MadGraph5_aMC@NLO

Top Pair Production with UFO, sm_v4 and usrmod

Asked by mathieu.buchkremer@uclouvain.be

Dear MG Team,

I am currently running various consistency tests between the default UFO Standard Model, the default v4 SM, and the usrmod. Considering top pair production, I obtain several discrepancies t tbar in the three above (SM) situations when decaying the W bosons or not.

Sqrt[s] = 14 TeV, 100k events

Process 1: generate p p > t t~

1) UFO: 554.3 ± 0.25 pb
2) sm_v4: 569.7 ± 0.24 pb
3) usrmod (only sm): 534.8 ± 0.23 pb

Process 2 : generate p p > t t~, (t > w+ b, w+ > f f) , (t~ > w- b~, w- > f f)

1) UFO (as it is): 473.1 ± 0.37 pb
2) sm_v4 (as it is): 497.9 ± 0.51 pb
3) usrmod (only sm): 458.1 ± 0.45 pb

where f is any fermion but the top quark. In order to work in the NWA, the run_card.dat is taken such that bwcutoff=1000 and cut_decays=F. Are these results to be expected, or am I missing something ?

Thanks,

Mathieu

Question information

Language:
English Edit question
Status:
Solved
For:
MadGraph5_aMC@NLO Edit question
Assignee:
No assignee Edit question
Solved by:
Olivier Mattelaer
Solved:
Last query:
Last reply:
Revision history for this message
Best Olivier Mattelaer (olivier-mattelaer) said : 		#1

Hi Matthieu,

Concerning the difference of cross-section between the three model, they probably depend of which value of the top mass you are using. I would not worry about that (except if you pay attention to the input parameter that you are using)

So now regarding your decay.
1) We found a glitch in version 1.5.8-1.5.10 concerning case with multiple W decay. which reduces (ususally slightly) the cross-section in this case. (The lepton cut were partly applied on the neutrino). So please use 1.5.11.

2) I first ran
p p > t t~, t > w+ b , t~ > w- b~
and the result for the UFO is 550.1(3)
So this is compatible with the width/mass expected error on the cross-section.

3) For the process that you indicate, you need not only to put cut_decays on F but also
the other cut (like missing et, deltaR ,...)
If I do I get: 539.9(5)
Which is again compatible with the expected error due to the inclusion of the finite width.

Cheers,

Olivier

Revision history for this message
mathieu.buchkremer@uclouvain.be (mathieu-buchkremer) said : 		#2

Thanks Olivier Mattelaer, that solved my question.