MadGraph5_aMC@NLO

ttbar production cross-section

Asked by Kirill Skovpen

Hello,

Sorry for one more question. I have been trying to compare the LO cross-sections @8TeV for ttbar production in MG5 and Pythia8. The value I get from MG5 is 135 pb while in Pythia8 I see the cross-section of 196 pb. I am sure I am missing something simple but I still can't figure out what it is that is causing such difference. I removed all cuts in MG5 run_card. Any ideas would be much welcomed. I also calculated the cross-section with changing the renormalisation and factorisation scale calculation in MG5 to match the one used in Pythia8 but I get quite similar results to the default method used in MG5.

Thanks,

Kirill

P.S. The exact process is just "p p > t t~"

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Olivier Mattelaer (olivier-mattelaer) said : 		#1

Hi Kirill,

I have no idea how you make your computation, (neither or pythia8 works for that matter)
So this is just the list of potential problem in such computation
1) The value of alphas (we use the one provided by the pdf set and not the one written in the param_card!)
2) The factorization/renormalization scale. Those are very large actually. So it is important to use the same scale (and a fix one) in both case.
3) the number of flavor (4flavor or 5 flavor computation)
4) Inclusion of the QED process for pythia (use QED=99 to include them in MG)
5) the mass spectrum
6) the cuts
7) the electroweak parameter (alpa_ew/MW/MZ/GF) particularly critical due to various choice of fixing the redundant parameter.
8) ...

Some of them didn't really apply in your case (all QED ones) but I put them here as references for future similar topic.

Cheers and good luck for understanding what you did wrong in your comparison,

Olivier

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