MadGraph5_aMC@NLO

onshell particle is not in the lhe file if width is larger than 10% of the mass

Asked by Meng Lu

Dear MG experts,

Recently i noticed there was a "10% threshold" on the lhe event records, i tried very simple process, e.g., p p > zp > e+ mu-, if the mass of zp is 3TeV, and following widths are set manually,
1. width = 301 GeV, onshell zp will not show up in the lhe
2. width = 300 GeV, onshell zp will not show up in the lhe
3. width = 299 GeV, onshell zp will show up in the lhe

is this due to some setup hardcoded in MG?

A follow-up question is, if i want the particle to show up in the lhe even if the width (wA) is larger than 10%*mass, could i simply set the width to some value which is smaller than 10%*mass, say wA', and finally scale the cross section by a factor (wA'/wA), since i checked that the kinematic distribution is independent with the width.

Cheers, Meng

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

Hi,

is this due to some setup hardcoded in MG?

Yes to avoid wrong behavior of the parton-shower.
The problem here is that for a width of 10%, we do consider the zp onshell from 0GeV up to 7.5TeV.
This is certainly not valid and the parton-shower will handle particle written in the lhef as "onshell" with special constraint.
Those constraints makes no sense when the width is larger than 10% (actually likely already the case at 7%)

A follow-up question is, if i want the particle to show up in the lhe even if the width (wA) is larger than 10%*mass, could i simply set the width to some value which is smaller than 10%*mass, say wA', and finally scale the cross section by a factor (wA'/wA), since i checked that the kinematic distribution is independent with the width.

This factor assumes narrow width approximation which is certainly not accurate in this regime.

The safest is likely to post-process the lhef file and added the particle that you want to add (if you do not use a parton-shower program obviously)

Cheers,

Olivier

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Meng Lu (meng-lu) said : 		#2

Thanks Olivier Mattelaer, that solved my question.