MadGraph5_aMC@NLO

Survey return zero cross section.

Asked by Kamran Ahmad

Survey return zero cross section. whats the reason for this message, i tried generating events for pp>thz, madgrapg generated feynman diagrams but launch commang gives this error

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

can be multiple things (as indicated in the message on screen)
1) that the result is indeed zero due to a coupling set to zero or impossible cut
2) that the cut are too strong for the phase-space integrator (then reduce the cut)
3) that your PDF is not compatible with your process (in your case you need a PDF that has non zero probability to have a b quark)

Likely 3 in your case no?

Cheers,

Olivier

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Kamran Ahmad (kamranahmmed786) said : 		#2

import model sm
define p = g u c d s u~ c~ d~ s~
define j = g u c d s u~ c~ d~ s~
define l+ = e+ mu+
define l- = e- mu-
define vl = ve vm vt
define vl~ = ve~ vm~ vt~
define p=p t b t~ b~
generate p p > h t z
set zerowidth_tchannel False
set run_mode 2
output htz
launch htz/

I used these commands to generate the given process, it displays the Feynman Diagrams for the process but gives zero cross-section when launched, I have tried using different PDFs on your instruction but that didn't solve the problem(also reduced the cuts at the same time)

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Best Olivier Mattelaer (olivier-mattelaer) said : 		#3

Hi,

If I follow your syntax, the only set of diagrams generated are for top gluon initial state.
So you need a PDF which consider the top as massless (so a 6 flavor PDF).
I know that such PDF were tested in the past for 100TeV collider but I personally do not know any.
(I think that the idea was dropped but I can be wrong)

Which PDF did you use? Did you check that they contain a contribution for the top?
Note, that such PDF will assume that the top mass is zero, but the model that you use is assuming that both b and t quark are massive which is inconsistent.

Please be sure to contact member of your team (or check litterature) on how to generate your sample since I doubt that you approach such generation correctly (I would need to do the same for that particular process)

Cheers,

Olivier

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Kamran Ahmad (kamranahmmed786) said : 		#4

Thanks Olivier Mattelaer, that solved my question.