MadGraph5_aMC@NLO

parton level cross section and matching

Asked by Sarah Malik

I noticed that the cross section I obtain at the parton level (so no pythia showering) changes a lot with and without matching. For instance if I'm generating W+1jet events, if I set ickkw=0, the cross -section is ~3700, with ickkw = 1, its 5700. My naive understanding was that if I'm only interested in parton level events and not doing any showering, the matching scheme should not matter. Is this not correct?

Thanks!

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Johan Alwall (johan-alwall) said : 		#1

Hello Sarah,

In fact this is not correct. With matching turned on, you get alpha_s and PDF reweighing inside MadEvent, which is a necessary preparation for the matching procedure, which is done inside the MadGraph-Pythia interface.

In principle, whenever you have final-state partons coming from QCD splittings qqg or ggg, you need to perform matching to get the correct Sudakov suppression, which regulates the divergent matrix element behavior near the soft and collinear limit. See my matching lectures at the Taiwan MadGraph school in May this year:
https://cp3.irmp.ucl.ac.be/projects/madgraph/wiki/SchoolNTU

If you still prefer not to run matching, you should set ickkw=0 for parton-level studies, bearing in mind that the result is not necessarily physically meaningful if you go to too small pt/deltaR.

All the best,
Johan

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Giancarlo Panizzo (giancarlo-panizzo-h) said : 		#2

Dear Jonathan,

I think there should be a clear last word on this topic. The correct parton level cross section should be computed with ickkw=1, right? This number has a meaning also without showering of pythia, or it needs the pythia showering to acquire a meaning?

Thanks

Giancarlo Panizzo

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Olivier Mattelaer (olivier-mattelaer) said : 		#3

Hi,

Yes this kind of computation are meaningful and actually even more than those with icckw=0.

Cheers,

Olivier

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