MadGraph5_aMC@NLO

Questionable Warning after FxFx Merging @ NLO with Pythia8

Asked by Richard Ruiz

Dear Experts,

I have a strange warning when attempting to FxFx Merge @ NLO with Pythia 8.212 that I hope some can clarify. I have followed the recommended instructions [1] as best as I could for the following process:

import model loop_sm-no_tau_mass
define p = g u c d s u~ c~ d~ s~
define j = g u c d s u~ c~ d~ s~
generate p p > t t~ [QCD] @0
add process p p > t t~ j [QCD] @1
output Test_SM_ttBar_nj_BMass

Just before starting the run (after param_card/run_card questions), I get this:
>
WARNING: You are running with FxFx merging enabled. To be able to merge
    samples of various multiplicities without double counting, you
    have to remove some events after showering 'by hand'. Please
    read http://amcatnlo.cern.ch/FxFx_merging.htm for more details.
INFO: Starting run

Things work as usual. I have a nice run summary, modest run time, and my events get showered. It all looks great:

INFO: The file .../Test_SM_ttBar_nj_BMass/Events/run_01/events_PYTHIA8_0.hepmc.gz has been generated.
It contains showered and hadronized events in the HEPMC format obtained showering the parton-level event file.../Test_SM_ttBar_nj_BMass/Events/run_01/events.lhe.gz with PYTHIA8
INFO: Run complete

However, at the end, I get this:

INFO:
WARNING: You are running with FxFx merging enabled. To be able to merge samples of various multiplicities without double counting, you have to remove some events after showering 'by hand'. Please read http://amcatnlo.cern.ch/FxFx_merging.htm for more details.
quit
INFO:

This is where I am confused. According to [1], I am done at this step. However, I am being told I need to remove events by hand. Is this just an artifact that applies to HERWIG and non-Pythia 8.2 showers, or are there actually additional steps that I have not yet performed?

[1] http://amcatnlo.web.cern.ch/amcatnlo/FxFx_merging.htm

Thank you for all the hard work.

Best,
Richard

Question information

Language:
English Edit question
Status:
Solved
For:
MadGraph5_aMC@NLO Edit question
Assignee:
Rikkert Frederix Edit question
Solved by:
Rikkert Frederix
Solved:
Last query:
Last reply:
Revision history for this message
Best Rikkert Frederix (frederix) said : 		#1

Dear Richard,

You have to make sure that you turn FxFx merging on in pythia8. This is done automatically when using the MG5_aMC build-in interface with pythia8 (and when setting the appropriate merging scale and maximum multiplicity parameters in the shower_card), but not when using pythia8 standalone using the generated LHE file for showering.

So, to be short, your HepMC file should be okay. The problem is the order of the messages, which is confusing. I'll see if I can update it.

Best regards,
Rikkert

Revision history for this message
Richard Ruiz (rruiz) said : 		#2

Thanks Rikkert Frederix, that solved my question.