Kingdom Nuclear Physics Group, STFC Daresbury Laboratory, Daresbury, United Kingdom 86 Nuclear Physics Institute of the Czech Academy of Sciences, Husinec-Řež, Czech Republic Nuclear Physics Institute of the Czech Academy of Sciences Husinec-Řež Czech Republic Nuclear Physics Institute of the Czech Academy of Sciences, Husinec-Řež, Czech Republic 87 Oak Ridge National Laboratory, Oak Ridge, TN, United States Oak Ridge National Laboratory Oak Ridge TN United States Oak Ridge National Laboratory, Oak Ridge, Tennessee, United States 88 Ohio State University, Columbus, OH, United States Ohio State University Columbus OH United States Ohio State University, Columbus, Ohio, United States 89 Physics department, Faculty of science, University of Zagreb, Zagreb, Croatia Physics department Faculty of science, University of Zagreb Zagreb Croatia Physics department, Faculty of science, University of Zagreb, Zagreb, Croatia 90 Physics Department, Panjab University, Chandigarh, India Physics Department Panjab University Chandigarh India Physics Department, Panjab University, Chandigarh, India 91 Physics Department, University of Jammu, Jammu, India Physics Department University of Jammu Jammu India Physics Department, University of Jammu, Jammu, India 92 Physics Department, University of Rajasthan, Jaipur, India Physics Department University of Rajasthan Jaipur India Physics Department, University of Rajasthan, Jaipur, India 93 Physics Program and International Institute for Sustainability with Knotted Chiral Meta Matter (SKCM2), Hiroshima University, Hiroshima, Japan Physics Program and International Institute for Sustainability with Knotted Chiral Meta Matter (SKCM2) Hiroshima University Hiroshima Japan Physics Program and International Institute for Sustainability with Knotted Chiral Meta Matter (SKCM2), Hiroshima University, Hiroshima, Japan 94 Physikalisches Institut, Eberhard-Karls-Universität Tübingen, Tübingen, Germany Physikalisches Institut Eberhard-Karls-Universität Tübingen Tübingen Germany Physikalisches Institut, Eberhard-Karls-Universität Tübingen, Tübingen, Germany 95 Physikalisches Institut, Ruprecht-Karls-Universität Heidelberg, Heidelberg, Germany Physikalisches Institut Ruprecht-Karls-Universität Heidelberg Heidelberg Germany Physikalisches Institut, Ruprecht-Karls-Universität Heidelberg, Heidelberg, Germany 96 Physik Department, Technische Universität München, Munich, Germany Physik Department Technische Universität München Munich Germany Physik Department, Technische Universität München, Munich, Germany 97 Politecnico di Bari and Sezione INFN, Bari, Italy Politecnico di Bari Sezione INFN Bari Italy Politecnico di Bari and Sezione INFN, Bari, Italy 98 Research Division and ExtreMe Matter Institute EMMI, GSI Helmholtzzentrum für Schwerionenforschung GmbH, Darmstadt, Germany Research Division ExtreMe Matter Institute EMMI GSI Helmholtzzentrum für Schwerionenforschung GmbH Darmstadt Germany Research Division and ExtreMe Matter Institute EMMI, GSI Helmholtzzentrum für Schwerionenforschung GmbH, Darmstadt, Germany 99 Saha Institute of Nuclear Physics, Homi Bhabha National Institute, Kolkata, India Saha Institute of Nuclear Physics Homi Bhabha National Institute Kolkata India Saha Institute of Nuclear Physics, Homi Bhabha National Institute, Kolkata, India 100 School of Physics and Astronomy, University of Birmingham, Birmingham, United Kingdom School of Physics and Astronomy University of Birmingham Birmingham United Kingdom School of Physics and Astronomy, University of Birmingham, Birmingham, United Kingdom 101 Sección Física, Departamento de Ciencias, Pontificia Universidad Católica del Perú, Lima, Peru Sección Física Departamento de Ciencias Pontificia Universidad Católica del Perú Lima Peru Sección Física, Departamento de Ciencias, Pontificia Universidad Católica del Perú, Lima, Peru 102 Stefan Meyer Institut für Subatomare Physik (SMI), Vienna, Austria Stefan Meyer Institut für Subatomare Physik (SMI) Vienna Austria Stefan Meyer Institut für Subatomare Physik (SMI), Vienna, Austria 103 SUBATECH, IMT Atlantique, Nantes Université, CNRS-IN2P3, Nantes, France SUBATECH IMT Atlantique Nantes Université CNRS-IN2P3 Nantes France SUBATECH, IMT Atlantique, Nantes Université, CNRS-IN2P3, Nantes, France 104 Suranaree University of Technology, Nakhon Ratchasima, Thailand Suranaree University of Technology Nakhon Ratchasima Thailand Suranaree University of Technology, Nakhon Ratchasima, Thailand 105 Technical University of Košice, Košice, Slovak Republic Technical University of Košice Košice Slovak Republic Technical University of Košice, Košice, Slovak Republic 106 The Henryk Niewodniczanski Institute of Nuclear Physics, Polish Academy of Sciences, Cracow, Poland The Henryk Niewodniczanski Institute of Nuclear Physics Polish Academy of Sciences Cracow Poland The Henryk Niewodniczanski Institute of Nuclear Physics, Polish Academy of Sciences, Cracow, Poland 107 The University of Texas at Austin, Austin, TX, United States The University of Texas at Austin Austin TX United States The University of Texas at Austin, Austin, Texas, United States 108 Universidad Autónoma de Sinaloa, Culiacán, Mexico Universidad Autónoma de Sinaloa Culiacán Mexico Universidad Autónoma de Sinaloa, Culiacán, Mexico 109 Universidade de São Paulo (USP), São Paulo, Brazil Universidade de São Paulo (USP) São Paulo Brazil Universidade de São Paulo (USP), São Paulo, Brazil 110 Universidade Estadual de Campinas (UNICAMP), Campinas, Brazil Universidade Estadual de Campinas (UNICAMP) Campinas Brazil Universidade Estadual de Campinas (UNICAMP), Campinas, Brazil 111 Universidade Federal do ABC, Santo Andre, Brazil Universidade Federal do ABC Santo Andre Brazil Universidade Federal do ABC, Santo Andre, Brazil 112 University of Cape Town, Cape Town, South Africa University of Cape Town Cape Town South Africa University of Cape Town, Cape Town, South Africa 113 University of Houston, Houston, TX, United States University of Houston Houston TX United States University of Houston, Houston, Texas, United States 114 University of Jyväskylä, Jyväskylä, Finland University of Jyväskylä Jyväskylä Finland University of Jyväskylä, Jyväskylä, Finland 115 University of Kansas, Lawrence, KS, United States University of Kansas Lawrence KS United States University of Kansas, Lawrence, Kansas, United States 116 University of Liverpool, Liverpool, United Kingdom University of Liverpool Liverpool United Kingdom University of Liverpool, Liverpool, United Kingdom 117 University of Science and Technology of China, Hefei, China University of Science and Technology of China Hefei China University of Science and Technology of China, Hefei, China 118 University of South-Eastern Norway, Kongsberg, Norway University of South-Eastern Norway Kongsberg Norway University of South-Eastern Norway, Kongsberg, Norway 119 University of Tennessee, Knoxville, TN, United States University of Tennessee Knoxville TN United States University of Tennessee, Knoxville, Tennessee, United States 120 University of the Witwatersrand, Johannesburg, South Africa University of the Witwatersrand Johannesburg South Africa University of the Witwatersrand, Johannesburg, South Africa 121 University of Tokyo, Tokyo, Japan University of Tokyo Tokyo Japan University of Tokyo, Tokyo, Japan 122 University of Tsukuba, Tsukuba, Japan University of Tsukuba Tsukuba Japan University of Tsukuba, Tsukuba, Japan 123 University Politehnica of Bucharest, Bucharest, Romania University Politehnica of Bucharest Bucharest Romania University Politehnica of Bucharest, Bucharest, Romania 124 Université Clermont Auvergne, CNRS/IN2P3, LPC, Clermont-Ferrand, France Université Clermont Auvergne CNRS/IN2P3 LPC Clermont-Ferrand France Université Clermont Auvergne, CNRS/IN2P3, LPC, Clermont-Ferrand, France 125 Université de Lyon, CNRS/IN2P3, Institut de Physique des 2 Infinis de Lyon, Lyon, France Université de Lyon CNRS/IN2P3 Institut de Physique des 2 Infinis de Lyon Lyon France Université de Lyon, CNRS/IN2P3, Institut de Physique des 2 Infinis de Lyon, Lyon, France 126 Université de Strasbourg, CNRS, IPHC UMR 7178, F-67000 Strasbourg, France Université de Strasbourg CNRS IPHC UMR 7178 Strasbourg F-67000 France Université de Strasbourg, CNRS, IPHC UMR 7178, F-67000 Strasbourg, France, Strasbourg, France 127 Université Paris-Saclay Centre d'Etudes de Saclay (CEA), IRFU, Départment de Physique Nucléaire (DPhN), Saclay, France Université Paris-Saclay Centre d'Etudes de Saclay (CEA) IRFU Départment de Physique Nucléaire (DPhN) Saclay France Université Paris-Saclay Centre d'Etudes de Saclay (CEA), IRFU, Départment de Physique Nucléaire (DPhN), Saclay, France 128 Università degli Studi di Foggia, Foggia, Italy Università degli Studi di Foggia Foggia Italy Università degli Studi di Foggia, Foggia, Italy 129 Università del Piemonte Orientale, Vercelli, Italy Università del Piemonte Orientale Vercelli Italy Università del Piemonte Orientale, Vercelli, Italy 130 Università di Brescia, Brescia, Italy Università di Brescia Brescia Italy Università di Brescia, Brescia, Italy 131 Variable Energy Cyclotron Centre, Homi Bhabha National Institute, Kolkata, India Variable Energy Cyclotron Centre Homi Bhabha National Institute Kolkata India Variable Energy Cyclotron Centre, Homi Bhabha National Institute, Kolkata, India 132 Warsaw University of Technology, Warsaw, Poland Warsaw University of Technology Warsaw Poland Warsaw University of Technology, Warsaw, Poland 133 Wayne State University, Detroit, MI, United States Wayne State University Detroit MI United States Wayne State University, Detroit, Michigan, United States 134 Westfälische Wilhelms-Universität Münster, Institut für Kernphysik, Münster, Germany Westfälische Wilhelms-Universität Münster Institut für Kernphysik Münster Germany Westfälische Wilhelms-Universität Münster, Institut für Kernphysik, Münster, Germany 135 Wigner Research Centre for Physics, Budapest, Hungary Wigner Research Centre for Physics Budapest Hungary Wigner Research Centre for Physics, Budapest, Hungary 136 Yale University, New Haven, CT, United States Yale University New Haven CT United States Yale University, New Haven, Connecticut, United States 137 Yonsei University, Seoul, Republic of Korea Yonsei University Seoul Republic of Korea Yonsei University, Seoul, Republic of Korea 138 Zentrum für Technologie und Transfer (ZTT), Worms, Germany Zentrum für Technologie und Transfer (ZTT) Worms Germany Zentrum für Technologie und Transfer (ZTT), Worms, Germany 139 Affiliated with an institute covered by a cooperation agreement with CERN Affiliated with an institute covered by a cooperation agreement with CERN Affiliated with an institute covered by a cooperation agreement with CERN 140 Affiliated with an international laboratory covered by a cooperation agreement with CERN Affiliated with an international laboratory covered by a cooperation agreement with CERN Affiliated with an international laboratory covered by a cooperation agreement with CERN I Deceased. II Also at: Max-Planck-Institut für Physik, Munich, Germany. III Also at: Italian National Agency for New Technologies, Energy and Sustainable Economic Development (ENEA), Bologna, Italy. IV Also at: Dipartimento DET del Politecnico di Torino, Turin, Italy. V Also at: Department of Applied Physics, Aligarh Muslim University, Aligarh, India. VI Also at: Institute of Theoretical Physics, University of Wroclaw, Poland. VII Also at: An institution covered by a cooperation agreement with CERN. ⋆ E-mail address: alice-publications@cern.ch .Editor: M. Pierini Abstract This letter reports measurements which characterize the underlying event associated with hard scatterings at mid-pseudorapidity (| η | < 0.8 ) in pp, p–Pb and Pb–Pb collisions at centre-of-mass energy per nucleon pair, s NN = 5.02 TeV. The measurements are performed with ALICE at the LHC. Different multiplicity classes are defined based on the event activity measured at forward rapidities. The hard scatterings are identified by the leading particle defined as the charged particle with the largest transverse momentum (p T ) in the collision and having 8 < p T < 15 GeV/c . The p T spectra of associated particles (0.5 ≤ p T < 6 GeV/c ) are measured in different azimuthal regions defined with respect to the leading particle direction: toward, transverse, and away. The associated charged particle yields in the transverse region are subtracted from those of the away and toward regions. The remaining jet-like yields are reported as a function of the multiplicity measured in the transverse region. The measurements show a suppression of the jet-like yield in the away region and an enhancement of high-p T associated particles in the toward region in central Pb–Pb collisions, as compared to minimum-bias pp collisions. These observations are consistent with previous measurements that used two-particle correlations, and with an interpretation in terms of parton energy loss in a high-density quark gluon plasma. These yield modifications vanish in peripheral Pb–Pb collisions and are not observed in either high-multiplicity pp or p–Pb collisions. Data availability This manuscript has associated data in a HEPData repository at https://www.hepdata.net/ . 1 Introduction In proton-proton (pp) collisions, jets, originating from partonic scatterings with large momentum transfer, are accompanied by particles produced by initial- and final-state radiation (ISR and FSR, respectively), as well as, by a plethora of other mechanisms. These include proton break-up, and, in a scenario incorporating multi-parton interactions (MPI) [1,2] , several semi-hard parton-parton scatterings in a single pp collision. These jet-accompanying particles experimentally make up the underlying event (UE) and are commonly studied via azimuthal separations from the jets to minimise the influence of hard scatterings. The present study follows the strategy originally introduced by the CDF collaboration [3] . First, the leading charged particle in the event is found, i.e., the charged particle with the highest transverse momentum in the collision (p T trig ). Secondly, the associated particles (p T < p T trig ) are measured in three topological regions depending on their azimuthal angle relative to the leading particle, | Δ φ | = | φ assoc − φ trig | , see Fig. 1 . The toward region contains the primary jet within the acceptance of the detector, while the away region contains the back-scattered particles of the recoil jet [4] . In contrast, the transverse region is dominated by the underlying-event dynamics, but it also includes contributions from ISR and FSR [5] . The measurements performed at RHIC and LHC in small systems (pp, p–A, and d–A collisions) have shown for high particle multiplicities similar phenomena as were originally observed only in A–A collisions and have been attributed there to the formation of the strongly interacting quark gluon plasma [6,7] , namely, long range angular correlations and collectivity [8] . The origin of these effects in small systems is still an open question; on one hand, hydrodynamical calculations describe some aspects of the data [9] ; on the other hand, mechanisms like colour reconnection [10] , rope hadronisation [11] , and string shoving [12] can produce collective-like effects in Monte Carlo event generators such as PYTHIA 8 [13] . Thus, investigating pp collisions as a function of the charged particle multiplicity has become ever more pertinent [9,14–18] . The interpretation of the results from the analysis of high-multiplicity pp collisions is challenging due to the selection biases of the sample towards events in which partonic scatterings with large momentum transfer (hard scatterings) occurred. To mitigate this inherent bias, Martin et al. [19] suggested to use the charged-particle multiplicity in the transverse region (N ch T ) as a classifier of the activity in the collisions, since the correlation between N ch T and the hardest scattering in the collision is small. The ALICE collaboration has reported the first N ch T spectra measured in pp collisions at centre-of-mass energy, s = 13 TeV [20] . Event generators, such as PYTHIA 8 [13] and EPOS-LHC [21] , do not provide a good description of the measured distribution of the ratio N ch T /〈 N ch T 〉 , where 〈 N ch T 〉 is the event-averaged charged-particle multiplicity in the transverse region, underestimating in particular the number of collisions with large N ch T ( > 3 × 〈 N ch T 〉 ) . In the framework of MPI-based models, like those implemented in PYTHIA 8 and HERWIG 7 [22] , the probability for a hard scattering in the collision increases with decreasing impact parameterVIII VIII In event generators like PYTHIA 8 the impact parameter profile is described by an overlap matter distribution of the two incoming hadrons. between the colliding protons. Thus, requiring a high-p T particle (e.g., p T trig > 8 GeV/c ) in a given pp collision biases the selection of collisions towards those with a smaller impact parameter [23] , which in turn biases the selection towards pp collisions with more MPI [20] . This feature of the N ch T -based analysis is important for the isolation of potential MPI and colour reconnection effects, which according to PYTHIA 8, produce effects resembling collective behaviour [10] . By construction, MPI and colour reconnection effects are expected to be more relevant in the transverse region than in the away and toward regions [24] . It is worth mentioning that the MPI picture has been used to explain the p T spectra in p–Pb collisions and peripheral Pb–Pb collisions [25–27] . Studies, as a function of N ch T , are therefore important to the understanding of the effects observed in high-multiplicity pp collisions. Last but not least, measurements of UE observables are also important to tune event generators [28] that include hard partonic scatterings and MPI. This letter reports the inclusive charged-particle transverse momentum spectra in pp, p–Pb and Pb–Pb collisions at centre-of-mass energy per nucleon pair s NN = 5.02 TeV containing a high-p T leading particle within the kinematic intervals 8 ≤ p T trig < 15 GeV/c and | η | < 0.8 . This guarantees the selection of collisions in which the average activity in the transverse region is roughly flat as a function of p T trig [20] , and therefore, any additional selection on the charged particle multiplicity will only modulate the UE activity. The measurements are performed considering different event classes defined in terms of the multiplicity registered in the forward detectors. The p T spectra of associated charged particles (0.5 ≤ p T < 6 GeV/c and | η | < 0.8 ) are measured in the toward, away, and transverse regions as a function of the average charged particle multiplicity in the transverse region. To further investigate the possible modification of the particles produced in the hard scattering in pp, p–Pb, and Pb–Pb collisions, the p T distributions in the toward (d N ch t / d p T ) and away (d N ch a / d p T ) regions obtained after the subtraction of the p T spectra in the transverse region (d N ch T / d p T ) are also reported. The subtracted yields (d N ch st , sa / d p T ) are further normalised to those measured in minimum-bias (MB) pp collisions,(1) I X t , a ≡ ( d N ch t , a / d p T − d N ch T / d p T ) | X ( d N ch t , a / d p T − d N ch T / d p T ) | pp , MB = ( d N ch st , sa / d p T ) | X ( d N ch st , sa / d p T ) | pp , MB , where X indicates the collision system and the event multiplicity class. In this way, the hard process p T spectra in the toward and away regions are isolated, and thus allowing us to study possible modifications to the produced particles due to medium effects in high-multiplicity pp, p–Pb, and Pb–Pb collisions. In heavy-ion collisions, this ratio is sensitive to the same effects which were studied using the I AA quantity [29–31] , where jets produced in the early stage of the collision propagate through the hot and dense quark–gluon plasma. Their interaction with the coloured medium lead to parton-energy loss (jet quenching) [32] which, for example, results in the suppression of the charged-particle yield at high p T [33] , and the suppression of the high-p T yield in the away region [29,30] . It is worth mentioning that jet quenching effects have not been observed so far in small systems [33,34] . 