MadGraph5_aMC@NLO

Leaving an off-shell vector boson in the final state

Asked by Stefan von Buddenbrock

Dear MadGraph experts

Is it possible in MadGraph to leave an off-shell vector boson in the final state of an ME as a result of some resonant decay, so that another program (like Pythia) could "continue" the decay process?

A simple example would be:

import model heft
generate g g > h, h > z z
launch

For something like this, the event generation will give a "zero phase space" error, which is true obviously. But is there some way to generate the four momenta of the Z bosons such that they would be like h -> ZZ*, and then Pythia could pick up the LHE file and decay them to leptons accordingly? This wouldn't technically be a matrix element, since the initial and final state need to be on-shell. I am just wondering if it is possible.

I also know that g g > h > z z works in this case. But this is a simplified example; the real problem which I am facing is a bit more complicated.

Cheers
Stefan

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Olivier Mattelaer (olivier-mattelaer) said : 		#1

Dear Stefan,

We do not have such mode available. I like the idea but never investigate time on how accurate such method would be.
The problem is obviously that you need that the second program needs to be able to handle such not standard output.

Note that I would never use such idea for the h -> ZZ*, in that case it is important to have the full 1 -> 4 matrix element. Indeed you have some spin-correlation effects that are include in that
decay which would be lost if you do not use the full matrix element. This should in general be True in presence of off-shell particles.

If you explain me your real situation, I might be able to give you some advise (and/or access to the beta version of MadSpin briefly mention in this paper:arXiv:1610.08985)

Cheers,

Olivier

> On Nov 2, 2016, at 14:43, Stefan von Buddenbrock <email address hidden> wrote:
>
> New question #403735 on MadGraph5_aMC@NLO:
> https://answers.launchpad.net/mg5amcnlo/+question/403735
>
> Dear MadGraph experts
>
> Is it possible in MadGraph to leave an off-shell vector boson in the final state of an ME as a result of some resonant decay, so that another program (like Pythia) could "continue" the decay process?
>
> A simple example would be:
>
> import model heft
> generate g g > h, h > z z
> launch
>
> For something like this, the event generation will give a "zero phase space" error, which is true obviously. But is there some way to generate the four momenta of the Z bosons such that they would be like h -> ZZ*, and then Pythia could pick up the LHE file and decay them to leptons accordingly? This wouldn't technically be a matrix element, since the initial and final state need to be on-shell. I am just wondering if it is possible.
>
> I also know that g g > h > z z works in this case. But this is a simplified example; the real problem which I am facing is a bit more complicated.
>
> Cheers
> Stefan
>
> --
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> contact for MadGraph5_aMC@NLO.

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