jetmatching w/ pythia8 giving same number of events

Asked by trenta coollime on 2020-01-30

Hello, I am a grad student using MG for HEP research.

I have a process, p p > h > z zp, z > l+ l-, zp > b b~, in Higgs Valley model, which I launch for 100 events w/pythia8 hadronization
(MLM procedure, jetetamax=5, nJetMax =2, kTscale =58, Qmatch=5- b is treated light for matching).

I expected at least some of events would fail and don't get saved in .hepmc file. However, when I check .hepmc output and run with Delphes, I see all 100 events.

In contrast, in my CERN CMS CMSSW, when I run GENSIM (equivalent to pythia8) with same Jetmatching set-up, it gives me only a half of events as expected.

What is going on with pythia8 in MG such that I get all 100 events even with parton-jet matching?

Thank you.

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Solved by:
Olivier Mattelaer
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Last reply:

By default PY8 is setup to put the weight to zero instead of trashing the event. (This has the same effect for distribution)
The advantage of this method is that you can then change the matching scale without the need to regenerate your sample
which is useful for systematic or simply to find the appropriate merging scale.

For full sim this is however not the best method since your full slim is too slow to support such zero weight event.



Thanks Olivier Mattelaer, that solved my question.