2 Experiment and data analysis This analysis is based on the data recorded by the ALICE apparatus during the pp and Pb–Pb runs at s NN = 5.02 TeV in 2015, and the p–Pb run at s NN = 5.02 TeV in 2016. The present study uses the V0 detector, and the Silicon Pixel Detector (SPD) for triggering and background rejection. The V0 consists of two arrays of scintillating tiles placed on each side of the interaction point covering the full azimuthal acceptance and the pseudorapidity intervals of 2.8 < η < 5.1 (V0A) and − 3.7 < η < − 1.7 (V0C). The SPD is the innermost part of the Inner Tracking System (ITS) and it is the closest detector to the interaction point. It consists of two cylindrical silicon pixel layers at radial distances of 3.9 and 7.6 cm from the beam line and the pseudorapidity coverages of the two layers are | η | < 2 and | η | < 1.4 , respectively. The data were collected using a minimum-bias trigger, which required a signal in both V0A and V0C detectors. The offline event selection was optimised to reject beam-induced background in all collision systems by utilising the timing signals in the two V0 detectors. In Pb–Pb collisions, the beam-induced background is further suppressed by correlating the timing signals of the neutron zero degree calorimeters, which are positioned on both sides of the interaction point at 112.5 m distance along the beam axis [35] . The signals from the zero degree calorimeters are also used to suppress the contamination from electromagnetic interactions. This is performed by requesting the coincidence of the signals coming from both side zero degree calorimeters by which the background due to single nucleus electromagnetic dissociation processes is excluded. A criterion based on the offline reconstruction of multiple primary vertices in the SPD is applied to reduce the pileup caused by multiple interactions in the same bunch crossing [36] . The results presented in this letter are for minimum-bias triggered pp collisions having at least one charged particle in the pseudorapidity interval | η | < 1 (INEL>0). The INEL>0 event class corresponds to about 75% of the total inelastic cross section [37] . For pp and Pb–Pb collisions, the sample is subdivided into different multiplicity classes based on the total charge deposited in both V0 sub-detectors, which is termed as V0M amplitude [38] . For p–Pb collisions, the sample is subdivided based on the total charge deposited in V0A sub-detector (V0A amplitude) [39] , which is located in the Pb-going direction. The V0A estimator has been implemented in previous measurements that used p–Pb data (see e.g. [40] ). This allows for comparisons with other observables for similar V0A multiplicity classes. To ensure that a hard scattering took place in the collision, events are required to have a trigger particle within 8 ≤ p T trig < 15 GeV/c . In this p T trig interval, the momentum resolution effects are negligible on the extracted yields, and therefore, no p T trig resolution correction is applied. The total number of analysed collisions before the trigger particle selection are about 108 , 108 , and 107 for pp, p–Pb, and Pb–Pb collisions, respectively. The transverse momentum of particles is determined from measurements in the central barrel with the ITS and the Time Projection Chamber (TPC). The ITS is a tracking detector which consists of six cylindrical layers of silicon detectors. The TPC is a cylindrical drift detector which covers a radial distance of 85-247 cm from the beam axis and it has longitudinal dimension extending from about -250 cm to +250 cm around the nominal interaction point. Primary charged particles are measured in the pseudorapidity range of | η | < 0.8 and with p T > 0.5 GeV/c , where η is measured in the laboratory frame for the three collision systems. The configuration for p–Pb collisions with protons at 4 TeV energy colliding with Pb ions that have per-nucleon energies of (Z / A ) × 4 TeV ∼ 1.58 TeV results in a shift in the rapidity of the nucleon–nucleon centre-of-mass system by 0.465 in the direction of the proton beam (negative z-direction). Here Z and A are the atomic and mass numbers of the Pb ion, respectively. Therefore, the detector coverage | η | < 0.8 corresponds to roughly − 0.3 < | η cms | < 1.3 for p–Pb collisions. The particles with mean proper lifetime larger than 1 cm/c , which are either produced directly in the interaction or from decays of particles with mean proper lifetime smaller than 1 cm/c are termed as primary particles [41] . The track selection follows a procedure similar to the one described in Ref. [42] and only few specific details are reported here. Tracks (N tracks ) are required to have two hits in the ITS, out of which at least one should be in either of the two innermost layers. The geometrical track length L is calculated in the TPC readout plane, excluding the information from the pads at the sector boundaries (≈3 cm from the sector edges). The trajectory lengths built from radial segments, i.e. the crossed TPC pad rows, traversed in the TPC by a particle are required to be larger than 85% of the geometrical track length. The pad rows are made of at least 3 neighbouring individual observations (clusters), and their height varies from 7.5 mm to 15 mm [43] . The trajectory lengths built from clusters (one cluster per pad row) is required to be larger than 0.7 × L . The fraction of TPC clusters shared with another track is required to be lower than 0.4. The fit quality for the ITS and TPC track points must satisfy χ ITS 2 / N hits < 36 and χ TPC 2 / N clusters < 4 , respectively, where N hits and N clusters are the numbers of hits in the ITS and the number of clusters in the TPC, respectively. Only tracks with χ TPC − ITS 2 < 36 are included in the analysis, where χ TPC − ITS 2 is calculated comparing the track parameters from the combined ITS and TPC track reconstruction to that derived only from the TPC and constrained to the interaction point. The definition of χ TPC − ITS 2 can be found in Ref. [44] . To reduce the contamination from secondary particles, tracks are accepted if their distance-of-closest-approach (DCA) to the reconstructed primary interaction vertex satisfies in the longitudinal (d z ) and transverse (d xy ) directions the conditions d z < 2 cm and d xy < 0.018 cm + 0.035 (cm×GeV/c )/ p T . The measurement of the transverse momentum spectra of charged particles follows the standard procedure of the ALICE collaboration [42,45] . The raw yields are corrected for efficiency and contamination from secondary particles. The efficiency correction is calculated from Monte Carlo simulations with GEANT3 [46] transport code, which made use of PYTHIA 8 (Monash) [28] , EPOS-LHC [21] and HIJING [47] event generators for pp, p–Pb and Pb–Pb collisions, respectively and incorporated a detailed description of the detector material, geometry and response. Since the event generators do not reproduce the relative abundances of different particle species in the real data, the efficiency obtained from the simulations is re-weighted considering the particle composition from data as outlined in [42] . A multi-component template fit based on the DCA distributions from the simulation is used for the estimation of secondary contamination [42] . The p T spectra for the toward and away regions include contributions from the jet fragmentation, ISR, and FSR, as well as, the contribution from the underlying event. In order to increase the sensitivity to the hardest process of the event, the particle yields measured in the transverse region are subtracted from the corresponding yields in both the toward and away regions: d N ch t , a / d p T − d N ch T / d p T . This approach assumes that the background (UE, ISR, and FSR) in the toward and away regions is similar to the activity in the transverse region. However, one has to keep in mind that in Pb–Pb collisions two-particle correlations are affected by anisotropic transverse flow. In particular, the main contribution is due to the elliptic flow, v 2 , which is the second order coefficient in the Fourier expansion of the azimuthal distribution of the particle momenta [48] . This elliptic azimuthal anisotropy modulates the background according to:(2) B ( Δ φ ) = B 0 ( 1 + 2 V 2 cos ( 2 Δ φ ) ) , where V 2 is approximately given by the product of anisotropic flow coefficients for trigger and associated particles at their respective momenta i.e. V 2 ≈ v 2 trig v 2 assoc . The existing v 2 measurements over a broad transverse momentum range [49] suggest that the effect of the v 2 modulation of background should be more relevant in semi-central Pb–Pb collisions. The effect is expected to be important at low and intermediate transverse momenta and decreases for high transverse momentum particles [30] . In the high-p T region of interest for the jet quenching studies, namely p T > 4 GeV/c , the effect of the v 2 modulation is estimated to be small (about 5%) for Pb–Pb collisions. Given that the v 2 effect is larger in Pb–Pb collisions than in pp and p–Pb collisions, no correction for the v 2 modulation is applied for pp and p–Pb collisions since its effect is smaller than the other sources of systematic uncertainty. The results are shown as a function of the average number of charged particles in the transverse region 〈 N ch T 〉 . The values of 〈 N ch T 〉 are extracted in each multiplicity class from the N tracks distributions in the transverse region that are corrected for detector effects using a Bayesian unfolding [50] . The Bayesian unfolding requires the multiplicity response matrix, which is built from the correlation between the measured multiplicity and the multiplicity at generator level (without detector effects) in the transverse region. This has been obtained from MC simulations which include the propagation of particles through the detector using GEANT 3. As a crosscheck, the 〈 N ch T 〉 values are also calculated by integrating the transverse momentum distributions in the interval 0.5 ≤ p T < 8 GeV/c . The difference between the results from the two strategies is assigned as the systematic uncertainty on 〈 N ch T 〉 , where the effects related to the discrepancy between data and MC in the particle composition and secondary contamination are considered. This uncertainty amounts up to 3.5%, 4% and 6.5% for pp, p–Pb and Pb–Pb collisions, respectively. The systematic uncertainties related to the track selection criteria were studied by repeating the analysis varying one-by-one the track selection criteria [42,45] . In particular, the upper limits of the track fit quality parameters in the ITS (χ ITS 2 / N hits ) and in the TPC (χ TPC 2 / N clusters ) were varied in the ranges of 25–49 and 3–5, respectively. The maximum fraction of shared TPC clusters was varied between 0.2 to 1 and the maximum d z was varied between 1 and 5 cm [42] . We have also quantified the impact of not including the ITS hit requirement in the track selection. The systematic uncertainty on the primary particle composition was estimated using a procedure similar to the one described in [42] . To quantify the uncertainty due to the imperfect simulation of the detector response, the track matching between the TPC and the ITS information in the data and in the simulation were compared. To achieve this, the fraction of secondary particles was rescaled according to fits to the measured DCA distributions. After this rescaling, the agreement between data and model was found to be within 3% for all collision systems. This value was assigned as an additional systematic uncertainty [42] . The systematic uncertainty on the secondary particle contamination considers the imperfection of the method (multi-component template fit) used to extract the correction. The fit ranges were varied and the fit was repeated using templates with two (primaries, secondaries) or three (primaries, secondaries from material, secondaries from weak decays) components. The maximum spread among these variations was assigned as the systematic uncertainty on the secondary contamination. This contribution dominates at low p T . The density of materials used in simulations of the experimental setup was varied by ± 4.5% [35] , resulting in a negligible systematic uncertainty in the considered p T range of 0.5 to 6.0 GeV/c . For the estimation of total systematic uncertainty, all the above listed contributions were summed in quadrature. The systematic uncertainties are independent of the difference between the azimuthal angle of the associated particle and that of the trigger particle. The estimated systematic uncertainties on the p T spectra significantly depend on p T , while the dependence on the multiplicity classes is mild. The ranges of systematic uncertainties in the three considered collision systems are reported in Table 1 for the various sources described above. 3 Results and discussion The p T spectra measured in the transverse region for pp, p–Pb, and Pb–Pb collisions are shown in Fig. 2 (top panel). Results are presented for different multiplicity classes. The ratios between the spectra in the individual multiplicity classes and the MB (0−100%) one are shown in the bottom panel. In the p T range 0.5 − 6 GeV/c , the ratios for the highest multiplicity class (0−5%) are larger than unity and show an increasing trend with increasing p T at low p T (< 2 − 3 GeV/c ) followed at higher p T by a slow decrease. Instead, for the lowest multiplicity classes (40−60% and 60−90%) the ratios are lower than unity and follow an opposite trend with p T , decreasing at low p T and increasing for p T > 3 GeV/c . The behaviour of the ratios as a function of the event activity is reminiscent of analogous ratios as a function of the number of MPI in pp collisions simulated with PYTHIA 8, including colour reconnection [51] . In particular, at p T ≈ 2 − 3 GeV/c the p T spectrum of pp collisions with large MPI activity exhibits an enhancement with respect to the p T spectrum of MB pp collisions. The effect was not observed before in data because, in contrast to the present analysis, the jet contribution was included in the p T spectra [45] . The top (bottom) panel of Fig. 3 shows the charged particle yields for the toward (away) region after the subtraction of the yields measured in the transverse region in pp, p–Pb and Pb–Pb collisions. Results are compared with the p T spectra measured for MB pp collisions (0−100% V0M pp event class) quantified with the ratio I X t , a , as defined in Eq. (1) . At low transverse momenta, p T < 4 GeV/c , I X t , a is close to unity in pp and p–Pb collisions. In contrast, I X t , a in Pb–Pb collisions exhibits a strong multiplicity dependence over the whole measured p T interval. The I X t , a magnitude is larger for semi-peripheral Pb–Pb collisions, the maximum is observed for 20−40% Pb–Pb collisions, and is smaller for the most central and most peripheral classes. Given that the v 2 contribution is not subtracted from the jet-like yields reported in Fig. 3 , the centrality dependence of I X t , a follows the behaviour of v 2 as a function of collision centrality and particle p T in Pb–Pb collisions at LHC energies [52] . Fig. 4 shows the measured values of I X t , a in the transverse momentum interval 4 < p T < 6 GeV/c as a function of the average multiplicity in the transverse region for all the multiplicity classes considered in pp, p–Pb and Pb–Pb collisions. The figure shows that, within uncertainties, the I X t , a values are close to unity for all the multiplicity classes measured in pp and p–Pb collisions. This indicates that effects induced by possible energy loss in these systems are not observed within uncertainties. This result is consistent with previous studies of nuclear modification factor [33] and hadron-jet recoil measurements [34] . By contrast, for Pb–Pb collisions the I X t , a values are compatible to unity for peripheral collisions, and show a gradual enhancement (reduction) with the increase in multiplicity for the toward (away) region. The behaviour is the same for the I X t , a values measured either assuming a flat background or a v 2 -modulated background. The v 2 -modulated background was estimated following the approach depicted in Eq. (2) and using the v 2 data reported in [49] . This behaviour is qualitatively similar to the di-hadron correlation results reported by the STAR and ALICE collaborations [29,30] . In Pb–Pb collisions, I X t provides information about the fragmenting jet leaving the medium, while on the away side, I X a reflects the survival probability of the recoiling parton during passage through the medium. Thus a suppression of I X a would indicate that fewer partons survive the passage through the medium and is expected from the strong in-medium energy loss. On the other hand, the enhancement observed in the toward region is also subject to medium effects. The ratio is sensitive to a) a possible change of the fragmentation functions, b) a possible modification of the quark to gluon jet ratio in the final state due to different coupling with medium, and c) a possible bias on the parton spectrum due to trigger particle selection. Moreover, given that I X t , a is sensitive to the same effects as I AA , the interpretation of the results is similar to that reported in [30] . It is likely that all three effects play a role [30] . A detailed quantification of the contribution of each effect is beyond the scope of the present paper.In order to get further insight into the effect, the measured I X t , a values are compared in Fig. 5 with model predictions. Following the similar treatment of the experimental data, for the models, the total sample is subdivided into different V0M classes and the 〈 N ch T 〉 is calculated for each class. For high-multiplicity pp collisions, although I X t , a is close to unity, a small trend with multiplicity is visible, which is not seen at similar multiplicities (20−90% V0A) in p–Pb data. To understand the source of these slight deviations from unity, the data are compared with the predictions from the PYTHIA 8 (Monash tune [28] ) and EPOS-LHC [21] event generators. In PYTHIA, the hadronization of quarks is simulated using the Lund string fragmentation model [53] . Various PYTHIA tunes have been developed through extensive comparison of Monte Carlo distributions with the minimum-bias data from different experiments. The Monash tune of PYTHIA 8 is tuned to LHC data and uses an updated set of hadronization parameters compared to the previous tunes [28] . EPOS-LHC is built on the Parton-Based Gribov Regge Theory. Utilising the colour exchange mechanism of string excitation, the model is tuned to LHC data [21] . In this model, a part of the collision system which has high parton densities becomes a “core” region that evolves hydrodynamically as a quark–gluon plasma and it is surrounded by a more dilute “corona” for which fragmentation occurs in the vacuum. The upper panel of Fig. 5 shows I X t , a for different multiplicity classes. The observed deviations from unity are reproduced by PYTHIA 8 for both the toward and away regions. Given that PYTHIA 8 does not incorporate any jet quenching mechanism, the origin of the effect in high 〈 N ch T 〉 collisions is related to a remaining bias towards harder fragmentation and more activity from initial and final state radiation [54] . These effects enhance the high-p T yield in the toward region, and produce a broadening in the away region [55] . The EPOS-LHC results in the away region are similar to both data and PYTHIA 8. However, for I X t EPOS-LHC exhibits a trend with a maximum at intermediate multiplicity and a reduction toward low and high multiplicities, which is not consistent with the measurements. The middle and bottom panels of Fig. 5 show I X t , a measured for p–Pb and Pb–Pb collisions, respectively. The data are compared to PYTHIA 8/Angantyr [56] and EPOS-LHC predictions. The Angantyr model in PYTHIA 8 extrapolates the dynamics from pp collisions to p–Pb and Pb–Pb collisions, generalising the formalism adopted for pp collisions by including a description of the nucleon positions within the colliding nuclei and utilising the Glauber model to calculate the number of interacting nucleons and binary nucleon–nucleon collisions. PYTHIA 8/Angantyr, which does not include jet quenching effects, predicts I X t , a values consistent with unity for all the multiplicity classes in Pb–Pb collisions. Whereas for p–Pb collisions I X a is consistent with unity, and I X t is slightly below unity. In EPOS-LHC, a certain p T cutoff is defined in such a way that, above this cutoff, a particle loses part of its momentum in the core but survives as an independent particle produced by a flux tube. Soft particles, which are below the p T cutoff, get completely absorbed and form the core. This sort of energy loss mechanism implemented in EPOS-LHC depends on the system size [21,57,58] . Fig. 5 (middle) shows that for p–Pb collisions, EPOS-LHC does not describe either the magnitude or the trend of the multiplicity dependence of the measured ratio in the toward region, I X t . However, the model is in reasonable agreement with data in the away region. For Pb–Pb collisions, EPOS-LHC predicts a significant enhancement of I X t , a for low 〈 N ch T 〉 ranges and deviates significantly from the experimental results. In summary, while the data from Pb–Pb collisions are in qualitative agreement with expectations from parton energy loss due to the presence of a hot and dense medium, pp and p–Pb data do not show any hint of medium effects in the multiplicity range which is reported. 