MadGraph5_aMC@NLO

NLO Mjj cut

Asked by James

Hello,

I am using MG5_aMC_v2_2_3 and would like to generate some NLO W+Jets events using:
generate p p > e- ve~ j j QED=3 [QCD] @1
add process p p > e+ ve j j QED=3 [QCD] @2
The compilation and generation work well but I would like to add a dijet mass cut to this generation. I understand this cut will be applied before the shower so I intend to put it significantly looser than the offline cut.
I can't find a built in way to do this with aMC@NLO so my question concerns adding this cut manually to SubProcess/cuts.f. I see two jet collections related to the fastJet clustering, which collection should I use to apply a dijet mass cut and can I rely on the jets to be pT ordered?

Many thanks,
Jim

Question information

Language:
English Edit question
Status:
Solved
For:
MadGraph5_aMC@NLO Edit question
Assignee:
Rikkert Frederix Edit question
Solved by:
Rikkert Frederix
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Last reply:
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Rikkert Frederix (frederix) said : 		#1

Dear Jim,

There is only one call to fastjet in SubProcesses/cuts.f. You should use the jets as they come out of that call and they are already ordered in pT.

Best regards,
Rikkert

Revision history for this message
James (jimhendy) said : 		#2

Dear Rikket,

Thank you for your response.
My apologies, my question was unclear. I meant that the fastjet clustering returns both "pjet" and "jet". Which of these is best to use for the cut?

Thanks again,
Jim

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Best Rikkert Frederix (frederix) said : 		#3

Dear Jim,

As should be clear from the comment:

c******************************************************************************
c call FASTJET to get all the jets
c
c INPUT:
c input momenta: pQCD(0:3,nexternal), energy is 0th component
c number of input momenta: nQCD
c radius parameter: rfj
c minumum jet pt: sycut
c jet algorithm: palg, 1.0=kt, 0.0=C/A, -1.0 = anti-kt
c
c OUTPUT:
c jet momenta: pjet(0:3,nexternal), E is 0th cmpnt
c the number of jets (with pt > SYCUT): njet
c the jet for a given particle 'i': jet(i), note that this is the
c particle in pQCD, which doesn't
c necessarily correspond to the particle
c label in the process
c
         call amcatnlo_fastjetppgenkt_etamax_timed(
     $ pQCD,nQCD,rfj,sycut,etaj,palg,pjet,njet,jet)
c
c*****************************************************************************

'jet' simply list the jet which contains parton 'i'. It's an integer, not the jet momentum. Hence, you should use pjet, which are the actual jet momenta.

Best regards,
Rikkert

Revision history for this message
James (jimhendy) said : 		#4

Dear Rikkert,

Thanks for this, I had not understood that comment.

Regards,
Jim

Revision history for this message
James (jimhendy) said : 		#5

Thanks Rikkert Frederix, that solved my question.