MadGraph5_aMC@NLO

Pythia/Delphes: Nb of events after matching/merging less than the actual cross-section

Asked by Esteban Chalbaud

Hello,

I would like to understand the following problem that I am having with Madgraph, while running the process:
generate p p > tt j $$ w+ w- @0

I am getting the following output in Pythia:

NFO: Running Pythia
INFO: Creating shower LHE File (for plot)
INFO: Warning! Never use this lhe file for detector studies!
INFO: finish
INFO: No pgs_card detected, so not run pgs
INFO: prepare delphes run
INFO: Running Delphes
INFO: If you are interested in lhco output. please run root2lhco converter.
INFO: or edit bin/internal/run_delphes3 to run the converter automatically.
INFO: delphes done
  === Results Summary for run: run_01_decayed_1 tag: tag_1 ===

     Cross-section : 55.08 +- 0.305 pb
     Nb of events : 1000
     Matched cross-section : 37.38 +- 0.8405 pb
     Nb of events after matching/merging : 678
     Nb of events after merging : 678

INFO: storing files of previous run
INFO: Storing Pythia files of previous run
INFO: Done

I don't understand why I am getting less number of events even without any cuts in the input card.

Best Regards,
Esteban

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Olivier Mattelaer (olivier-mattelaer) said : 		#1

Hi,

You do have merging activated. (either ickkw=1 in the run_card or some flag in the pythia_card asking for merging computation).
In merging procedure, the parton-shower has to remove the double counting between the various sample computed in MG5aMC.
and therefore has to apply an accept/reject algorithm to include the sudakov form factor that avoid such double counting.
Therefore in such procedure it is normal that the cross-section decreases and that you are "loosing" some events.

It seems here that you have only one multiplicity
generate p p > tt j $$ w+ w- @0
so in that case, asking for merging should have been a consious choice and you had to specify in the pythia_card what is the highest multiplicity included in the merging procedure (did you?)

Cheers,

Olivier

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