4 Summary The transverse momentum spectra (0.5 ≤ p T < 6 GeV/c ) of primary charged particles in three azimuthal regions (toward, away and transverse) defined with respect to the direction of the particle with the highest transverse momentum in the event (8 ≤ p T trig < 15 GeV/c ) are reported. The spectra are studied in intervals of the multiplicity measured at forward pseudorapidities for pp, p–Pb, and Pb–Pb collisions at s NN = 5.02 TeV. The p T spectra in the transverse region are subtracted from those of the away and toward regions. This is based on the assumption that the transverse side provides a good estimation of the underlying event contribution in both the toward and away regions. However, for the interpretation of the results one has to keep in mind that v 2 modulates the background and this effect is important for semi-central Pb–Pb collisions and for p T > 4 GeV/c the effect is less than 5% in central and peripheral Pb–Pb collisions. Ratios to MB pp (I X t , a ), i.e., the multiplicity dependent yields normalised to the yield measured in MB pp collisions, are reported. At low transverse momentum (p T < 2 GeV/c ), within 20%, the I X t , a values are multiplicity independent for both the toward and away regions in pp and p–Pb collisions. In contrast, in Pb–Pb collisions for both toward and away regions the I X t , a values exhibit a centrality dependence which is expected given the residual presence of elliptic flow. In the highest transverse momentum interval (4 < p T < 6 GeV/c ), the I X t , a values in pp collisions are closer to unity but they exhibit a small reduction (increase) towards high V0 activity in pp collisions. This trend is well reproduced by PYTHIA 8. In the model, it is due to a selection bias towards pp collisions with harder fragmentation and larger activity from initial and final state radiation. For p–Pb collisions, within uncertainties, the I X t , a values are consistent with unity and do not show a multiplicity dependence. PYTHIA 8/Angantyr fairly describes I X a , but it underestimates by about 10% the I X t values in the low multiplicity classes (40−90% V0A event class). For Pb–Pb collisions, the I X t , a values are close to unity for peripheral collisions, and show a gradual increase (reduction) in the toward (away) region with increasing multiplicity. A similar observable, I AA , based on the per-trigger yield of associated particles in di-hadron correlation has been studied for central and peripheral Pb–Pb collisions at s NN = 2.76 TeV. The behaviour of I X t , a exhibits the same features as I AA : in central collisions, on the away-side, a suppression is observed as expected from strong in-medium energy loss. In the toward region, an enhancement is observed. PYTHIA 8/Angantyr predicts I X t , a ≈ 1 for all multiplicity intervals, and it does not reproduce the observed away-side suppression or toward-side enhancement. Generally, EPOS-LHC does not describe the measured I X t , a ratios. In summary, within the multiplicity reach reported in this paper, no jet quenching effects are observed in pp and p–Pb collisions within uncertainties. Further studies are required to extend the present analysis to higher multiplicities, which are currently limited by the event selection based on the forward V0 detector. The analysis of future pp and p–Pb collisions with much larger integrated luminosity may remove this limitation. Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgements The ALICE Collaboration would like to thank all its engineers and technicians for their invaluable contributions to the construction of the experiment and the CERN accelerator teams for the outstanding performance of the LHC complex. The ALICE Collaboration gratefully acknowledges the resources and support provided by all Grid centres and the Worldwide LHC Computing Grid (WLCG) collaboration. The ALICE Collaboration acknowledges the following funding agencies for their support in building and running the ALICE detector: A.I. Alikhanyan National Science Laboratory (Yerevan Physics Institute) Foundation (ANSL) , State Committee of Science and World Federation of Scientists (WFS) , Armenia; Austrian Academy of Sciences , Austrian Science Fund (FWF): [M 2467-N36 ] and Nationalstiftung für Forschung, Technologie und Entwicklung , Austria; Ministry of Communications and High Technologies, National Nuclear Research Center , Azerbaijan; Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq ), Financiadora de Estudos e Projetos (Finep), Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP ) and Universidade Federal do Rio Grande do Sul (UFRGS ), Brazil; Bulgarian Ministry of Education and Science , within the National Roadmap for Research Infrastructures 2020–2027 (object CERN), Bulgaria; Ministry of Education of China (MOEC), Ministry of Science & Technology of China (MSTC) and National Natural Science Foundation of China (NSFC), China; Ministry of Science and Education and Croatian Science Foundation , Croatia; Centro de Aplicaciones Tecnológicas y Desarrollo Nuclear (CEADEN), Cubaenergía , Cuba; Ministry of Education, Youth and Sports of the Czech Republic , Czech Republic; The Danish Council for Independent Research | Natural Sciences , the Villum Fonden and Danish National Research Foundation (DNRF), Denmark; Helsinki Institute of Physics (HIP), Finland; Commissariat à l'Energie Atomique (CEA ) and Institut National de Physique Nucléaire et de Physique des Particules (IN2P3) and Centre National de la Recherche Scientifique (CNRS), France; Bundesministerium für Bildung und Forschung (BMBF ) and GSI Helmholtzzentrum für Schwerionenforschung GmbH , Germany; General Secretariat for Research and Technology , Ministry of Education, Research and Religions, Greece; National Research, Development and Innovation Office , Hungary; Department of Atomic Energy Government of India (DAE ), Department of Science and Technology, Government of India (DST ), University Grants Commission , Government of India (UGC) and Council of Scientific and Industrial Research (CSIR), India; National Research and Innovation Agency - BRIN , Indonesia; Istituto Nazionale di Fisica Nucleare (INFN ), Italy; Japanese Ministry of Education, Culture, Sports, Science and Technology (MEXT) and Japan Society for the Promotion of Science (JSPS) KAKENHI, Japan; Consejo Nacional de Ciencia (CONACYT ) y Tecnología, through Fondo de Cooperación Internacional en Ciencia y Tecnología (FONCICYT) and Dirección General de Asuntos del Personal Académico (DGAPA), Mexico; Nederlandse Organisatie voor Wetenschappelijk Onderzoek (NWO), Netherlands; The Research Council of Norway , Norway; Commission on Science and Technology for Sustainable Development in the South (COMSATS), Pakistan; Pontificia Universidad Católica del Perú , Peru; Ministry of Education and Science , National Science Centre and WUT ID-UB , Poland; Korea Institute of Science and Technology Information and National Research Foundation of Korea (NRF), Republic of Korea; Ministry of Education and Scientific Research , Institute of Atomic Physics , Ministry of Research and Innovation and Institute of Atomic Physics and University Politehnica of Bucharest , Romania; Ministry of Education, Science, Research and Sport of the Slovak Republic , Slovakia; National Research Foundation of South Africa , South Africa; Swedish Research Council (VR) and Knut & Alice Wallenberg Foundation (KAW), Sweden; European Organization for Nuclear Research , Switzerland; Suranaree University of Technology (SUT), National Science and Technology Development Agency (NSTDA), Thailand Science Research and Innovation (TSRI) and National Science, Research and Innovation Fund (NSRF), Thailand; Turkish Energy, Nuclear and Mineral Research Agency (TENMAK), Turkey; National Academy of Sciences of Ukraine , Ukraine; Science and Technology Facilities Council (STFC), United Kingdom; National Science Foundation of the United States of America (NSF ) and United States Department of Energy , Office of Nuclear Physics (DOE NP), United States of America. 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+]>PLB 137649 137649 S0370-2693(22)00783-3 10.1016/j.physletb.2022.137649 The Author(s) Experiments https://www.hepdata.net/ Fig. 1 Illustration of toward, away and transverse regions with respect to the leading particle in a collision. Fig. 1 Fig. 2 Top panels: transverse momentum spectra of charged particles in the transverse region for different multiplicity classes in pp (left), p–Pb (middle) and Pb–Pb (right) collisions at s NN = 5.02 TeV. The p T spectra are measured at mid pseudorapidity (|η | < 0.8). Lower panels: Ratio of p T spectra in different multiplicity classes to the p T spectrum in the 0−100% multiplicity class for the corresponding collision systems. The statistical and systematic uncertainties are shown by bars and boxes, respectively. Fig. 2 Fig. 3 Transverse momentum spectra of charged particles in Toward-Transverse, d N ch st / d p T (top plot) and Away-Transverse, d N ch sa / d p T (bottom plot) regions for different multiplicity classes in pp (left), p–Pb (middle) and Pb–Pb (right) collisions at s NN = 5.02 TeV. The p T spectra are measured at mid pseudorapidity (|η | < 0.8). The lower panels of both plots show the ratio to minimum bias pp collisions. The statistical and systematic uncertainties are shown by bars and boxes, respectively. Fig. 3 Fig. 4 The I X t (left) and I X a (right) as a function of 〈 N ch T 〉 in 4 <p T < 6 GeV/c for different multiplicity classes in pp, p–Pb and Pb–Pb collisions at s NN = 5.02 TeV. Pb–Pb results are shown assuming a flat background (filled markers), and assuming a v 2 -modulated background (empty markers). The statistical and systematic uncertainties are shown by bars and boxes, respectively. Fig. 4 Fig. 5 Comparison of the measured the I X t (left) and I X a (right) in 4 <p T < 6 GeV/c with model predictions. The results are shown as a function of 〈 N ch T 〉 for different multiplicity classes in pp (top panel), p–Pb (middle panel) and Pb–Pb (bottom panel) collisions at s NN = 5.02 TeV. The red and magenta lines show the PYTHIA 8 (Monash) [28] and PYTHIA 8/Angantyr [28] predictions, respectively. The blue lines show the EPOS-LHC [21] results. The statistical and systematic uncertainties are shown by bars and boxes, respectively. Fig. 5 Table 1 Contributions to the relative (%) systematic uncertainty on the p T spectra of primary charged particles in pp, p–Pb, and Pb–Pb collisions at s NN = 5.02 TeV. Just for illustration, the range in the table corresponds to the lowest and highest relative systematic uncertainty in the considered p T range. The individual contributions are summed in quadrature to obtain the total uncertainty. Table 1 Source of uncertainty pp p–Pb Pb–Pb |
Track selection 2.1–8.2 2.4–5.8 3.0–9.9 |
Particle composition 0.3–1.8 0.5–1.9 0.3–2.4 |
Secondary particles 0.0–0.4 0.0–2.4 0.0–1.9 |
Matching efficiency 2.0–4.2 0.7–3.7 0.6–3.7 |
Total 3.2–8.8 3.6–6.3 3.5–10.0 |
Total (N ch -dependent) 2.0–4.5 1.7–4.0 1.1–3.7 |
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Graczykowski 132 E. Grecka 86 L. Greiner 74 A. Grelli 58 C. Grigoras 32 V. Grigoriev 139 S. Grigoryan 140 1 F. Grosa 32 J.F. Grosse-Oetringhaus 32 R. Grosso 98 D. Grund 35 G.G. Guardiano 110 R. Guernane 73 M. Guilbaud 103 K. Gulbrandsen 83 T. Gunji 121 W. Guo 6 A. Gupta 91 R. Gupta 91 S.P. Guzman 44 L. Gyulai 135 M.K. Habib 98 C. Hadjidakis 72 H. Hamagaki 76 M. Hamid 6 Y. Han 137 R. Hannigan 107 M.R. Haque 132 A. Harlenderova 98 J.W. Harris 136 A. Harton 9 J.A. Hasenbichler 32 H. Hassan 87 D. Hatzifotiadou 50 P. Hauer 42 L.B. Havener 136 S.T. Heckel 96 E. Hellbär 98 H. Helstrup 34 T. Herman 35 G. Herrera Corral 8 F. Herrmann 134 K.F. Hetland 34 B. Heybeck 63 H. Hillemanns 32 C. Hills 116 B. Hippolyte 126 B. Hofman 58 B. Hohlweger 84 J. Honermann 134 G.H. Hong 137 D. Horak 35 A. Horzyk 2 R. Hosokawa 14 Y. Hou 6 P. Hristov 32 C. Hughes 119 P. Huhn 63 L.M. Huhta 114 C.V. Hulse 72 T.J. Humanic 88 H. Hushnud 99 A. Hutson 113 D. Hutter 38 J.P. Iddon 116 R. Ilkaev 139 H. Ilyas 13 M. Inaba 122 G.M. Innocenti 32 M. Ippolitov 139 A. Isakov 86 T. Isidori 115 M.S. Islam 99 M. Ivanov 98 V. Ivanov 139 V. Izucheev 139 M. Jablonski 2 B. Jacak 74 N. Jacazio 32 P.M. Jacobs 74 S. Jadlovska 105 J. Jadlovsky 105 L. Jaffe 38 C. Jahnke 110 M.A. Janik 132 T. Janson 69 M. Jercic 89 O. Jevons 100 A.A.P. Jimenez 64 F. Jonas 87 134 P.G. Jones 100 J.M. Jowett 32 98 J. Jung 63 M. Jung 63 A. Junique 32 A. Jusko 100 M.J. Kabus 32 132 J. Kaewjai 104 P. Kalinak 59 A.S. Kalteyer 98 A. Kalweit 32 V. Kaplin 139 A. Karasu Uysal 71 D. Karatovic 89 O. Karavichev 139 T. Karavicheva 139 P. Karczmarczyk 132 E. Karpechev 139 V. Kashyap 80 A. Kazantsev 139 U. Kebschull 69 R. Keidel 138 D.L.D. Keijdener 58 M. Keil 32 B. Ketzer 42 A.M. Khan 6 S. Khan 15 A. Khanzadeev 139 Y. Kharlov 139 A. Khatun 15 A. Khuntia 106 B. Kileng 34 B. Kim 16 C. Kim 16 D.J. Kim 114 E.J. Kim 68 J. Kim 137 J.S. Kim 40 J. Kim 95 J. Kim 68 M. Kim 95 S. Kim 17 T. Kim 137 S. Kirsch 63 I. Kisel 38 S. Kiselev 139 A. Kisiel 132 J.P. Kitowski 2 J.L. Klay 5 J. Klein 32 S. Klein 74 C. Klein-Bösing 134 M. Kleiner 63 T. Klemenz 96 A. Kluge 32 A.G. Knospe 113 C. Kobdaj 104 T. Kollegger 98 A. Kondratyev 140 N. Kondratyeva 139 E. Kondratyuk 139 J. Konig 63 S.A. Konigstorfer 96 P.J. Konopka 32 G. Kornakov 132 S.D. Koryciak 2 A. Kotliarov 86 O. Kovalenko 79 V. Kovalenko 139 M. Kowalski 106 I. Králik 59 A. Kravčáková 37 L. Kreis 98 M. Krivda 100 59 F. Krizek 86 K. Krizkova Gajdosova 35 M. Kroesen 95 M. Krüger 63 D.M. Krupova 35 E. Kryshen 139 M. Krzewicki 38 V. Kučera 32 C. Kuhn 126 P.G. Kuijer 84 T. Kumaoka 122 D. Kumar 131 L. Kumar 90 N. Kumar 90 S. Kundu 32 P. Kurashvili 79 A. Kurepin 139 A.B. Kurepin 139 S. Kushpil 86 J. Kvapil 100 M.J. Kweon 57 J.Y. Kwon 57 Y. Kwon 137 S.L. La Pointe 38 P. La Rocca 26 Y.S. Lai 74 A. Lakrathok 104 M. Lamanna 32 R. Langoy 118 P. Larionov 48 E. Laudi 32 L. Lautner 32 96 R. Lavicka 102 T. Lazareva 139 R. Lea 130 54 J. Lehrbach 38 R.C. Lemmon 85 I. León Monzón 108 M.M. Lesch 96 E.D. Lesser 18 M. Lettrich 96 P. Lévai 135 X. Li 10 X.L. Li 6 J. Lien 118 R. Lietava 100 B. Lim 16 S.H. Lim 16 V. Lindenstruth 38 A. Lindner 45 C. Lippmann 98 A. Liu 18 D.H. Liu 6 J. Liu 116 I.M. Lofnes 20 V. Loginov 139 C. Loizides 87 P. Loncar 33 J.A. Lopez 95 X. Lopez 124 E. López Torres 7 P. Lu 98 117 J.R. Luhder 134 M. Lunardon 27 G. Luparello 56 Y.G. Ma 39 A. Maevskaya 139 M. Mager 32 T. Mahmoud 42 A. Maire 126 M. Malaev 139 N.M. Malik 91 Q.W. Malik 19 S.K. Malik 91 L. Malinina 140 VII D. Mal'Kevich 139 D. Mallick 80 N. Mallick 47 G. Mandaglio 30 52 V. Manko 139 F. Manso 124 V. Manzari 49 Y. Mao 6 G.V. Margagliotti 23 A. Margotti 50 A. Marín 98 C. Markert 107 M. Marquard 63 N.A. Martin 95 P. Martinengo 32 J.L. Martinez 113 M.I. Martínez 44 G. Martínez García 103 S. Masciocchi 98 M. Masera 24 A. Masoni 51 L. Massacrier 72 A. Mastroserio 128 49 A.M. Mathis 96 O. Matonoha 75 P.F.T. Matuoka 109 A. Matyja 106 C. Mayer 106 A.L. Mazuecos 32 F. Mazzaschi 24 M. Mazzilli 32 J.E. Mdhluli 120 A.F. Mechler 63 Y. Melikyan 139 A. Menchaca-Rocha 66 E. Meninno 102 28 A.S. Menon 113 M. Meres 12 S. Mhlanga 112 67 Y. Miake 122 L. Micheletti 55 L.C. Migliorin 125 D.L. Mihaylov 96 K. Mikhaylov 140 139 A.N. Mishra 135 D. Miśkowiec 98 A. Modak 4 A.P. Mohanty 58 B. Mohanty 80 M. Mohisin Khan 15 V M.A. Molander 43 Z. Moravcova 83 C. Mordasini 96 D.A. Moreira De Godoy 134 I. Morozov 139 A. Morsch 32 T. Mrnjavac 32 V. Muccifora 48 E. Mudnic 33 S. Muhuri 131 J.D. Mulligan 74 A. Mulliri 22 M.G. Munhoz 109 R.H. Munzer 63 H. Murakami 121 S. Murray 112 L. Musa 32 J. Musinsky 59 J.W. Myrcha 132 B. Naik 120 R. Nair 79 B.K. Nandi 46 R. Nania 50 E. Nappi 49 A.F. Nassirpour 75 A. Nath 95 C. Nattrass 119 A. Neagu 19 A. Negru 123 L. Nellen 64 S.V. Nesbo 34 G. Neskovic 38 D. Nesterov 139 B.S. Nielsen 83 E.G. Nielsen 83 S. Nikolaev 139 S. Nikulin 139 V. Nikulin 139 F. Noferini 50 S. Noh 11 P. Nomokonov 140 J. Norman 116 N. Novitzky 122 P. Nowakowski 132 A. Nyanin 139 J. Nystrand 20 M. Ogino 76 A. Ohlson 75 V.A. Okorokov 139 J. Oleniacz 132 A.C. Oliveira Da Silva 119 M.H. Oliver 136 A. Onnerstad 114 C. Oppedisano 55 A. Ortiz Velasquez 64 A. Oskarsson 75 J. Otwinowski 106 M. Oya 93 K. Oyama 76 Y. Pachmayer 95 S. Padhan 46 D. Pagano 130 54 G. Paić 64 A. Palasciano 49 S. Panebianco 127 J. Park 57 J.E. Parkkila 32 114 S.P. Pathak 113 R.N. Patra 91 B. Paul 22 H. Pei 6 T. Peitzmann 58 X. Peng 6 L.G. Pereira 65 H. Pereira Da Costa 127 D. Peresunko 139 G.M. Perez 7 S. Perrin 127 Y. Pestov 139 V. Petráček 35 V. Petrov 139 M. Petrovici 45 R.P. Pezzi 103 65 S. Piano 56 M. Pikna 12 P. Pillot 103 O. Pinazza 50 32 L. Pinsky 113 C. Pinto 96 26 S. Pisano 48 M. Płoskoń 74 M. Planinic 89 F. Pliquett 63 M.G. Poghosyan 87 S. Politano 29 N. Poljak 89 A. Pop 45 S. Porteboeuf-Houssais 124 J. Porter 74 V. Pozdniakov 140 S.K. Prasad 4 S. Prasad 47 R. Preghenella 50 F. Prino 55 C.A. Pruneau 133 I. Pshenichnov 139 M. Puccio 32 S. Qiu 84 L. Quaglia 24 R.E. Quishpe 113 S. Ragoni 100 A. Rakotozafindrabe 127 L. Ramello 129 55 F. Rami 126 S.A.R. Ramirez 44 T.A. Rancien 73 R. Raniwala 92 S. Raniwala 92 S.S. Räsänen 43 R. Rath 47 I. Ravasenga 84 K.F. Read 87 119 A.R. Redelbach 38 K. Redlich 79 VI A. Rehman 20 P. Reichelt 63 F. Reidt 32 H.A. Reme-Ness 34 Z. Rescakova 37 K. Reygers 95 A. Riabov 