MadGraph5_aMC@NLO

"No phase space" in cascade with many off-shell particles

Asked by Nick

Hello,

I'm working with a model where a1 and a2 are two additional scalars, and I'm generating the following cascade, where the additional scalars are offshell: (ma1 = 130 and ma2 = 170)

generate p p > a1 a2, (a1 > w+ w-, w+ > e+ ve, w- > u~ d), ( a2 > w+ a1, (w+ > e+ ve), (a1 > w+ w-, w+ > e+ ve, w- > u~ d))

The diagrams are generated, but the process stops with the message:
"Zero result detected: No Phase Space. Please check particle masses."

Why is this the case, if the final state are just light particles?

Thank you!

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Valentin Hirschi
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Best Valentin Hirschi (valentin-hirschi) said : 		#1

The decay chain syntax is intended for a cascade of onshell decays, so that phase-space is generated in such a way that MG5aMC assumes that particles can be produced onshell before decaying (which is not possible for a1 in your case given that it has ma1=130 and decays to two W-boson with a combined mass of 160 or so).

What you really want here is to select your signal contribution with either coupling order specification or you can probably simply do this with required s-channels in your case (always check the produced diagrams with 'display diagrams' to make sure you have the intended selection); that is

generate p p > a1 a2 > w+ w- w+ w-, (w+ > e+ ve, w- > u~ d)

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Nick (nick1) said : 		#2

Thanks a lot! it works perfectly now.

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Nick (nick1) said : 		#3

Thanks Valentin Hirschi, that solved my question.