MadGraph5_aMC@NLO

xqcut in combination with ptj

Asked by Maikel de Vries

Hello,

I have a question regarding the matching procedure between pythia and MadGraph. I want to create a qcd 4jet sample from 2j 3j and 4j matrix elements. So I generate those via:
generate p p > j j @0
add process p p > j j j @1
add process p p > j j j j @250
I set MG to MLM matching and change the xqcut in MG and the QCUT in the pythia card to some values, let's say 40 and 50 respectively. Now, however, I want to look at qcd samples with the jet pT's with values above 100. How can I do this consistently with the qcut variables? Since Madgraph overrides for ptj values if I am not mistaken.

Thanks in advance,

Maikel

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Johan Alwall
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Olivier Mattelaer (olivier-mattelaer) said : 		#1

Hi Maikel,

Not sure that your question makes sense. In fact I'm not sure to understand which type of sample of events you want.

If you want to put a pt cut on the leading jet (and /or) subleading jet, you can use
 0 = ptj1min ! minimum pt for the leading jet in pt
 0 = ptj2min ! minimum pt for the second jet in pt

Cheers,

Olivier

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Johan Alwall (johan-alwall) said : 		#2

Hello Maikel,

Sorry, in fact I think I should be the one answering this. I'd like to add to Olivier's answer, that in practice the method to add a cut on the N first jets might work, but it's safer to use an ht-based cut (in your case, set htjmin to 200). That way, you are sure to get also events where two particles from the parton level reconstruct to a single jet at the detector level. This also provides a hard scale (harder than the matching scale), which is necessary for the matching to make sense.

All the best,
Johan

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Maikel de Vries (mdt-maikel) said : 		#3

Hello Johan and Olivier,

Thanks, that is very helpful, I have one more question though. This QCD (htjmin = 200) sample still has a the third and fourth jet with momentum less than 100 GeV. Is there a way to generate consistently a four jet QCD sample with all jet pT's above some value? Maybe a combination of htjmin and the ptjmin's?

Best, Maikel

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Best Johan Alwall (johan-alwall) said : 		#4

Hello Maikel,

You can generate just the 4-jet sample (i.e., veto any events that, at parton level, have less than 4 jets) by setting ptj4min to 100. However, note that you might get "holes" in your phase space due to different jet definitions at parton level and detector level if you do this, so you might want to use the most similar jet definition (i.e., if ktjets use xqcut, if cone jets use dRjj and ptj for matching). In this case, there's obviously no point to include the 2 and 3 jet samples in your process. Note that you need to use a threshold at parton level that is suitably smaller (probably, at least 20%) than the pt cut you want at detector level, to allow for smearing effects.

All the best,
Johan

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Maikel de Vries (mdt-maikel) said : 		#5

Thanks a lot, I think I understand the logic now.

Best, Maikel

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Maikel de Vries (mdt-maikel) said : 		#6

Thanks Johan Alwall, that solved my question.