139 V. Riabov 139 R. Ricci 28 T. Richert 75 M. Richter 19 W. Riegler 32 F. Riggi 26 C. Ristea 62 M. Rodríguez Cahuantzi 44 K. Røed 19 R. Rogalev 139 E. Rogochaya 140 T.S. Rogoschinski 63 D. Rohr 32 D. Röhrich 20 P.F. Rojas 44 S. Rojas Torres 35 P.S. Rokita 132 F. Ronchetti 48 A. Rosano 30 52 E.D. Rosas 64 A. Rossi 53 A. Roy 47 P. Roy 99 S. Roy 46 N. Rubini 25 O.V. Rueda 75 D. Ruggiano 132 R. Rui 23 B. Rumyantsev 140 P.G. Russek 2 R. Russo 84 A. Rustamov 81 E. Ryabinkin 139 Y. Ryabov 139 A. Rybicki 106 H. Rytkonen 114 W. Rzesa 132 O.A.M. Saarimaki 43 R. Sadek 103 S. Sadovsky 139 J. Saetre 20 K. Šafařík 35 S.K. Saha 131 S. Saha 80 B. Sahoo 46 P. Sahoo 46 R. Sahoo 47 S. Sahoo 60 D. Sahu 47 P.K. Sahu 60 J. Saini 131 K. Sajdakova 37 S. Sakai 122 M.P. Salvan 98 S. Sambyal 91 T.B. Saramela 109 D. Sarkar 133 N. Sarkar 131 P. Sarma 41 V. Sarritzu 22 V.M. Sarti 96 M.H.P. Sas 136 J. Schambach 87 H.S. Scheid 63 C. Schiaua 45 R. Schicker 95 A. Schmah 95 C. Schmidt 98 H.R. Schmidt 94 M.O. Schmidt 32 M. Schmidt 94 N.V. Schmidt 87 63 A.R. Schmier 119 R. Schotter 126 J. Schukraft 32 K. Schwarz 98 K. Schweda 98 G. Scioli 25 E. Scomparin 55 J.E. Seger 14 Y. Sekiguchi 121 D. Sekihata 121 I. Selyuzhenkov 98 139 S. Senyukov 126 J.J. Seo 57 D. Serebryakov 139 L. Šerkšnytė 96 A. Sevcenco 62 T.J. Shaba 67 A. Shabanov 139 A. Shabetai 103 R. Shahoyan 32 W. Shaikh 99 A. Shangaraev 139 A. Sharma 90 D. Sharma 46 H. Sharma 106 M. Sharma 91 N. Sharma 90 S. Sharma 91 U. Sharma 91 A. Shatat 72 O. Sheibani 113 K. Shigaki 93 M. Shimomura 77 S. Shirinkin 139 Q. Shou 39 Y. Sibiriak 139 S. Siddhanta 51 T. Siemiarczuk 79 T.F. Silva 109 D. Silvermyr 75 T. Simantathammakul 104 R. Simeonov 36 G. Simonetti 32 B. Singh 91 B. Singh 96 R. Singh 80 R. Singh 91 R. Singh 47 V.K. Singh 131 V. Singhal 131 T. Sinha 99 B. Sitar 12 M. Sitta 129 55 T.B. Skaali 19 G. Skorodumovs 95 M. Slupecki 43 N. Smirnov 136 R.J.M. Snellings 58 E.H. Solheim 19 C. Soncco 101 J. Song 113 A. Songmoolnak 104 F. Soramel 27 S. Sorensen 119 R. Spijkers 84 I. Sputowska 106 J. Staa 75 J. Stachel 95 I. Stan 62 P.J. Steffanic 119 S.F. Stiefelmaier 95 D. Stocco 103 I. Storehaug 19 M.M. Storetvedt 34 P. Stratmann 134 S. Strazzi 25 C.P. Stylianidis 84 A.A.P. Suaide 109 C. Suire 72 M. Sukhanov 139 M. Suljic 32 V. Sumberia 91 S. Sumowidagdo 82 S. Swain 60 A. Szabo 12 I. Szarka 12 U. Tabassam 13 S.F. Taghavi 96 G. Taillepied 98 124 J. Takahashi 110 G.J. Tambave 20 S. Tang 124 6 Z. Tang 117 J.D. Tapia Takaki 115 N. Tapus 123 L.A. Tarasovicova 134 M.G. Tarzila 45 A. Tauro 32 A. Telesca 32 L. Terlizzi 24 C. Terrevoli 113 G. Tersimonov 3 S. Thakur 131 D. Thomas 107 R. Tieulent 125 A. Tikhonov 139 A.R. Timmins 113 M. Tkacik 105 T. Tkacik 105 A. Toia 63 N. Topilskaya 139 M. Toppi 48 F. Torales-Acosta 18 T. Tork 72 A.G. Torres Ramos 31 A. Trifiró 30 52 A.S. Triolo 30 52 S. Tripathy 50 T. Tripathy 46 S. Trogolo 32 V. Trubnikov 3 W.H. Trzaska 114 T.P. Trzcinski 132 R. Turrisi 53 T.S. Tveter 19 K. Ullaland 20 B. Ulukutlu 96 A. Uras 125 M. Urioni 54 130 G.L. Usai 22 M. Vala 37 N. Valle 21 S. Vallero 55 L.V.R. van Doremalen 58 M. van Leeuwen 84 C.A. van Veen 95 R.J.G. van Weelden 84 P. Vande Vyvre 32 D. Varga 135 Z. Varga 135 M. Varga-Kofarago 135 M. Vasileiou 78 A. Vasiliev 139 O. Vázquez Doce 96 V. Vechernin 139 E. Vercellin 24 S. Vergara Limón 44 L. Vermunt 58 R. Vértesi 135 M. Verweij 58 L. Vickovic 33 Z. Vilakazi 120 O. Villalobos Baillie 100 G. Vino 49 A. Vinogradov 139 T. Virgili 28 V. Vislavicius 83 A. Vodopyanov 140 B. Volkel 32 M.A. Völkl 95 K. Voloshin 139 S.A. Voloshin 133 G. Volpe 31 B. von Haller 32 I. Vorobyev 96 N. Vozniuk 139 J. Vrláková 37 B. Wagner 20 C. Wang 39 D. Wang 39 M. Weber 102 A. Wegrzynek 32 F.T. Weiglhofer 38 S.C. Wenzel 32 J.P. Wessels 134 S.L. Weyhmiller 136 J. Wiechula 63 J. Wikne 19 G. Wilk 79 J. Wilkinson 98 G.A. Willems 134 B. Windelband 95 M. Winn 127 J.R. Wright 107 W. Wu 39 Y. Wu 117 R. Xu 6 A.K. Yadav 131 S. Yalcin 71 Y. Yamaguchi 93 K. Yamakawa 93 S. Yang 20 S. Yano 93 Z. Yin 6 I.-K. Yoo 16 J.H. Yoon 57 S. Yuan 20 A. Yuncu 95 V. Zaccolo 23 C. Zampolli 32 H.J.C. Zanoli 58 F. Zanone 95 N. Zardoshti 32 100 A. Zarochentsev 139 P. Závada 61 N. Zaviyalov 139 M. Zhalov 139 B. Zhang 6 S. Zhang 39 X. Zhang 6 Y. Zhang 117 M. Zhao 10 V. Zherebchevskii 139 Y. Zhi 10 N. Zhigareva 139 D. Zhou 6 Y. Zhou 83 J. Zhu 98 6 Y. Zhu 6 G. Zinovjev 3 I N. Zurlo 130 54 1 A.I. Alikhanyan National Science Laboratory (Yerevan Physics Institute) Foundation, Yerevan, Armenia A.I. Alikhanyan National Science Laboratory (Yerevan Physics Institute) Foundation Yerevan Armenia A.I. Alikhanyan National Science Laboratory (Yerevan Physics Institute) Foundation, Yerevan, Armenia 2 AGH University of Science and Technology, Cracow, Poland AGH University of Science and Technology Cracow Poland AGH University of Science and Technology, Cracow, Poland 3 Bogolyubov Institute for Theoretical Physics, National Academy of Sciences of Ukraine, Kiev, Ukraine Bogolyubov Institute for Theoretical Physics National Academy of Sciences of Ukraine Kiev Ukraine Bogolyubov Institute for Theoretical Physics, National Academy of Sciences of Ukraine, Kiev, Ukraine 4 Bose Institute, Department of Physics and Centre for Astroparticle Physics and Space Science (CAPSS), Kolkata, India Bose Institute Department of Physics Centre for Astroparticle Physics and Space Science (CAPSS) Kolkata India Bose Institute, Department of Physics and Centre for Astroparticle Physics and Space Science (CAPSS), Kolkata, India 5 California Polytechnic State University, San Luis Obispo, CA, United States California Polytechnic State University San Luis Obispo CA United States California Polytechnic State University, San Luis Obispo, California, United States 6 Central China Normal University, Wuhan, China Central China Normal University Wuhan China Central China Normal University, Wuhan, China 7 Centro de Aplicaciones Tecnológicas y Desarrollo Nuclear (CEADEN), Havana, Cuba Centro de Aplicaciones Tecnológicas y Desarrollo Nuclear (CEADEN) Havana Cuba Centro de Aplicaciones Tecnológicas y Desarrollo Nuclear (CEADEN), Havana, Cuba 8 Centro de Investigación y de Estudios Avanzados (CINVESTAV), Mexico City and Mérida, Mexico Centro de Investigación y de Estudios Avanzados (CINVESTAV) Mexico City and Mérida Mexico Centro de Investigación y de Estudios Avanzados (CINVESTAV), Mexico City and Mérida, Mexico 9 Chicago State University, Chicago, IL, United States Chicago State University Chicago IL United States Chicago State University, Chicago, Illinois, United States 10 China Institute of Atomic Energy, Beijing, China China Institute of Atomic Energy Beijing China China Institute of Atomic Energy, Beijing, China 11 Chungbuk National University, Cheongju, Republic of Korea Chungbuk National University Cheongju Republic of Korea Chungbuk National University, Cheongju, Republic of Korea 12 Comenius University Bratislava, Faculty of Mathematics, Physics and Informatics, Bratislava, Slovak Republic Comenius University Bratislava Faculty of Mathematics, Physics and Informatics Bratislava Slovak Republic Comenius University Bratislava, Faculty of Mathematics, Physics and Informatics, Bratislava, Slovak Republic 13 COMSATS University Islamabad, Islamabad, Pakistan COMSATS University Islamabad Islamabad Pakistan COMSATS University Islamabad, Islamabad, Pakistan 14 Creighton University, Omaha, NE, United States Creighton University Omaha NE United States Creighton University, Omaha, Nebraska, United States 15 Department of Physics, Aligarh Muslim University, Aligarh, India Department of Physics Aligarh Muslim University Aligarh India Department of Physics, Aligarh Muslim University, Aligarh, India 16 Department of Physics, Pusan National University, Pusan, Republic of Korea Department of Physics Pusan National University Pusan Republic of Korea Department of Physics, Pusan National University, Pusan, Republic of Korea 17 Department of Physics, Sejong University, Seoul, Republic of Korea Department of Physics Sejong University Seoul Republic of Korea Department of Physics, Sejong University, Seoul, Republic of Korea 18 Department of Physics, University of California, Berkeley, CA, United States Department of Physics University of California Berkeley CA United States Department of Physics, University of California, Berkeley, California, United States 19 Department of Physics, University of Oslo, Oslo, Norway Department of Physics University of Oslo Oslo Norway Department of Physics, University of Oslo, Oslo, Norway 20 Department of Physics and Technology, University of Bergen, Bergen, Norway Department of Physics and Technology University of Bergen Bergen Norway Department of Physics and Technology, University of Bergen, Bergen, Norway 21 Dipartimento di Fisica, Università di Pavia, Pavia, Italy Dipartimento di Fisica Università di Pavia Pavia Italy Dipartimento di Fisica, Università di Pavia, Pavia, Italy 22 Dipartimento di Fisica dell'Università and Sezione INFN, Cagliari, Italy Dipartimento di Fisica dell'Università Sezione INFN Cagliari Italy Dipartimento di Fisica dell'Università and Sezione INFN, Cagliari, Italy 23 Dipartimento di Fisica dell'Università and Sezione INFN, Trieste, Italy Dipartimento di Fisica dell'Università Sezione INFN Trieste Italy Dipartimento di Fisica dell'Università and Sezione INFN, Trieste, Italy 24 Dipartimento di Fisica dell'Università and Sezione INFN, Turin, Italy Dipartimento di Fisica dell'Università Sezione INFN Turin Italy Dipartimento di Fisica dell'Università and Sezione INFN, Turin, Italy 25 Dipartimento di Fisica e Astronomia dell'Università and Sezione INFN, Bologna, Italy Dipartimento di Fisica e Astronomia dell'Università Sezione INFN Bologna Italy Dipartimento di Fisica e Astronomia dell'Università and Sezione INFN, Bologna, Italy 26 Dipartimento di Fisica e Astronomia dell'Università and Sezione INFN, Catania, Italy Dipartimento di Fisica e Astronomia dell'Università Sezione INFN Catania Italy Dipartimento di Fisica e Astronomia dell'Università and Sezione INFN, Catania, Italy 27 Dipartimento di Fisica e Astronomia dell'Università and Sezione INFN, Padova, Italy Dipartimento di Fisica e Astronomia dell'Università Sezione INFN Padova Italy Dipartimento di Fisica e Astronomia dell'Università and Sezione INFN, Padova, Italy 28 Dipartimento di Fisica ‘E.R. Caianiello’ dell'Università and Gruppo Collegato INFN, Salerno, Italy Dipartimento di Fisica ‘E.R. Caianiello’ dell'Università Gruppo Collegato INFN Salerno Italy Dipartimento di Fisica ‘E.R. Caianiello’ dell'Università and Gruppo Collegato INFN, Salerno, Italy 29 Dipartimento DISAT del Politecnico and Sezione INFN, Turin, Italy Dipartimento DISAT del Politecnico Sezione INFN Turin Italy Dipartimento DISAT del Politecnico and Sezione INFN, Turin, Italy 30 Dipartimento di Scienze MIFT, Università di Messina, Messina, Italy Dipartimento di Scienze MIFT Università di Messina Messina Italy Dipartimento di Scienze MIFT, Università di Messina, Messina, Italy 31 Dipartimento Interateneo di Fisica ‘M. Merlin’ and Sezione INFN, Bari, Italy Dipartimento Interateneo di Fisica ‘M. Merlin’ Sezione INFN Bari Italy Dipartimento Interateneo di Fisica ‘M. Merlin’ and Sezione INFN, Bari, Italy 32 European Organization for Nuclear Research (CERN), Geneva, Switzerland European Organization for Nuclear Research (CERN) Geneva Switzerland European Organization for Nuclear Research (CERN), Geneva, Switzerland 33 Faculty of Electrical Engineering, Mechanical Engineering and Naval Architecture, University of Split, Split, Croatia Faculty of Electrical Engineering, Mechanical Engineering and Naval Architecture University of Split Split Croatia Faculty of Electrical Engineering, Mechanical Engineering and Naval Architecture, University of Split, Split, Croatia 34 Faculty of Engineering and Science, Western Norway University of Applied Sciences, Bergen, Norway Faculty of Engineering and Science Western Norway University of Applied Sciences Bergen Norway Faculty of Engineering and Science, Western Norway University of Applied Sciences, Bergen, Norway 35 Faculty of Nuclear Sciences and Physical Engineering, Czech Technical University in Prague, Prague, Czech Republic Faculty of Nuclear Sciences and Physical Engineering Czech Technical University in Prague Prague Czech Republic Faculty of Nuclear Sciences and Physical Engineering, Czech Technical University in Prague, Prague, Czech Republic 36 Faculty of Physics, Sofia University, Sofia, Bulgaria Faculty of Physics Sofia University Sofia Bulgaria Faculty of Physics, Sofia University, Sofia, Bulgaria 37 Faculty of Science, P.J. Šafárik University, Košice, Slovak Republic Faculty of Science P.J. Šafárik University Košice Slovak Republic Faculty of Science, P.J. Šafárik University, Košice, Slovak Republic 38 Frankfurt Institute for Advanced Studies, Johann Wolfgang Goethe-Universität Frankfurt, Frankfurt, Germany Frankfurt Institute for Advanced Studies Johann Wolfgang Goethe-Universität Frankfurt Frankfurt Germany Frankfurt Institute for Advanced Studies, Johann Wolfgang Goethe-Universität Frankfurt, Frankfurt, Germany 39 Fudan University, Shanghai, China Fudan University Shanghai China Fudan University, Shanghai, China 40 Gangneung-Wonju National University, Gangneung, Republic of Korea Gangneung-Wonju National University Gangneung Republic of Korea Gangneung-Wonju National University, Gangneung, Republic of Korea 41 Gauhati University, Department of Physics, Guwahati, India Gauhati University Department of Physics Guwahati India Gauhati University, Department of Physics, Guwahati, India 42 Helmholtz-Institut für Strahlen- und Kernphysik, Rheinische Friedrich-Wilhelms-Universität Bonn, Bonn, Germany Helmholtz-Institut für Strahlen- und Kernphysik Rheinische Friedrich-Wilhelms-Universität Bonn Bonn Germany Helmholtz-Institut für Strahlen- und Kernphysik, Rheinische Friedrich-Wilhelms-Universität Bonn, Bonn, Germany 43 Helsinki Institute of Physics (HIP), Helsinki, Finland Helsinki Institute of Physics (HIP) Helsinki Finland Helsinki Institute of Physics (HIP), Helsinki, Finland 44 High Energy Physics Group, Universidad Autónoma de Puebla, Puebla, Mexico High Energy Physics Group Universidad Autónoma de Puebla Puebla Mexico High Energy Physics Group, Universidad Autónoma de Puebla, Puebla, Mexico 45 Horia Hulubei National Institute of Physics and Nuclear Engineering, Bucharest, Romania Horia Hulubei National Institute of Physics and Nuclear Engineering Bucharest Romania Horia Hulubei National Institute of Physics and Nuclear Engineering, Bucharest, Romania 46 Indian Institute of Technology Bombay (IIT), Mumbai, India Indian Institute of Technology Bombay (IIT) Mumbai India Indian Institute of Technology Bombay (IIT), Mumbai, India 47 Indian Institute of Technology Indore, Indore, India Indian Institute of Technology Indore Indore India Indian Institute of Technology Indore, Indore, India 48 INFN, Laboratori Nazionali di Frascati, Frascati, Italy INFN, Laboratori Nazionali di Frascati Frascati Italy INFN, Laboratori Nazionali di Frascati, Frascati, Italy 49 INFN, Sezione di Bari, Bari, Italy INFN, Sezione di Bari Bari Italy INFN, Sezione di Bari, Bari, Italy 50 INFN, Sezione di Bologna, Bologna, Italy INFN, Sezione di Bologna Bologna Italy INFN, Sezione di Bologna, Bologna, Italy 51 INFN, Sezione di Cagliari, Cagliari, Italy INFN, Sezione di Cagliari Cagliari Italy INFN, Sezione di Cagliari, Cagliari, Italy 52 INFN, Sezione di Catania, Catania, Italy INFN, Sezione di Catania Catania Italy INFN, Sezione di Catania, Catania, Italy 53 INFN, Sezione di Padova, Padova, Italy INFN, Sezione di Padova Padova Italy INFN, Sezione di Padova, Padova, Italy 54 INFN, Sezione di Pavia, Pavia, Italy INFN, Sezione di Pavia Pavia Italy INFN, Sezione di Pavia, Pavia, Italy 55 INFN, Sezione di Torino, Turin, Italy INFN, Sezione di Torino Turin Italy INFN, Sezione di Torino, Turin, Italy 56 INFN, Sezione di Trieste, Trieste, Italy INFN, Sezione di Trieste Trieste Italy INFN, Sezione di Trieste, Trieste, Italy 57 Inha University, Incheon, Republic of Korea Inha University Incheon Republic of Korea Inha University, Incheon, Republic of Korea 58 Institute for Gravitational and Subatomic Physics (GRASP), Utrecht University/Nikhef, Utrecht, Netherlands Institute for Gravitational and Subatomic Physics (GRASP) Utrecht University/Nikhef Utrecht Netherlands Institute for Gravitational and Subatomic Physics (GRASP), Utrecht University/Nikhef, Utrecht, Netherlands 59 Institute of Experimental Physics, Slovak Academy of Sciences, Košice, Slovak Republic Institute of Experimental Physics Slovak Academy of Sciences Košice Slovak Republic Institute of Experimental Physics, Slovak Academy of Sciences, Košice, Slovak Republic 60 Institute of Physics, Homi Bhabha National Institute, Bhubaneswar, India Institute of Physics Homi Bhabha National Institute Bhubaneswar India Institute of Physics, Homi Bhabha National Institute, Bhubaneswar, India 61 Institute of Physics of the Czech Academy of Sciences, Prague, Czech Republic Institute of Physics of the Czech Academy of Sciences Prague Czech Republic Institute of Physics of the Czech Academy of Sciences, Prague, Czech Republic 62 Institute of Space Science (ISS), Bucharest, Romania Institute of Space Science (ISS) Bucharest Romania Institute of Space Science (ISS), Bucharest, Romania 63 Institut für Kernphysik, Johann Wolfgang Goethe-Universität Frankfurt, Frankfurt, Germany Institut für Kernphysik Johann Wolfgang Goethe-Universität Frankfurt Frankfurt Germany Institut für Kernphysik, Johann Wolfgang Goethe-Universität Frankfurt, Frankfurt, Germany 64 Instituto de Ciencias Nucleares, Universidad Nacional Autónoma de México, Mexico City, Mexico Instituto de Ciencias Nucleares Universidad Nacional Autónoma de México Mexico City Mexico Instituto de Ciencias Nucleares, Universidad Nacional Autónoma de México, Mexico City, Mexico 65 Instituto de Física, Universidade Federal do Rio Grande do Sul (UFRGS), Porto Alegre, Brazil Instituto de Física Universidade Federal do Rio Grande do Sul (UFRGS) Porto Alegre Brazil Instituto de Física, Universidade Federal do Rio Grande do Sul (UFRGS), Porto Alegre, Brazil 66 Instituto de Física, Universidad Nacional Autónoma de México, Mexico City, Mexico Instituto de Física Universidad Nacional Autónoma de México Mexico City Mexico Instituto de Física, Universidad Nacional Autónoma de México, Mexico City, Mexico 67 iThemba LABS, National Research Foundation, Somerset West, South Africa iThemba LABS National Research Foundation Somerset West South Africa iThemba LABS, National Research Foundation, Somerset West, South Africa 68 Jeonbuk National University, Jeonju, Republic of Korea Jeonbuk National University Jeonju Republic of Korea Jeonbuk National University, Jeonju, Republic of Korea 69 Johann-Wolfgang-Goethe Universität Frankfurt Institut für Informatik, Fachbereich Informatik und Mathematik, Frankfurt, Germany Johann-Wolfgang-Goethe Universität Frankfurt Institut für Informatik Fachbereich Informatik und Mathematik Frankfurt Germany Johann-Wolfgang-Goethe Universität Frankfurt Institut für Informatik, Fachbereich Informatik und Mathematik, Frankfurt, Germany 70 Korea Institute of Science and Technology Information, Daejeon, Republic of Korea Korea Institute of Science and Technology Information Daejeon Republic of Korea Korea Institute of Science and Technology Information, Daejeon, Republic of Korea 71 KTO Karatay University, Konya, Turkey KTO Karatay University Konya Turkey KTO Karatay University, Konya, Turkey 72 Laboratoire de Physique des 2 Infinis, Irène Joliot-Curie, Orsay, France Laboratoire de Physique des 2 Infinis, Irène Joliot-Curie Orsay France Laboratoire de Physique des 2 Infinis, Irène Joliot-Curie, Orsay, France 73 Laboratoire de Physique Subatomique et de