MadGraph5_aMC@NLO

onshell particles with large widths

Asked by Dave Lister

I'm generating a c tau nu final state via an leptoquark (something like pp > ta+ lq , lq > c vt) , by generating pp > ta+ c vt, I include also non resonant processes.
Going to large coupling parameters I encountered problems, when the width of the resonantly produced (intermediate) leptoquark got larger then 1/10 of its mass. Ther main issue is that wheres the process still runs through, the particle is no longer classified as on-shell and therefore not stored.
If I understand it correctly this seems to be caused by myamp.f, where seems to be also an option to bypass this (gForceBW), but I found nothing on how to activate this.
Can please somebody a hint on how get this particles stored.

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Olivier Mattelaer (olivier-mattelaer) said : 		#1

The choice of writting or not a particle within the lhef file is related to the way parton-shower program should handle an onshell particles. If an additional particle is written within the file, this impact in two way the parton-shower. First all reshuffling of momenta will be done in a way that preserve such mass and second, the parton-shower will respect the leading-color information for that particle.

For a particle with a width larger than 10% than the mass none of the above really make sense. So it does not make sense to write such information inside the file. (Without even stating the obvious that such information is not physical due to interference term).

Cheers,

Olivier

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