Find the parton an MC particle comes from

[gordonwatts]

Assumptions

1. You have an MC file which contains a vertex -> daughters -> vertex type data structure. This is often found in an MC Les Houches generator files.
2. For each tree, you have some way of telling if a particle is something that came from the hard scatter. That is, an outgoing parton.
3. It is the case that in NLO and NNLO, etc., you might have more than one parton as a parent, but for the sake of this let's pretend that that doesn't happen.

What you want

1. Match the closest MC particle with a jet axis
2. Find the 'parton' that particle is descended from

Why this is tricky

You have to traverse parent vertices and particles until you find what the parton is. This is a loop that doesn't have a fixed number of iterations. You have to keep walking up the tree until you find the MC particle that satisfies the I am a parton condition.