Cosmologie, Université Grenoble-Alpes, CNRS-IN2P3, Grenoble, France Laboratoire de Physique Subatomique et de Cosmologie Université Grenoble-Alpes CNRS-IN2P3 Grenoble France Laboratoire de Physique Subatomique et de Cosmologie, Université Grenoble-Alpes, CNRS-IN2P3, Grenoble, France 74 Lawrence Berkeley National Laboratory, Berkeley, CA, United States Lawrence Berkeley National Laboratory Berkeley CA United States Lawrence Berkeley National Laboratory, Berkeley, California, United States 75 Lund University Department of Physics, Division of Particle Physics, Lund, Sweden Lund University Department of Physics Division of Particle Physics Lund Sweden Lund University Department of Physics, Division of Particle Physics, Lund, Sweden 76 Nagasaki Institute of Applied Science, Nagasaki, Japan Nagasaki Institute of Applied Science Nagasaki Japan Nagasaki Institute of Applied Science, Nagasaki, Japan 77 Nara Women's University (NWU), Nara, Japan Nara Women's University (NWU) Nara Japan Nara Women's University (NWU), Nara, Japan 78 National and Kapodistrian University of Athens, School of Science, Department of Physics , Athens, Greece National and Kapodistrian University of Athens School of Science Department of Physics Athens Greece National and Kapodistrian University of Athens, School of Science, Department of Physics , Athens, Greece 79 National Centre for Nuclear Research, Warsaw, Poland National Centre for Nuclear Research Warsaw Poland National Centre for Nuclear Research, Warsaw, Poland 80 National Institute of Science Education and Research, Homi Bhabha National Institute, Jatni, India National Institute of Science Education and Research Homi Bhabha National Institute Jatni India National Institute of Science Education and Research, Homi Bhabha National Institute, Jatni, India 81 National Nuclear Research Center, Baku, Azerbaijan National Nuclear Research Center Baku Azerbaijan National Nuclear Research Center, Baku, Azerbaijan 82 National Research and Innovation Agency - BRIN, Jakarta, Indonesia National Research and Innovation Agency - BRIN Jakarta Indonesia National Research and Innovation Agency - BRIN, Jakarta, Indonesia 83 Niels Bohr Institute, University of Copenhagen, Copenhagen, Denmark Niels Bohr Institute University of Copenhagen Copenhagen Denmark Niels Bohr Institute, University of Copenhagen, Copenhagen, Denmark 84 Nikhef, National institute for subatomic physics, Amsterdam, Netherlands Nikhef, National institute for subatomic physics Amsterdam Netherlands Nikhef, National institute for subatomic physics, Amsterdam, Netherlands 85 Nuclear Physics Group, STFC Daresbury Laboratory, Daresbury, United Kingdom Nuclear Physics Group STFC Daresbury Laboratory Daresbury United Kingdom Nuclear Physics Group, STFC Daresbury Laboratory, Daresbury, United Kingdom 86 Nuclear Physics Institute of the Czech Academy of Sciences, Husinec-Řež, Czech Republic Nuclear Physics Institute of the Czech Academy of Sciences Husinec-Řež Czech Republic Nuclear Physics Institute of the Czech Academy of Sciences, Husinec-Řež, Czech Republic 87 Oak Ridge National Laboratory, Oak Ridge, TN, United States Oak Ridge National Laboratory Oak Ridge TN United States Oak Ridge National Laboratory, Oak Ridge, Tennessee, United States 88 Ohio State University, Columbus, OH, United States Ohio State University Columbus OH United States Ohio State University, Columbus, Ohio, United States 89 Physics department, Faculty of science, University of Zagreb, Zagreb, Croatia Physics department Faculty of science, University of Zagreb Zagreb Croatia Physics department, Faculty of science, University of Zagreb, Zagreb, Croatia 90 Physics Department, Panjab University, Chandigarh, India Physics Department Panjab University Chandigarh India Physics Department, Panjab University, Chandigarh, India 91 Physics Department, University of Jammu, Jammu, India Physics Department University of Jammu Jammu India Physics Department, University of Jammu, Jammu, India 92 Physics Department, University of Rajasthan, Jaipur, India Physics Department University of Rajasthan Jaipur India Physics Department, University of Rajasthan, Jaipur, India 93 Physics Program and International Institute for Sustainability with Knotted Chiral Meta Matter (SKCM2), Hiroshima University, Hiroshima, Japan Physics Program and International Institute for Sustainability with Knotted Chiral Meta Matter (SKCM2) Hiroshima University Hiroshima Japan Physics Program and International Institute for Sustainability with Knotted Chiral Meta Matter (SKCM2), Hiroshima University, Hiroshima, Japan 94 Physikalisches Institut, Eberhard-Karls-Universität Tübingen, Tübingen, Germany Physikalisches Institut Eberhard-Karls-Universität Tübingen Tübingen Germany Physikalisches Institut, Eberhard-Karls-Universität Tübingen, Tübingen, Germany 95 Physikalisches Institut, Ruprecht-Karls-Universität Heidelberg, Heidelberg, Germany Physikalisches Institut Ruprecht-Karls-Universität Heidelberg Heidelberg Germany Physikalisches Institut, Ruprecht-Karls-Universität Heidelberg, Heidelberg, Germany 96 Physik Department, Technische Universität München, Munich, Germany Physik Department Technische Universität München Munich Germany Physik Department, Technische Universität München, Munich, Germany 97 Politecnico di Bari and Sezione INFN, Bari, Italy Politecnico di Bari Sezione INFN Bari Italy Politecnico di Bari and Sezione INFN, Bari, Italy 98 Research Division and ExtreMe Matter Institute EMMI, GSI Helmholtzzentrum für Schwerionenforschung GmbH, Darmstadt, Germany Research Division ExtreMe Matter Institute EMMI GSI Helmholtzzentrum für Schwerionenforschung GmbH Darmstadt Germany Research Division and ExtreMe Matter Institute EMMI, GSI Helmholtzzentrum für Schwerionenforschung GmbH, Darmstadt, Germany 99 Saha Institute of Nuclear Physics, Homi Bhabha National Institute, Kolkata, India Saha Institute of Nuclear Physics Homi Bhabha National Institute Kolkata India Saha Institute of Nuclear Physics, Homi Bhabha National Institute, Kolkata, India 100 School of Physics and Astronomy, University of Birmingham, Birmingham, United Kingdom School of Physics and Astronomy University of Birmingham Birmingham United Kingdom School of Physics and Astronomy, University of Birmingham, Birmingham, United Kingdom 101 Sección Física, Departamento de Ciencias, Pontificia Universidad Católica del Perú, Lima, Peru Sección Física Departamento de Ciencias Pontificia Universidad Católica del Perú Lima Peru Sección Física, Departamento de Ciencias, Pontificia Universidad Católica del Perú, Lima, Peru 102 Stefan Meyer Institut für Subatomare Physik (SMI), Vienna, Austria Stefan Meyer Institut für Subatomare Physik (SMI) Vienna Austria Stefan Meyer Institut für Subatomare Physik (SMI), Vienna, Austria 103 SUBATECH, IMT Atlantique, Nantes Université, CNRS-IN2P3, Nantes, France SUBATECH IMT Atlantique Nantes Université CNRS-IN2P3 Nantes France SUBATECH, IMT Atlantique, Nantes Université, CNRS-IN2P3, Nantes, France 104 Suranaree University of Technology, Nakhon Ratchasima, Thailand Suranaree University of Technology Nakhon Ratchasima Thailand Suranaree University of Technology, Nakhon Ratchasima, Thailand 105 Technical University of Košice, Košice, Slovak Republic Technical University of Košice Košice Slovak Republic Technical University of Košice, Košice, Slovak Republic 106 The Henryk Niewodniczanski Institute of Nuclear Physics, Polish Academy of Sciences, Cracow, Poland The Henryk Niewodniczanski Institute of Nuclear Physics Polish Academy of Sciences Cracow Poland The Henryk Niewodniczanski Institute of Nuclear Physics, Polish Academy of Sciences, Cracow, Poland 107 The University of Texas at Austin, Austin, TX, United States The University of Texas at Austin Austin TX United States The University of Texas at Austin, Austin, Texas, United States 108 Universidad Autónoma de Sinaloa, Culiacán, Mexico Universidad Autónoma de Sinaloa Culiacán Mexico Universidad Autónoma de Sinaloa, Culiacán, Mexico 109 Universidade de São Paulo (USP), São Paulo, Brazil Universidade de São Paulo (USP) São Paulo Brazil Universidade de São Paulo (USP), São Paulo, Brazil 110 Universidade Estadual de Campinas (UNICAMP), Campinas, Brazil Universidade Estadual de Campinas (UNICAMP) Campinas Brazil Universidade Estadual de Campinas (UNICAMP), Campinas, Brazil 111 Universidade Federal do ABC, Santo Andre, Brazil Universidade Federal do ABC Santo Andre Brazil Universidade Federal do ABC, Santo Andre, Brazil 112 University of Cape Town, Cape Town, South Africa University of Cape Town Cape Town South Africa University of Cape Town, Cape Town, South Africa 113 University of Houston, Houston, TX, United States University of Houston Houston TX United States University of Houston, Houston, Texas, United States 114 University of Jyväskylä, Jyväskylä, Finland University of Jyväskylä Jyväskylä Finland University of Jyväskylä, Jyväskylä, Finland 115 University of Kansas, Lawrence, KS, United States University of Kansas Lawrence KS United States University of Kansas, Lawrence, Kansas, United States 116 University of Liverpool, Liverpool, United Kingdom University of Liverpool Liverpool United Kingdom University of Liverpool, Liverpool, United Kingdom 117 University of Science and Technology of China, Hefei, China University of Science and Technology of China Hefei China University of Science and Technology of China, Hefei, China 118 University of South-Eastern Norway, Kongsberg, Norway University of South-Eastern Norway Kongsberg Norway University of South-Eastern Norway, Kongsberg, Norway 119 University of Tennessee, Knoxville, TN, United States University of Tennessee Knoxville TN United States University of Tennessee, Knoxville, Tennessee, United States 120 University of the Witwatersrand, Johannesburg, South Africa University of the Witwatersrand Johannesburg South Africa University of the Witwatersrand, Johannesburg, South Africa 121 University of Tokyo, Tokyo, Japan University of Tokyo Tokyo Japan University of Tokyo, Tokyo, Japan 122 University of Tsukuba, Tsukuba, Japan University of Tsukuba Tsukuba Japan University of Tsukuba, Tsukuba, Japan 123 University Politehnica of Bucharest, Bucharest, Romania University Politehnica of Bucharest Bucharest Romania University Politehnica of Bucharest, Bucharest, Romania 124 Université Clermont Auvergne, CNRS/IN2P3, LPC, Clermont-Ferrand, France Université Clermont Auvergne CNRS/IN2P3 LPC Clermont-Ferrand France Université Clermont Auvergne, CNRS/IN2P3, LPC, Clermont-Ferrand, France 125 Université de Lyon, CNRS/IN2P3, Institut de Physique des 2 Infinis de Lyon, Lyon, France Université de Lyon CNRS/IN2P3 Institut de Physique des 2 Infinis de Lyon Lyon France Université de Lyon, CNRS/IN2P3, Institut de Physique des 2 Infinis de Lyon, Lyon, France 126 Université de Strasbourg, CNRS, IPHC UMR 7178, F-67000 Strasbourg, France Université de Strasbourg CNRS IPHC UMR 7178 Strasbourg F-67000 France Université de Strasbourg, CNRS, IPHC UMR 7178, F-67000 Strasbourg, France, Strasbourg, France 127 Université Paris-Saclay Centre d'Etudes de Saclay (CEA), IRFU, Départment de Physique Nucléaire (DPhN), Saclay, France Université Paris-Saclay Centre d'Etudes de Saclay (CEA) IRFU Départment de Physique Nucléaire (DPhN) Saclay France Université Paris-Saclay Centre d'Etudes de Saclay (CEA), IRFU, Départment de Physique Nucléaire (DPhN), Saclay, France 128 Università degli Studi di Foggia, Foggia, Italy Università degli Studi di Foggia Foggia Italy Università degli Studi di Foggia, Foggia, Italy 129 Università del Piemonte Orientale, Vercelli, Italy Università del Piemonte Orientale Vercelli Italy Università del Piemonte Orientale, Vercelli, Italy 130 Università di Brescia, Brescia, Italy Università di Brescia Brescia Italy Università di Brescia, Brescia, Italy 131 Variable Energy Cyclotron Centre, Homi Bhabha National Institute, Kolkata, India Variable Energy Cyclotron Centre Homi Bhabha National Institute Kolkata India Variable Energy Cyclotron Centre, Homi Bhabha National Institute, Kolkata, India 132 Warsaw University of Technology, Warsaw, Poland Warsaw University of Technology Warsaw Poland Warsaw University of Technology, Warsaw, Poland 133 Wayne State University, Detroit, MI, United States Wayne State University Detroit MI United States Wayne State University, Detroit, Michigan, United States 134 Westfälische Wilhelms-Universität Münster, Institut für Kernphysik, Münster, Germany Westfälische Wilhelms-Universität Münster Institut für Kernphysik Münster Germany Westfälische Wilhelms-Universität Münster, Institut für Kernphysik, Münster, Germany 135 Wigner Research Centre for Physics, Budapest, Hungary Wigner Research Centre for Physics Budapest Hungary Wigner Research Centre for Physics, Budapest, Hungary 136 Yale University, New Haven, CT, United States Yale University New Haven CT United States Yale University, New Haven, Connecticut, United States 137 Yonsei University, Seoul, Republic of Korea Yonsei University Seoul Republic of Korea Yonsei University, Seoul, Republic of Korea 138 Zentrum für Technologie und Transfer (ZTT), Worms, Germany Zentrum für Technologie und Transfer (ZTT) Worms Germany Zentrum für Technologie und Transfer (ZTT), Worms, Germany 139 Affiliated with an institute covered by a cooperation agreement with CERN Affiliated with an institute covered by a cooperation agreement with CERN Affiliated with an institute covered by a cooperation agreement with CERN 140 Affiliated with an international laboratory covered by a cooperation agreement with CERN Affiliated with an international laboratory covered by a cooperation agreement with CERN Affiliated with an international laboratory covered by a cooperation agreement with CERN I Deceased. II Also at: Max-Planck-Institut für Physik, Munich, Germany. III Also at: Italian National Agency for New Technologies, Energy and Sustainable Economic Development (ENEA), Bologna, Italy. IV Also at: Dipartimento DET del Politecnico di Torino, Turin, Italy. V Also at: Department of Applied Physics, Aligarh Muslim University, Aligarh, India. VI Also at: Institute of Theoretical Physics, University of Wroclaw, Poland. VII Also at: An institution covered by a cooperation agreement with CERN. ⋆ E-mail address: alice-publications@cern.ch .Editor: M. Pierini Abstract This letter reports measurements which characterize the underlying event associated with hard scatterings at mid-pseudorapidity (| η | < 0.8 ) in pp, p–Pb and Pb–Pb collisions at centre-of-mass energy per nucleon pair, s NN = 5.02 TeV. The measurements are performed with ALICE at the LHC. Different multiplicity classes are defined based on the event activity measured at forward rapidities. The hard scatterings are identified by the leading particle defined as the charged particle with the largest transverse momentum (p T ) in the collision and having 8 < p T < 15 GeV/c . The p T spectra of associated particles (0.5 ≤ p T < 6 GeV/c ) are measured in different azimuthal regions defined with respect to the leading particle direction: toward, transverse, and away. The associated charged particle yields in the transverse region are subtracted from those of the away and toward regions. The remaining jet-like yields are reported as a function of the multiplicity measured in the transverse region. The measurements show a suppression of the jet-like yield in the away region and an enhancement of high-p T associated particles in the toward region in central Pb–Pb collisions, as compared to minimum-bias pp collisions. These observations are consistent with previous measurements that used two-particle correlations, and with an interpretation in terms of parton energy loss in a high-density quark gluon plasma. These yield modifications vanish in peripheral Pb–Pb collisions and are not observed in either high-multiplicity pp or p–Pb collisions. Data availability This manuscript has associated data in a HEPData repository at https://www.hepdata.net/ . 1 Introduction In proton-proton (pp) collisions, jets, originating from partonic scatterings with large momentum transfer, are accompanied by particles produced by initial- and final-state radiation (ISR and FSR, respectively), as well as, by a plethora of other mechanisms. These include proton break-up, and, in a scenario incorporating multi-parton interactions (MPI) [1,2] , several semi-hard parton-parton scatterings in a single pp collision. These jet-accompanying particles experimentally make up the underlying event (UE) and are commonly studied via azimuthal separations from the jets to minimise the influence of hard scatterings. The present study follows the strategy originally introduced by the CDF collaboration [3] . First, the leading charged particle in the event is found, i.e., the charged particle with the highest transverse momentum in the collision (p T trig ). Secondly, the associated particles (p T < p T trig ) are measured in three topological regions depending on their azimuthal angle relative to the leading particle, | Δ φ | = | φ assoc − φ trig | , see Fig. 1 . The toward region contains the primary jet within the acceptance of the detector, while the away region contains the back-scattered particles of the recoil jet [4] . In contrast, the transverse region is dominated by the underlying-event dynamics, but it also includes contributions from ISR and FSR [5] . The measurements performed at RHIC and LHC in small systems (pp, p–A, and d–A collisions) have shown for high particle multiplicities similar phenomena as were originally observed only in A–A collisions and have been attributed there to the formation of the strongly interacting quark gluon plasma [6,7] , namely, long range angular correlations and collectivity [8] . The origin of these effects in small systems is still an open question; on one hand, hydrodynamical calculations describe some aspects of the data [9] ; on the other hand, mechanisms like colour reconnection [10] , rope hadronisation [11] , and string shoving [12] can produce collective-like effects in Monte Carlo event generators such as PYTHIA 8 [13] . Thus, investigating pp collisions as a function of the charged particle multiplicity has become ever more pertinent [9,14–18] . The interpretation of the results from the analysis of high-multiplicity pp collisions is challenging due to the selection biases of the sample towards events in which partonic scatterings with large momentum transfer (hard scatterings) occurred. To mitigate this inherent bias, Martin et al. [19] suggested to use the charged-particle multiplicity in the transverse region (N ch T ) as a classifier of the activity in the collisions, since the correlation between N ch T and the hardest scattering in the collision is small. The ALICE collaboration has reported the first N ch T spectra measured in pp collisions at centre-of-mass energy, s = 13 TeV [20] . Event generators, such as PYTHIA 8 [13] and EPOS-LHC [21] , do not provide a good description of the measured distribution of the ratio N ch T /〈 N ch T 〉 , where 〈 N ch T 〉 is the event-averaged charged-particle multiplicity in the transverse region, underestimating in particular the number of collisions with large N ch T ( > 3 × 〈 N ch T 〉 ) . In the framework of MPI-based models, like those implemented in PYTHIA 8 and HERWIG 7 [22] , the probability for a hard scattering in the collision increases with decreasing impact parameterVIII VIII In event generators like PYTHIA 8 the impact parameter profile is described by an overlap matter distribution of the two incoming hadrons. between the colliding protons. Thus, requiring a high-p T particle (e.g., p T trig > 8 GeV/c ) in a given pp collision biases the selection of collisions towards those with a smaller impact parameter [23] , which in turn biases the