MadGraph5_aMC@NLO

Zero cross section for e+ e- > zp

Asked by marlon

Good day sir. I am newbee to Madgraph. I am current trying to generate e+ e- > zp, zp > e+ e- . I am using the BL_N_UFO. So what i notice is that if i set the mass of Z' = 1500 GeV and apply a center of mass energy of 3000 GeV , i get a zero cross section with e reasons.
Survey return zero cross section.
   Typical reasons are the following:
   1) A massive s-channel particle has a width set to zero.
   2) The pdf are zero for at least one of the initial state particles
      or you are using maxjetflavor=4 for initial state b:s.
   3) The cuts are too strong.
   Please check/correct your param_card and/or your run_card.
Furthermore, if i try center of mass energy equal the mass of z prime , i can get a cross section. I suspect that it is because the mass and width of electron and positron is set to zero in parameter.py in BL_N_UFO.

So my question is what should i do to generate the process stated above using larger center mass energy maybe up to 50 TeV

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marlon (chuchupachula) said : 		#1

i edit my question : what should i do to generate the process stated above using larger center mass energy maybe up 50 Tev with my z prime mass still 1500 GeV . Is it possible?

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Olivier Mattelaer (olivier-mattelaer) said : 		#2

Hi,

using the syntax e+ e- > zp, zp > e+ e-
request the particle zp to be onshell.
This allows to be slightly offshell but not too much (a phase-space cut is applied).
So setting the sqrts outside that range provides a zero result by definition.

If you do not want to onshell part of the diagram, then you can use the following syntax
e+ e- > zp > e+ e-
this syntax is just doing a diagram filtering (and can therefore often breaks gauge and lorentz invariance )

Cheers,

Olivier

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