MadGraph5_aMC@NLO

FxFx merging parameters

Asked by Michele Lupattelli

Hi,

I have two questions:

1) The NLO runcard states that

# 3: FxFx Merging - WARNING! Applies merging only at the hard-event *
# level. After showering an MLM-type merging should be applied as *
# well. See http://amcatnlo.cern.ch/FxFx_merging.htm for details. *

and when I launch the event generation the following message is printed

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.

but checking the webpage, it seems that using PYTHIA8 everything should work correctly. Is it right?

2) http://amcatnlo.web.cern.ch/amcatnlo/FxFx_merging.htm says that

### = ptj
where ### is a number at most half as large as the merging scale.

Is ptj relevant for the merging procedure? If Qcut = 30 in the shower card, can I put, for instance, 0 = ptj? If I vary Qcut, shall I vary ptj, too?

Cheers,

Michele

Question information

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MadGraph5_aMC@NLO Edit question
Assignee:
Rikkert Frederix Edit question
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Rikkert Frederix (frederix) said : 		#1

Dear Michele,

1. You have to make sure that you turn FxFx merging on for Pythia8 as well. See the pythia8 manual for details for that.

2. You can put it to zero, but this will be rather inefficient. Better to keep ptj equal to about half of the smallest merging scale (i.e., Qcut) you would like to use. In that setup, you can vary the merging scale without introducing any bias due to a too large ptj cutoff.

Best,
Rikkert

Revision history for this message
Michele Lupattelli (michelel94) said : 		#2

Hi,

1) Do I have to do it even if I installed Pythia8 within mg5_aMC@NLO? Because in the shower_card I have the settings

#***********************************************************************
# FxFx merging parameters !ONLY FOR PYTHIA8!
#***********************************************************************
Qcut = 30 # Merging scale
njmax = 2 # Maximal multiplicity in the merging

And in the runcard I specified that I use Pythia8 parton shower

  PYTHIA8 = parton_shower

2) Do you mean that ptj = 0 would be inefficient when I analyse the events? And, if I want to compute a cross section without any cuts on the final state particles, should not I put ptj = 0?

Cheers,
Michele

Revision history for this message
Michele Lupattelli (michelel94) said : 		#3

2) or is ptj a cut just on the jets produced at the Matrix Element level? In this case it would be clear for me the inefficiency of puttin ptj=0.

Cheers,
Michele

Revision history for this message
Rikkert Frederix (frederix) said : 		#4

Hi Michele,

1. If you use the pythia8 installed with mg5_aMC, then, indeed, it's just a matter of putting the relevant information in the shower_card.

2. Yeah... it's just a jet cut at the matrix element level.

Best,
Rikkert

Revision history for this message
Michele Lupattelli (michelel94) said : 		#5

Hi,

That solved my question.

Cheers,
Michele

Revision history for this message
Michele Lupattelli (michelel94) said : 		#6

Hi again,

I have a further question concerning the parameters in the shower_card.

I was checking the photon emission during the shower, and I found a file contained in ./bin/"outputdirectory"/MCatNLO/RUN_PYTHIA8_# named Pythia8.cmd which contains all the commands used to run the shower using Pythia8. There are some default parameters, as, for instance,

TimeShower:QEDshowerByQ = off ! Prevent quarks from radiating photons
                                   ! ** USE on FOR REALISTIC SHOWER SIMULATION ** !!!
TimeShower:QEDshowerByL = off ! Prevent leptons from radiating photons
                                   ! ** USE on FOR REALISTIC SHOWER SIMULATION ** !!!

Is it possible to change their value within the shower_card?

Best,

Michele

Revision history for this message
Rikkert Frederix (frederix) said : 		#7

Hi Michele,

No, I don't think it's possible from the shower_card.dat directly.

Easiest is to run pythia independently from madgraph, I guess. Just use the executable that is available together with the pythia8.cmd file.

Best,
Rikkert

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