selection towards pp collisions with more MPI [20] . This feature of the N ch T -based analysis is important for the isolation of potential MPI and colour reconnection effects, which according to PYTHIA 8, produce effects resembling collective behaviour [10] . By construction, MPI and colour reconnection effects are expected to be more relevant in the transverse region than in the away and toward regions [24] . It is worth mentioning that the MPI picture has been used to explain the p T spectra in p–Pb collisions and peripheral Pb–Pb collisions [25–27] . Studies, as a function of N ch T , are therefore important to the understanding of the effects observed in high-multiplicity pp collisions. Last but not least, measurements of UE observables are also important to tune event generators [28] that include hard partonic scatterings and MPI. This letter reports the inclusive charged-particle transverse momentum spectra in pp, p–Pb and Pb–Pb collisions at centre-of-mass energy per nucleon pair s NN = 5.02 TeV containing a high-p T leading particle within the kinematic intervals 8 ≤ p T trig < 15 GeV/c and | η | < 0.8 . This guarantees the selection of collisions in which the average activity in the transverse region is roughly flat as a function of p T trig [20] , and therefore, any additional selection on the charged particle multiplicity will only modulate the UE activity. The measurements are performed considering different event classes defined in terms of the multiplicity registered in the forward detectors. The p T spectra of associated charged particles (0.5 ≤ p T < 6 GeV/c and | η | < 0.8 ) are measured in the toward, away, and transverse regions as a function of the average charged particle multiplicity in the transverse region. To further investigate the possible modification of the particles produced in the hard scattering in pp, p–Pb, and Pb–Pb collisions, the p T distributions in the toward (d N ch t / d p T ) and away (d N ch a / d p T ) regions obtained after the subtraction of the p T spectra in the transverse region (d N ch T / d p T ) are also reported. The subtracted yields (d N ch st , sa / d p T ) are further normalised to those measured in minimum-bias (MB) pp collisions,(1) I X t , a ≡ ( d N ch t , a / d p T − d N ch T / d p T ) | X ( d N ch t , a / d p T − d N ch T / d p T ) | pp , MB = ( d N ch st , sa / d p T ) | X ( d N ch st , sa / d p T ) | pp , MB , where X indicates the collision system and the event multiplicity class. In this way, the hard process p T spectra in the toward and away regions are isolated, and thus allowing us to study possible modifications to the produced particles due to medium effects in high-multiplicity pp, p–Pb, and Pb–Pb collisions. In heavy-ion collisions, this ratio is sensitive to the same effects which were studied using the I AA quantity [29–31] , where jets produced in the early stage of the collision propagate through the hot and dense quark–gluon plasma. Their interaction with the coloured medium lead to parton-energy loss (jet quenching) [32] which, for example, results in the suppression of the charged-particle yield at high p T [33] , and the suppression of the high-p T yield in the away region [29,30] . It is worth mentioning that jet quenching effects have not been observed so far in small systems [33,34] . 2 Experiment and data analysis This analysis is based on the data recorded by the ALICE apparatus during the pp and Pb–Pb runs at s NN = 5.02 TeV in 2015, and the p–Pb run at s NN = 5.02 TeV in 2016. The present study uses the V0 detector, and the Silicon Pixel Detector (SPD) for triggering and background rejection. The V0 consists of two arrays of scintillating tiles placed on each side of the interaction point covering the full azimuthal acceptance and the pseudorapidity intervals of 2.8 < η < 5.1 (V0A) and − 3.7 < η < − 1.7 (V0C). The SPD is the innermost part of the Inner Tracking System (ITS) and it is the closest detector to the interaction point. It consists of two cylindrical silicon pixel layers at radial distances of 3.9 and 7.6 cm from the beam line and the pseudorapidity coverages of the two layers are | η | < 2 and | η | < 1.4 , respectively. The data were collected using a minimum-bias trigger, which required a signal in both V0A and V0C detectors. The offline event selection was optimised to reject beam-induced background in all collision systems by utilising the timing signals in the two V0 detectors. In Pb–Pb collisions, the beam-induced background is further suppressed by correlating the timing signals of the neutron zero degree calorimeters, which are positioned on both sides of the interaction point at 112.5 m distance along the beam axis [35] . The signals from the zero degree calorimeters are also used to suppress the contamination from electromagnetic interactions. This is performed by requesting the coincidence of the signals coming from both side zero degree calorimeters by which the background due to single nucleus electromagnetic dissociation processes is excluded. A criterion based on the offline reconstruction of multiple primary vertices in the SPD is applied to reduce the pileup caused by multiple interactions in the same bunch crossing [36] . The results presented in this letter are for minimum-bias triggered pp collisions having at least one charged particle in the pseudorapidity interval | η | < 1 (INEL>0). The INEL>0 event class corresponds to about 75% of the total inelastic cross section [37] . For pp and Pb–Pb collisions, the sample is subdivided into different multiplicity classes based on the total charge deposited in both V0 sub-detectors, which is termed as V0M amplitude [38] . For p–Pb collisions, the sample is subdivided based on the total charge deposited in V0A sub-detector (V0A amplitude) [39] , which is located in the Pb-going direction. The V0A estimator has been implemented in previous measurements that used p–Pb data (see e.g. [40] ). This allows for comparisons with other observables for similar V0A multiplicity classes. To ensure that a hard scattering took place in the collision, events are required to have a trigger particle within 8 ≤ p T trig < 15 GeV/c . In this p T trig interval, the momentum resolution effects are negligible on the extracted yields, and therefore, no p T trig resolution correction is applied. The total number of analysed collisions before the trigger particle selection are about 108 , 108 , and 107 for pp, p–Pb, and Pb–Pb collisions, respectively. The transverse momentum of particles is determined from measurements in the central barrel with the ITS and the Time Projection Chamber (TPC). The ITS is a tracking detector which consists of six cylindrical layers of silicon detectors. The TPC is a cylindrical drift detector which covers a radial distance of 85-247 cm from the beam axis and it has longitudinal dimension extending from about -250 cm to +250 cm around the nominal interaction point. Primary charged particles are measured in the pseudorapidity range of | η | < 0.8 and with p T > 0.5 GeV/c , where η is measured in the laboratory frame for the three collision systems. The configuration for p–Pb collisions with protons at 4 TeV energy colliding with Pb ions that have per-nucleon energies of (Z / A ) × 4 TeV ∼ 1.58 TeV results in a shift in the rapidity of the nucleon–nucleon centre-of-mass system by 0.465 in the direction of the proton beam (negative z-direction). Here Z and A are the atomic and mass numbers of the Pb ion, respectively. Therefore, the detector coverage | η | < 0.8 corresponds to roughly − 0.3 < | η cms | < 1.3 for p–Pb collisions. The particles with mean proper lifetime larger than 1 cm/c , which are either produced directly in the interaction or from decays of particles with mean proper lifetime smaller than 1 cm/c are termed as primary particles [41] . The track selection follows a procedure similar to the one described in Ref. [42] and only few specific details are reported here. Tracks (N tracks ) are required to have two hits in the ITS, out of which at least one should be in either of the two innermost layers. The geometrical track length L is calculated in the TPC readout plane, excluding the information from the pads at the sector boundaries (≈3 cm from the sector edges). The trajectory lengths built from radial segments, i.e. the crossed TPC pad rows, traversed in the TPC by a particle are required to be larger than 85% of the geometrical track length. The pad rows are made of at least 3 neighbouring individual observations (clusters), and their height varies from 7.5 mm to 15 mm [43] . The trajectory lengths built from clusters (one cluster per pad row) is required to be larger than 0.7 × L . The fraction of TPC clusters shared with another track is required to be lower than 0.4. The fit quality for the ITS and TPC track points must satisfy χ ITS 2 / N hits < 36 and χ TPC 2 / N clusters < 4 , respectively, where N hits and N clusters are the numbers of hits in the ITS and the number of clusters in the TPC, respectively. Only tracks with χ TPC − ITS 2 < 36 are included in the analysis, where χ TPC − ITS 2 is calculated comparing the track parameters from the combined ITS and TPC track reconstruction to that derived only from the TPC and constrained to the interaction point. The definition of χ TPC − ITS 2 can be found in Ref. [44] . To reduce the contamination from secondary particles, tracks are accepted if their distance-of-closest-approach (DCA) to the reconstructed primary interaction vertex satisfies in the longitudinal (d z ) and transverse (d xy ) directions the conditions d z < 2 cm and d xy < 0.018 cm + 0.035 (cm×GeV/c )/ p T . The measurement of the transverse momentum spectra of charged particles follows the standard procedure of the ALICE collaboration [42,45] . The raw yields are corrected for efficiency and contamination from secondary particles. The efficiency correction is calculated from Monte Carlo simulations with GEANT3 [46] transport code, which made use of PYTHIA 8 (Monash) [28] , EPOS-LHC [21] and HIJING [47] event generators for pp, p–Pb and Pb–Pb collisions, respectively and incorporated a detailed description of the detector material, geometry and response. Since the event generators do not reproduce the relative abundances of different particle species in the real data, the efficiency obtained from the simulations is re-weighted considering the particle composition from data as outlined in [42] . A multi-component template fit based on the DCA distributions from the simulation is used for the estimation of secondary contamination [42] . The p T spectra for the toward and away regions include contributions from the jet fragmentation, ISR, and FSR, as well as, the contribution from the underlying event. In order to increase the sensitivity to the hardest process of the event, the particle yields measured in the transverse region are subtracted from the corresponding yields in both the toward and away regions: d N ch t , a / d p T − d N ch T / d p T . This approach assumes that the background (UE, ISR, and FSR) in the toward and away regions is similar to the activity in the transverse region. However, one has to keep in mind that in Pb–Pb collisions two-particle correlations are affected by anisotropic transverse flow. In particular, the main contribution is due to the elliptic flow, v 2 , which is the second order coefficient in the Fourier expansion of the azimuthal distribution of the particle momenta [48] . This elliptic azimuthal anisotropy modulates the background according to:(2) B ( Δ φ ) = B 0 ( 1 + 2 V 2 cos ( 2 Δ φ ) ) , where V 2 is approximately given by the product of anisotropic flow coefficients for trigger and associated particles at their respective momenta i.e. V 2 ≈ v 2 trig v 2 assoc . The existing v 2 measurements over a broad transverse momentum range [49] suggest that the effect of the v 2 modulation of background should be more relevant in semi-central Pb–Pb collisions. The effect is expected to be important at low and intermediate transverse momenta and decreases for high transverse momentum particles [30] . In the high-p T region of interest for the jet quenching studies, namely p T > 4 GeV/c , the effect of the v 2 modulation is estimated to be small (about 5%) for Pb–Pb collisions. Given that the v 2 effect is larger in Pb–Pb collisions than in pp and p–Pb collisions, no correction for the v 2 modulation is applied for pp and p–Pb collisions since its effect is smaller than the other sources of systematic uncertainty. The results are shown as a function of the average number of charged particles in the transverse region 〈 N ch T 〉 . The values of 〈 N ch T 〉 are extracted in each multiplicity class from the N tracks distributions in the transverse region that are corrected for detector effects using a Bayesian unfolding [50] . The Bayesian unfolding requires the multiplicity response matrix, which is built from the correlation between the measured multiplicity and the multiplicity at generator level (without detector effects) in the transverse region. This has been obtained from MC simulations which include the propagation of particles through the detector using GEANT 3. As a crosscheck, the 〈 N ch T 〉 values are also calculated by integrating the transverse momentum distributions in the interval 0.5 ≤ p T < 8 GeV/c . The difference between the results from the two strategies is assigned as the systematic uncertainty on 〈 N ch T 〉 , where the effects related to the discrepancy between data and MC in the particle composition and secondary contamination are considered. This uncertainty amounts up to 3.5%, 4% and 6.5% for pp, p–Pb and Pb–Pb collisions, respectively. The systematic uncertainties related to the track selection criteria were studied by repeating the analysis varying one-by-one the track selection criteria [42,45] . In particular, the upper limits of the track fit quality parameters in the ITS (χ ITS 2 / N hits ) and in the TPC (χ TPC 2 / N clusters ) were varied in the ranges of 25–49 and 3–5, respectively. The maximum fraction of shared TPC clusters was varied between 0.2 to 1 and the maximum d z was varied between 1 and 5 cm [42] . We have also quantified the impact of not including the ITS hit requirement in the track selection. The systematic uncertainty on the primary particle composition was estimated using a procedure similar to the one described in [42] . To quantify the uncertainty due to the imperfect simulation of the detector response, the track matching between the TPC and the ITS information in the data and in the simulation were compared. To achieve this, the fraction of secondary particles was rescaled according to fits to the measured DCA distributions. After this rescaling, the agreement between data and model was found to be within 3% for all collision systems. This value was assigned as an additional systematic uncertainty [42] . The systematic uncertainty on the secondary particle contamination considers the imperfection of the method (multi-component template fit) used to extract the correction. The fit ranges were varied and the fit was repeated using templates with two (primaries, secondaries) or three (primaries, secondaries from material, secondaries from weak decays) components. The maximum spread among these variations was assigned as the systematic uncertainty on the secondary contamination. This contribution dominates at low p T . The density of materials used in simulations of the experimental setup was varied by ± 4.5% [35] , resulting in a negligible systematic uncertainty in the considered p T range of 0.5 to 6.0 GeV/c . For the estimation of total systematic uncertainty, all the above listed contributions were summed in quadrature. The systematic uncertainties are independent of the difference between the azimuthal angle of the associated particle and that of the trigger particle. The estimated systematic uncertainties on the p T spectra significantly depend on p T , while the dependence on the multiplicity classes is mild. The ranges of systematic uncertainties in the three considered collision systems are reported in Table 1 for the various sources described above. 3 Results and discussion The p T spectra measured in the transverse region for pp, p–Pb, and Pb–Pb collisions are shown in Fig. 2 (top panel). Results are presented for different multiplicity classes. The ratios between the spectra in the individual multiplicity classes and the MB (0−100%) one are shown in the bottom panel. In the p T range 0.5 − 6 GeV/c , the ratios for the highest multiplicity class (0−5%) are larger than unity and show an increasing trend with increasing p T at low p T (< 2 − 3 GeV/c ) followed at higher p T by a slow decrease. Instead, for the lowest multiplicity classes (40−60% and 60−90%) the ratios are lower than unity and follow an opposite trend with p T , decreasing at low p T and increasing for p T > 3 GeV/c . The behaviour of the ratios as a function of the event activity is reminiscent of analogous ratios as a function of the number of MPI in pp collisions simulated with PYTHIA 8, including colour reconnection [51] . In particular, at p T ≈ 2 − 3 GeV/c the p T spectrum of pp collisions with large MPI activity exhibits an enhancement with respect to the p T spectrum of MB pp collisions. The effect was not observed before in data because, in contrast to the present analysis, the jet contribution was included in the p T spectra [45] . The top (bottom) panel of Fig. 3 shows the charged particle yields for the toward (away) region after the subtraction of the yields measured in the transverse region in pp, p–Pb and Pb–Pb collisions. Results are compared with the p T spectra measured for MB pp collisions (0−100% V0M pp event class) quantified with the ratio I X t , a , as defined in Eq. (1) . At low transverse momenta, p T < 4 GeV/c , I X t , a is close to unity in pp and p–Pb collisions. In contrast, I X t , a in Pb–Pb collisions exhibits a strong multiplicity dependence over the whole measured p T interval. The I X t , a magnitude is larger for semi-peripheral Pb–Pb collisions, the maximum is observed for 20−40% Pb–Pb collisions, and is smaller for the most central and most peripheral classes. Given that the v 2 contribution is not subtracted from the jet-like yields reported in Fig. 3 , the centrality dependence of I X t , a follows the behaviour of v 2 as a function of collision centrality and particle p T in Pb–Pb collisions at LHC energies [52] . Fig. 4 shows the measured values of I X t , a in the transverse momentum interval 4 < p T < 6 GeV/c as a function of the average multiplicity in the transverse region for all the multiplicity classes considered in pp, p–Pb and Pb–Pb collisions. The figure shows that, within uncertainties, the I X t , a values are close to unity for all the multiplicity classes measured in pp and p–Pb collisions. This indicates that effects induced by possible energy loss in these systems are not observed within uncertainties. This result is consistent with previous studies of nuclear modification factor [33] and hadron-jet recoil measurements [34] . By contrast, for Pb–Pb collisions the I X t , a values are compatible to unity for peripheral collisions, and show a gradual enhancement (reduction) with the increase in multiplicity for the toward (away) region. The behaviour is the same for the I X t , a values measured either assuming a flat background or a v 2 -modulated background. The v 2 -modulated background was estimated following the approach depicted in Eq. (2) and using the v 2 data reported in [49] . This behaviour is qualitatively similar to the di-hadron correlation results reported by the STAR and ALICE collaborations [29,30] . In Pb–Pb collisions, I X t provides information about the fragmenting jet leaving the medium, while on the away side, I X a reflects the survival probability of the recoiling parton during passage through the medium. Thus a suppression of I X a would indicate that fewer partons survive the passage through the medium and is expected from the strong in-medium energy loss. On the other hand, the enhancement observed in the toward region is also subject to medium effects. The ratio is sensitive to a) a possible change of the fragmentation functions, b) a possible modification of the quark to gluon jet ratio in the final state due to different coupling with medium, and c) a possible bias on the parton spectrum due to trigger particle selection. Moreover, given that I X t , a is sensitive to the same effects as I AA , the interpretation of the results is similar to that reported in [30] . It is likely that all three effects play a role [30] . A detailed quantification of the contribution of each effect is beyond the scope of the present paper.In order to get further insight into the effect, the measured I X t , a values are compared in Fig. 5 with model predictions. Following the similar treatment of the experimental data, for the models, the total sample is subdivided into different V0M classes and the 〈 N ch T 〉 is calculated for each class. For high-multiplicity pp collisions, although I X t , a is close to unity, a small trend with multiplicity is visible, which is not seen at similar multiplicities (20−90% V0A) in p–Pb data. To understand the source of these slight deviations from unity, the data are compared with the predictions from the PYTHIA 8 (Monash tune [28] ) and EPOS-LHC [21] event generators. In PYTHIA, the hadronization of quarks is simulated using the Lund string fragmentation model [53] . Various PYTHIA tunes have been developed through extensive comparison of Monte Carlo distributions with the minimum-bias data from different experiments. The Monash tune of PYTHIA 8 is tuned to LHC data and uses an updated set of hadronization parameters compared to the previous tunes [28] . EPOS-LHC is built on the Parton-Based Gribov Regge Theory. Utilising the colour exchange mechanism of string excitation, the model is tuned to LHC data [21] . In this model, a part of the collision system which has high parton densities becomes a “core” region that evolves hydrodynamically as a quark–gluon plasma and it is surrounded by a more dilute “corona” for which fragmentation occurs in the vacuum. The upper panel of Fig. 5 shows I X t , a for different multiplicity classes. The observed deviations from unity are reproduced by PYTHIA 8 for both the toward and away regions. Given that PYTHIA 8 does not incorporate any jet quenching mechanism, the origin of the effect in high 〈 N ch T 〉 collisions is related to a remaining bias towards harder fragmentation and more activity from initial and final state radiation [54] . These effects enhance the high-p T yield in the toward region, and produce a broadening in the away region [55] . The EPOS-LHC results in the away region are similar to both data and PYTHIA 8. However, for I X t EPOS-LHC exhibits a trend with a maximum at intermediate multiplicity and a reduction toward low and high multiplicities, which is not consistent with the measurements. The middle and bottom panels of Fig. 5 show I X t , a measured for p–Pb and Pb–Pb collisions, respectively. The data are compared to PYTHIA 8/Angantyr [56] and EPOS-LHC predictions. The Angantyr model in PYTHIA 8 extrapolates the dynamics from pp collisions to p–Pb and Pb–Pb collisions, generalising the formalism adopted for pp collisions by including a description of the nucleon positions within the colliding nuclei and utilising the Glauber model to calculate the number of interacting nucleons and binary nucleon–nucleon collisions. PYTHIA 8/Angantyr, which does not include jet quenching effects, predicts I X t , a values consistent with unity for all the multiplicity classes in Pb–Pb collisions. Whereas for p–Pb collisions I X a is consistent with unity, and I X t is slightly below unity. In EPOS-LHC, a certain p T cutoff is defined in such a way that, above this cutoff, a particle loses part of its momentum in the core but survives as an independent particle produced by a flux tube. Soft particles, which are below the p T cutoff, get completely absorbed and form the core. This sort of energy loss mechanism implemented in EPOS-LHC depends on the system size [21,57,58] . Fig. 5 (middle) shows that for p–Pb collisions, EPOS-LHC does not describe either the magnitude or the trend of the multiplicity dependence of the measured ratio in the toward region, I X t . However, the model is in reasonable agreement with data in the away region. For Pb–Pb collisions, EPOS-LHC predicts a significant enhancement of I X t , a for low 〈 N ch T 〉 ranges and deviates significantly from the experimental results. In summary, while the data from Pb–Pb collisions are in qualitative agreement with expectations from parton energy loss due to the presence of a hot and dense medium, pp and p–Pb data do not show any hint of medium effects in the multiplicity range which is reported. 4 Summary The transverse momentum spectra (0.5 ≤ p T < 6 GeV/c ) of primary charged particles in three azimuthal regions (toward, away and transverse) defined with respect to the direction of the particle with the highest transverse momentum in the event (8 ≤ p T trig < 15 GeV/c ) are reported. The spectra are studied in intervals of the multiplicity measured at forward pseudorapidities for pp, p–Pb, and Pb–Pb collisions at s NN = 5.02 TeV. The p T spectra in the transverse region are subtracted from those of the away and toward regions. This is based on the assumption that the transverse side provides a good estimation of the underlying event contribution in both the toward and away regions. However, for the interpretation of the results one has to keep in mind that v 2 modulates the background and this effect is important for semi-central Pb–Pb collisions and for p T > 4 GeV/c the effect is less than 5% in central and peripheral Pb–Pb collisions. Ratios to MB pp (I X t , a ), i.e., the multiplicity dependent yields normalised to the yield measured in MB pp collisions, are reported. At low transverse momentum (p T < 2 GeV/c ), within 20%, the I X t , a values are multiplicity independent for both the toward and away regions in pp and p–Pb collisions. In contrast, in Pb–Pb collisions for both toward and away regions the I X t , a values exhibit a centrality dependence which is expected given the residual presence of elliptic flow. In the highest transverse momentum interval (4 < p T < 6 GeV/c ), the I X t , a values in pp collisions are closer to unity but they exhibit a small reduction (increase) towards high V0 activity in pp collisions. This trend is well reproduced by PYTHIA 8. In the model, it is due to a selection bias towards pp collisions with harder fragmentation and larger activity from initial and final state radiation. For p–Pb collisions, within uncertainties, the I X t , a values are consistent with unity and do not show a multiplicity dependence. PYTHIA 8/Angantyr fairly describes I X a , but it underestimates by about 10% the I X t values in the low multiplicity classes (40−90% V0A event class). For Pb–Pb collisions, the I X t , a values are close to unity for peripheral collisions, and show a gradual increase (reduction) in the toward (away) region with increasing multiplicity. A similar observable, I AA , based on the per-trigger yield of associated particles in di-hadron correlation has been studied for central and peripheral Pb–Pb collisions at s NN = 2.76 TeV. The behaviour of I X t , a exhibits the same features as I AA : in central collisions, on the away-side, a suppression is observed as expected from strong in-medium energy loss. In the toward region, an enhancement is observed. PYTHIA 8/Angantyr predicts I X t , a ≈ 1 for all multiplicity intervals, and it does not reproduce the observed away-side suppression or toward-side enhancement. Generally, EPOS-LHC does not describe the measured I X t , a ratios. In summary, within the multiplicity reach reported in this paper, no jet quenching effects are observed in pp and p–Pb collisions within uncertainties. Further studies are required to extend the present analysis to higher multiplicities, which are currently limited by the event selection based on the forward V0 detector. The analysis of future pp and p–Pb collisions with much larger integrated luminosity may remove this limitation. Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgements The ALICE Collaboration would like to thank all its engineers and technicians for their invaluable contributions to the construction of the experiment and the CERN accelerator teams for the outstanding performance of the LHC complex. The ALICE Collaboration gratefully acknowledges the resources and support provided by all Grid centres and the Worldwide LHC Computing Grid (WLCG) collaboration. The ALICE Collaboration acknowledges the following funding agencies for their support in building and running the ALICE detector: A.I. Alikhanyan National Science Laboratory (Yerevan Physics Institute) Foundation (ANSL) , State Committee of Science and World Federation of Scientists (WFS) , Armenia; Austrian Academy of Sciences , Austrian Science Fund (FWF): [M 2467-N36 ] and Nationalstiftung für Forschung, Technologie und Entwicklung , Austria; Ministry of Communications and High Technologies, National Nuclear Research Center , Azerbaijan; Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq ), Financiadora de Estudos e Projetos (Finep), Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP ) and Universidade Federal do Rio Grande do Sul (UFRGS ), Brazil; Bulgarian Ministry of Education and Science , within the National Roadmap for Research Infrastructures 2020–2027 (object CERN), Bulgaria; Ministry of Education of China (MOEC), Ministry of Science & Technology of China (MSTC) and National Natural Science Foundation of China (NSFC), China; Ministry of Science and Education and Croatian Science Foundation , Croatia; Centro de Aplicaciones Tecnológicas y Desarrollo Nuclear (CEADEN), Cubaenergía , Cuba; Ministry of Education, Youth and Sports of the Czech Republic , Czech Republic; The Danish Council for Independent Research | Natural Sciences , the Villum Fonden and Danish National Research Foundation (DNRF), Denmark; Helsinki Institute of Physics (HIP), Finland; Commissariat à l'Energie Atomique (CEA ) and Institut National de Physique Nucléaire et de Physique des Particules (IN2P3) and Centre National de la Recherche Scientifique (CNRS), France; Bundesministerium für Bildung und Forschung (BMBF ) and GSI Helmholtzzentrum für Schwerionenforschung GmbH , Germany; General Secretariat for Research and Technology , Ministry of Education, Research and Religions, Greece; National Research, Development and Innovation Office , Hungary; Department of Atomic Energy Government of India (DAE ), Department of Science and Technology, Government of India (DST ), University Grants Commission , Government of India (UGC) and Council of Scientific and Industrial Research (CSIR), India; National Research and Innovation Agency - BRIN , Indonesia; Istituto Nazionale di Fisica Nucleare (INFN ), Italy; Japanese Ministry of Education, Culture, Sports, Science and Technology (MEXT) and Japan Society for the Promotion of Science (JSPS) KAKENHI, Japan; Consejo Nacional de Ciencia (CONACYT ) y Tecnología, through Fondo de Cooperación Internacional en Ciencia y Tecnología (FONCICYT) and Dirección General de Asuntos del Personal Académico (DGAPA), Mexico; Nederlandse Organisatie voor Wetenschappelijk Onderzoek (NWO), Netherlands; The Research Council of Norway , Norway; Commission on Science and Technology for Sustainable Development in the South (COMSATS), Pakistan; Pontificia Universidad Católica del Perú , Peru; Ministry of Education and Science , National Science Centre and WUT ID-UB , Poland; Korea Institute of Science and Technology Information and National Research Foundation of Korea (NRF), Republic of Korea; Ministry of Education and Scientific Research , Institute of Atomic Physics , Ministry of Research and Innovation and Institute of Atomic Physics and University Politehnica of Bucharest , Romania; Ministry of Education, Science, Research and Sport of the Slovak Republic , Slovakia; National Research Foundation of South Africa , South Africa; Swedish Research Council (VR) and Knut & Alice Wallenberg Foundation (KAW), Sweden; European Organization for Nuclear Research , Switzerland; Suranaree University of Technology (SUT), National Science and Technology Development Agency (NSTDA), Thailand Science Research and Innovation (TSRI) and National Science, Research and Innovation Fund (NSRF), Thailand; Turkish Energy, Nuclear and Mineral Research Agency (TENMAK), Turkey; National Academy of Sciences of Ukraine , Ukraine; Science and Technology Facilities Council (STFC), United Kingdom; National Science Foundation of the United States of America (NSF ) and United States Department of Energy , Office of Nuclear Physics (DOE NP), United States of America. 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Qin and X.-N. Wang, “Jet quenching in high-energy heavy-ion collisions”, Int. J. Mod. Phys. E 24 no. 11, (2015) 1530014, arXiv:1511.00790 [hep-ph]. [33] ALICE Collaboration S. Acharya Transverse momentum spectra and nuclear modification factors of charged particles in pp, p-Pb and Pb-Pb collisions at the LHC J. High Energy Phys. 11 2018 013 arXiv:1802.09145 [nucl-ex] ALICE Collaboration, S. Acharya et al., “Transverse momentum spectra and nuclear modification factors of charged particles in pp, p-Pb and Pb-Pb collisions at the LHC”, JHEP 11 (2018) 013, arXiv:1802.09145 [nucl-ex]. [34] ALICE Collaboration S. Acharya Constraints on jet quenching in p-Pb collisions at s NN = 5.02 TeV measured by the event-activity dependence of semi-inclusive hadron-jet distributions Phys. Lett. B 783 2018 95 113 arXiv:1712.05603 [nucl-ex] ALICE Collaboration, S. 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[40] ALICE Collaboration J. Adam Multiplicity dependence of charged pion, kaon, and (anti)proton production at large transverse momentum in p-Pb collisions at s NN = 5.02 TeV Phys. Lett. B 760 2016 720 735 arXiv:1601.03658 [nucl-ex] ALICE Collaboration, J. Adam et al., “Multiplicity dependence of charged pion, kaon, and (anti)proton production at large transverse momentum in p-Pb collisions at sNN = 5.02 TeV”, Phys. Lett. B 760 (2016) 720–735, arXiv:1601.03658 [nucl-ex]. [41] ALICE Collaboration The ALICE definition of primary particles ALICE-PUBLIC-2017-005 https://cds.cern.ch/record/2270008 Jun 2017 ALICE Collaboration, “The ALICE definition of primary particles”, ALICE-PUBLIC-2017-005 (Jun, 2017). https://cds.cern.ch/record/2270008. [42] ALICE Collaboration S. Acharya Transverse momentum spectra and nuclear modification factors of charged particles in pp, p-Pb and Pb-Pb collisions at the LHC J. High Energy Phys. 11 2018 013 arXiv:1802.09145 [nucl-ex] ALICE Collaboration, S. 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Xu Hadron production in p+p, p+Pb, and Pb+Pb collisions with the HIJING 2.0 model at energies available at the CERN large hadron collider Phys. Rev. C 83 2011 014915 arXiv:1008.1841 [hep-ph] W.-T. Deng, X.-N. Wang, and R. Xu, “Hadron production in p+p, p+Pb, and Pb+Pb collisions with the HIJING 2.0 model at energies available at the CERN Large Hadron Collider”, Phys. Rev. C 83 (2011) 014915, arXiv:1008.1841 [hep-ph]. [48] ALICE Collaboration K. Aamodt Harmonic decomposition of two-particle angular correlations in Pb-Pb collisions at s NN = 2.76 TeV Phys. Lett. B 708 2012 249 264 arXiv:1109.2501 [nucl-ex] ALICE Collaboration, K. Aamodt et al., “Harmonic decomposition of two-particle angular correlations in Pb-Pb collisions at sNN= 2.76 TeV”, Phys. Lett. B 708 (2012) 249–264, arXiv:1109.2501 [nucl-ex]. [49] ALICE Collaboration B. Abelev Anisotropic flow of charged hadrons, pions and (anti-)protons measured at high transverse momentum in Pb-Pb collisions at s NN = 2.76 TeV Phys. Lett. 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diff --git a/tests/units/hindawi/data/example1.xml b/tests/units/hindawi/data/example1.xml
index 79a771c4..d69e5f08 100644
--- a/tests/units/hindawi/data/example1.xml
+++ b/tests/units/hindawi/data/example1.xml
@@ -66,4 +66,3 @@
-
diff --git a/tests/units/hindawi/data/example2.xml b/tests/units/hindawi/data/example2.xml
index 5c2bfacf..0adc5a8c 100644
--- a/tests/units/hindawi/data/example2.xml
+++ b/tests/units/hindawi/data/example2.xml
@@ -182,4 +182,3 @@
-
diff --git a/tests/units/hindawi/data/example3.xml b/tests/units/hindawi/data/example3.xml
index 40b8645b..95511b1f 100644
--- a/tests/units/hindawi/data/example3.xml
+++ b/tests/units/hindawi/data/example3.xml
@@ -55,4 +55,3 @@
-
diff --git a/tests/units/hindawi/data/without_doi.xml b/tests/units/hindawi/data/without_doi.xml
index 40b8645b..95511b1f 100644
--- a/tests/units/hindawi/data/without_doi.xml
+++ b/tests/units/hindawi/data/without_doi.xml
@@ -55,4 +55,3 @@
-
diff --git a/tests/units/hindawi/data/without_page_nr.xml b/tests/units/hindawi/data/without_page_nr.xml
index 717e6940..b31202ac 100644
--- a/tests/units/hindawi/data/without_page_nr.xml
+++ b/tests/units/hindawi/data/without_page_nr.xml
@@ -181,4 +181,3 @@
-
diff --git a/tests/units/hindawi/test_hindawi_enhance.py b/tests/units/hindawi/test_hindawi_enhance.py
index 623925a5..f8e71fb2 100644
--- a/tests/units/hindawi/test_hindawi_enhance.py
+++ b/tests/units/hindawi/test_hindawi_enhance.py
@@ -1,10 +1,7 @@
import xml.etree.ElementTree as ET
import pytest
-from common.parsing.xml_extractors import RequiredFieldNotFoundExtractionError
-from hindawi.hindawi_file_processing import (
- enhance_hindawi,
-)
+from hindawi.hindawi_file_processing import enhance_hindawi
from hindawi.parser import HindawiParser
diff --git a/tests/units/iop/data/aca95c.xml b/tests/units/iop/data/aca95c.xml
index 39528652..e6179bb5 100644
--- a/tests/units/iop/data/aca95c.xml
+++ b/tests/units/iop/data/aca95c.xml
@@ -24,14 +24,14 @@
symmetry, we show that the Kahler potential and superpotential in the RNMSSM change as follows:
-
- 1
+
+ 1
-
- 2
+
+ 2
@@ -102,24 +102,24 @@
-
+
-
+
- Q
+ Q
-
+
@@ -129,12 +129,12 @@
-
+
-
+
@@ -144,24 +144,24 @@
-
+
-
+
- L
+ L
-
+
@@ -171,12 +171,12 @@
-
+
-
+
@@ -186,12 +186,12 @@
-
+
-
+
@@ -201,24 +201,24 @@
-
+
-
+
- N
+ N
-
+
@@ -228,12 +228,12 @@
-
+
-
+
@@ -243,24 +243,24 @@
-
+
-
+
- T
+ T
-
+
@@ -270,12 +270,12 @@
-
+
-
+
@@ -297,8 +297,8 @@
as
-
- 3
+
+ 3
@@ -306,8 +306,8 @@
in Eq. (3) are read from soft SUSY-breaking operators such as
-
- 4
+
+ 4
@@ -329,8 +329,8 @@
triplet T shown in Table 1 , the determinant of the charged chargino mass matrix vanishes owing to the unbroken R symmetry. This mass issue can be resolved by an extension of the matter content. The simplest way to achieve this is via the addition of the triplet T [16 ], which couples to the Higgs doublets as
-
- 5
+
+ 5
@@ -338,8 +338,8 @@
. With the quantum numbers of T , one can infer its soft masses from the SUSY-breaking operators,
-
- 6
+
+ 6
@@ -368,14 +368,14 @@
, respectively. With the new superpotential term in Eq. (5) taken into account, the chargino and neutralino mass matrices are now given as
-
- 7
+
+ 7
-
- 8
+
+ 8
@@ -389,8 +389,8 @@
, and we simply assume the singlet vevs
-
- 9
+
+ 9
@@ -542,7 +542,7 @@
-
+
@@ -552,7 +552,7 @@
-
+
@@ -562,7 +562,7 @@
-
+
@@ -572,7 +572,7 @@
-
+
@@ -582,14 +582,14 @@
-
+
- λ
+ λ
@@ -599,7 +599,7 @@
-
+
@@ -642,8 +642,8 @@
pair is dominated by lepton final states [36 ]
-
- 10
+
+ 10
@@ -738,14 +738,14 @@
(in purple).
![]()
![]()
C. Higgs Compared to the R -symmetric MSSM (RMSSM) [43 –47 ] or conventional NMSSM, the scalar mass spectrum of the Higgs sector in the minimal RNMSSM is different. Let us begin with relevant soft masses in our model.
-
- 11
+
+ 11
where the first three terms arise from the following soft SUSY-breaking operators:
-
- 12
+
+ 12
@@ -759,14 +759,14 @@
are given by
-
- 13
+
+ 13
and
-
- 14
+
+ 14
@@ -777,14 +777,14 @@
are eliminated by the b term because of the conditions of electroweak symmetry breaking.
Second, the mass squared matrices for the CP -odd scalars are
-
- 15
+
+ 15
which contain a massless Goldstone mode, and
-
- 16
+
+ 16
@@ -792,8 +792,8 @@
is given by
-
- 17
+
+ 17
@@ -893,14 +893,14 @@
-
+
- b
+ b
@@ -910,14 +910,14 @@
-
+
- λ
+ λ
@@ -927,7 +927,7 @@
-
+
@@ -937,8 +937,8 @@
, with
-
- 18
+
+ 18
@@ -949,14 +949,14 @@
.
D. Sfermions Apart from Majorana gaugino masses, R symmetry also prohibits holomorphic soft masses, such as A terms related to sfermions. It only allows scalar soft masses
-
- 19
+
+ 19
which arise from soft SUSY-breaking operators such as
-
- 20
+
+ 20
@@ -975,7 +975,7 @@
-
+
@@ -1000,7 +1000,7 @@
-
+
@@ -1025,7 +1025,7 @@
-
+
@@ -1050,7 +1050,7 @@
-
+
diff --git a/tests/units/iop/data/title_and_abstract_with_cdata.xml b/tests/units/iop/data/title_and_abstract_with_cdata.xml
index 8608149c..b935d886 100644
--- a/tests/units/iop/data/title_and_abstract_with_cdata.xml
+++ b/tests/units/iop/data/title_and_abstract_with_cdata.xml
@@ -4766,4 +4766,4 @@
PTEP 2020 2020 8 083C 10.1093/ptep/ptaa104 [116]
L. Workman R.
et al
- PTEP 2022 2022 083C 10.1093/ptep/ptac097
\ No newline at end of file
+ PTEP 2022 2022 083C 10.1093/ptep/ptac097
diff --git a/tests/units/iop/test_iop_parser.py b/tests/units/iop/test_iop_parser.py
index 8f00bd09..51326b65 100644
--- a/tests/units/iop/test_iop_parser.py
+++ b/tests/units/iop/test_iop_parser.py
@@ -1,13 +1,13 @@
import xml.etree.ElementTree as ET
+from common.cleanup import replace_cdata_format
from common.constants import ARXIV_EXTRACTION_PATTERN
from common.enhancer import Enhancer
from common.exceptions import UnknownLicense
from common.parsing.xml_extractors import RequiredFieldNotFoundExtractionError
from common.utils import parse_element_text, parse_to_ET_element
-from common.cleanup import replace_cdata_format
-from iop.parser import IOPParser
from iop.iop_process_file import process_xml
+from iop.parser import IOPParser
from pytest import fixture, mark, param, raises
@@ -541,7 +541,6 @@ def test_no_authors(shared_datadir, parser):
parser._publisher_specific_parsing(article)
-
def test_title(shared_datadir, parser):
content = (shared_datadir / "title_and_abstract_with_cdata.xml").read_text()
content = replace_cdata_format(content)
@@ -805,7 +804,7 @@ def test_no_collaborations_value(shared_datadir, parser):
assert "collaborations" not in parsed_article
-def test_title(shared_datadir, parser):
+def test_title_required(shared_datadir, parser):
content = (shared_datadir / "just_required_fields.xml").read_text()
article = ET.fromstring(content)
parsed_article = parser._publisher_specific_parsing(article)
@@ -894,10 +893,10 @@ def test_cdata_abstract_title(shared_datadir):
content = replace_cdata_format(content)
ET_article = parse_to_ET_element(content)
-
abstract_element = ET_article.find("front/article-meta/abstract/p")
abstract_text = parse_element_text(abstract_element)
- assert (abstract_text
+ assert (
+ abstract_text
== "Recently, the experimental measurements of the branching ratios and different"
" polarization asymmetries for processes occurring through flavor-changing-charged"
" current $ (b\\rightarrow c\\tau\\overline{\\nu}_{\\tau}) $ transitions by BABAR,"
@@ -931,12 +930,14 @@ def test_cdata_abstract_title(shared_datadir):
)
title_element = ET_article.find("front/article-meta/title-group/article-title")
title_text = parse_element_text(title_element)
- assert (title_text
- == "Analysis of ${{\\boldsymbol b}{\\bf\\rightarrow} {\\boldsymbol c}{\\boldsymbol\\tau}\\bar"
- "{\\boldsymbol\\nu}_{\\boldsymbol\\tau}}$ anomalies using weak effective Hamiltonian with "
- "complex couplings and their impact on various physical observables"
+ assert (
+ title_text
+ == "Analysis of ${{\\boldsymbol b}{\\bf\\rightarrow} {\\boldsymbol c}{\\boldsymbol\\tau}\\bar"
+ "{\\boldsymbol\\nu}_{\\boldsymbol\\tau}}$ anomalies using weak effective Hamiltonian with "
+ "complex couplings and their impact on various physical observables"
)
+
def test_cdata_without_regex():
paseudo_aricle = """
diff --git a/tests/units/oup/data/oup_orcid.xml b/tests/units/oup/data/oup_orcid.xml
index 12c3643c..81365ed5 100644
--- a/tests/units/oup/data/oup_orcid.xml
+++ b/tests/units/oup/data/oup_orcid.xml
@@ -3085,4 +3085,4 @@ m_{H^+_1},\; m_{H^+_2}, \; m_{H^{++}_2} &\in& [1,\; 20]\;\mathrm{ TeV},\; v_R \i
-
\ No newline at end of file
+
diff --git a/tests/units/oup/data/ptab170_no_journal_title_and_pubmed_value.xml b/tests/units/oup/data/ptab170_no_journal_title_and_pubmed_value.xml
index b61bac9b..0b5fd1b4 100644
--- a/tests/units/oup/data/ptab170_no_journal_title_and_pubmed_value.xml
+++ b/tests/units/oup/data/ptab170_no_journal_title_and_pubmed_value.xml
@@ -37,7 +37,7 @@
- Detectable electric current induced by the dark matter axion in a
+ Detectable electric current induced by the dark matter axion in a
conductor
diff --git a/tests/units/oup/test_oup_parser.py b/tests/units/oup/test_oup_parser.py
index 7d6d9656..d20f9898 100644
--- a/tests/units/oup/test_oup_parser.py
+++ b/tests/units/oup/test_oup_parser.py
@@ -635,54 +635,36 @@ def test_authors_parsing_with_orcid(article_with_orcid):
"surname": "Hong",
"given_names": "T T",
"affiliations": [
- {
- "organization": "An Giang University",
- "country": "Vietnam"
- },
- {
- "organization": "Vietnam National University",
- "country": "Vietnam"
- }
+ {"organization": "An Giang University", "country": "Vietnam"},
+ {"organization": "Vietnam National University", "country": "Vietnam"},
],
"orcid": "0000-0002-7719-4160",
- "full_name": "Hong, T T"
+ "full_name": "Hong, T T",
},
{
"surname": "Le",
"given_names": "V K",
"affiliations": [
- {
- "organization": "An Giang University",
- "country": "Vietnam"
- },
- {
- "organization": "Binh Thuy Junior High School",
- "country": "Vietnam"
- }
+ {"organization": "An Giang University", "country": "Vietnam"},
+ {"organization": "Binh Thuy Junior High School", "country": "Vietnam"},
],
- "full_name": "Le, V K"
+ "full_name": "Le, V K",
},
{
"surname": "Phuong",
"given_names": "L T T",
"affiliations": [
- {
- "organization": "An Giang University",
- "country": "Vietnam"
- }
+ {"organization": "An Giang University", "country": "Vietnam"}
],
- "full_name": "Phuong, L T T"
+ "full_name": "Phuong, L T T",
},
{
"surname": "Hoi",
"given_names": "N C",
"affiliations": [
- {
- "organization": "An Giang University",
- "country": "Vietnam"
- }
+ {"organization": "An Giang University", "country": "Vietnam"}
],
- "full_name": "Hoi, N C"
+ "full_name": "Hoi, N C",
},
{
"surname": "Ngan",
@@ -690,10 +672,10 @@ def test_authors_parsing_with_orcid(article_with_orcid):
"affiliations": [
{
"organization": "Department of Physics, Can Tho University",
- "country": "Vietnam"
+ "country": "Vietnam",
}
],
- "full_name": "Ngan, N T K"
+ "full_name": "Ngan, N T K",
},
{
"surname": "Nha",
@@ -702,15 +684,15 @@ def test_authors_parsing_with_orcid(article_with_orcid):
"affiliations": [
{
"organization": "Subatomic Physics Research Group, Science and Technology Advanced Institute, Van Lang University",
- "country": "Vietnam"
+ "country": "Vietnam",
},
{
"organization": "Faculty of Applied Technology, School of Engineering and Technology, Van Lang University",
- "country": "Vietnam"
- }
+ "country": "Vietnam",
+ },
],
"orcid": "0009-0005-5993-6895",
- "full_name": "Nha, N H T"
- }
+ "full_name": "Nha, N H T",
+ },
]
assert article_with_orcid["authors"] == expected_output
diff --git a/tests/units/springer/test_parser.py b/tests/units/springer/test_parser.py
index 77146de0..18bf50ef 100644
--- a/tests/units/springer/test_parser.py
+++ b/tests/units/springer/test_parser.py
@@ -1,9 +1,9 @@
import xml.etree.ElementTree as ET
from os import listdir
+from common.enhancer import Enhancer
from pytest import fixture
from springer.parser import SpringerParser
-from common.enhancer import Enhancer
@fixture(scope="module")
@@ -333,90 +333,94 @@ def test_abstract(parsed_articles):
@fixture
def article_with_orcid(parser, datadir):
with open(datadir / "s10052-024-12692-y.xml") as file:
- yield parser._generic_parsing(parser._publisher_specific_parsing(ET.fromstring(file.read())))
+ yield parser._generic_parsing(
+ parser._publisher_specific_parsing(ET.fromstring(file.read()))
+ )
def test_article_with_cleaned_orcid(article_with_orcid):
- expected_output = [{
- "surname": "Hong",
- "given_names": "T.",
- "email": "tthong@agu.edu.vn",
- "affiliations": [
- {
- "value": "An Giang University, Long Xuyen, 880000, Vietnam",
- "organization": "An Giang University",
- "country": "Vietnam"
- },
- {
- "value": "Vietnam National University, Ho Chi Minh City, 700000, Vietnam",
- "organization": "Vietnam National University",
- "country": "Vietnam"
- }
- ],
- "full_name": "Hong, T."
- },
+ expected_output = [
{
- "surname": "Tran",
- "given_names": "Q.",
- "email": "tqduyet@agu.edu.vn",
- "affiliations": [
- {
- "value": "An Giang University, Long Xuyen, 880000, Vietnam",
- "organization": "An Giang University",
- "country": "Vietnam"
- },
- {
- "value": "Vietnam National University, Ho Chi Minh City, 700000, Vietnam",
- "organization": "Vietnam National University",
- "country": "Vietnam"
- }
- ],
- "full_name": "Tran, Q."
- },
+ "surname": "Hong",
+ "given_names": "T.",
+ "email": "tthong@agu.edu.vn",
+ "affiliations": [
+ {
+ "value": "An Giang University, Long Xuyen, 880000, Vietnam",
+ "organization": "An Giang University",
+ "country": "Vietnam",
+ },
+ {
+ "value": "Vietnam National University, Ho Chi Minh City, 700000, Vietnam",
+ "organization": "Vietnam National University",
+ "country": "Vietnam",
+ },
+ ],
+ "full_name": "Hong, T.",
+ },
{
- "surname": "Nguyen",
- "given_names": "T.",
- "email": "thanhphong@ctu.edu.vn",
- "affiliations": [
- {
- "value": "Department of Physics, Can Tho University, 3/2 Street, Can Tho, Vietnam",
- "organization": "Can Tho University",
- "country": "Vietnam"
- }
- ],
- "full_name": "Nguyen, T."
- },
+ "surname": "Tran",
+ "given_names": "Q.",
+ "email": "tqduyet@agu.edu.vn",
+ "affiliations": [
+ {
+ "value": "An Giang University, Long Xuyen, 880000, Vietnam",
+ "organization": "An Giang University",
+ "country": "Vietnam",
+ },
+ {
+ "value": "Vietnam National University, Ho Chi Minh City, 700000, Vietnam",
+ "organization": "Vietnam National University",
+ "country": "Vietnam",
+ },
+ ],
+ "full_name": "Tran, Q.",
+ },
{
- "surname": "Hue",
- "given_names": "L.",
- "email": "lethohue@vlu.edu.vn",
- "affiliations": [
- {
- "value": "Subatomic Physics Research Group, Science and Technology Advanced Institute, Van Lang University, Ho Chi Minh City, Vietnam",
- "organization": "Van Lang University",
- "country": "Vietnam"
- }
- ],
- "full_name": "Hue, L."
- },
+ "surname": "Nguyen",
+ "given_names": "T.",
+ "email": "thanhphong@ctu.edu.vn",
+ "affiliations": [
+ {
+ "value": "Department of Physics, Can Tho University, 3/2 Street, Can Tho, Vietnam",
+ "organization": "Can Tho University",
+ "country": "Vietnam",
+ }
+ ],
+ "full_name": "Nguyen, T.",
+ },
{
- "orcid": "0009-0005-5993-6895",
- "surname": "Nha",
- "given_names": "N.",
- "email": "nguyenhuathanhnha@vlu.edu.vn",
- "affiliations": [
- {
- "value": "Subatomic Physics Research Group, Science and Technology Advanced Institute, Van Lang University, Ho Chi Minh City, Vietnam",
- "organization": "Van Lang University",
- "country": "Vietnam"
- },
- {
- "value": "Faculty of Applied Technology, School of Technology, Van Lang University, Ho Chi Minh City, Vietnam",
- "organization": "Van Lang University",
- "country": "Vietnam"
- }
- ],
- "full_name": "Nha, N."
- }]
+ "surname": "Hue",
+ "given_names": "L.",
+ "email": "lethohue@vlu.edu.vn",
+ "affiliations": [
+ {
+ "value": "Subatomic Physics Research Group, Science and Technology Advanced Institute, Van Lang University, Ho Chi Minh City, Vietnam",
+ "organization": "Van Lang University",
+ "country": "Vietnam",
+ }
+ ],
+ "full_name": "Hue, L.",
+ },
+ {
+ "orcid": "0009-0005-5993-6895",
+ "surname": "Nha",
+ "given_names": "N.",
+ "email": "nguyenhuathanhnha@vlu.edu.vn",
+ "affiliations": [
+ {
+ "value": "Subatomic Physics Research Group, Science and Technology Advanced Institute, Van Lang University, Ho Chi Minh City, Vietnam",
+ "organization": "Van Lang University",
+ "country": "Vietnam",
+ },
+ {
+ "value": "Faculty of Applied Technology, School of Technology, Van Lang University, Ho Chi Minh City, Vietnam",
+ "organization": "Van Lang University",
+ "country": "Vietnam",
+ },
+ ],
+ "full_name": "Nha, N.",
+ },
+ ]
assert expected_output == article_with_orcid["authors"]
diff --git a/tests/units/springer/test_parser/s10052-024-12692-y.xml b/tests/units/springer/test_parser/s10052-024-12692-y.xml
index c99a7c13..15af4e09 100644
--- a/tests/units/springer/test_parser/s10052-024-12692-y.xml
+++ b/tests/units/springer/test_parser/s10052-024-12692-y.xml
@@ -490,4 +490,4 @@
-
\ No newline at end of file
+