contribution of Feynman diagram of process

Asked by lata on 2017-11-29

Hello,

When I generate a process in MG there are two feynman diagrams related to it since final states particles are same for both diagrams, one is t-channel VBF production mode other one is s-channel with same particles. So I just want to confirm if in LHE file there will be contribution of both channels. If yes then, can you please tell me if there is any way to produce lhe only for t-channel?

please let me know if further information is needed for this question.

Thanks in advance,

Lata Panwar

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Last query:
2017-11-29
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2017-11-29

Dear Lata,

From the LHE file, you have the information on which particle need to be considered as on-shell by the parton-shower.
Therefore some of the events will have an additional line in the lhe event.

> If yes then, can you please tell me if there is any way to produce lhe only for t-channel?

The first question to ask is: Does it make sense to do such computation?
Splitting the t and s channel diagram can be very dangerous since it often break gauge and lorentz invariance of the computation.
Making the computation piece by piece quite tricky to handle since you have to ensure that gauge and lorentz invariance are restored when you summing up the piece.
(which is actually extremely difficult to ensure and impose kind of unphysical constraint/setup for the computation)

This word of caution said, you can use one of the following syntax:
generate p p > e+ e- $$ Z # full forbid the Z particle to be on s-channel
generate p p > e+ e- $ Z # forbid the Z to be consider as on-shell (the behavior is currently quite tricky if the Z width is more than 10% of the Z-mass since in that case the Z is never consider onshell)

To check gauge and lorentz invariance of your process, you can run the following command:

check p p > e+ e- $$ z
check p p > e+ e- $ Z

Cheers,

Olivier

> On Nov 29, 2017, at 07:46, lata <email address hidden> wrote:
>
> New question #661231 on MadGraph5_aMC@NLO:
> https://answers.launchpad.net/mg5amcnlo/+question/661231
>
> Hello,
>
> When I generate a process in MG there are two feynman diagrams related to it since final states particles are same for both diagrams, one is t-channel VBF production mode other one is s-channel with same particles. So I just want to confirm if in LHE file there will be contribution of both channels. If yes then, can you please tell me if there is any way to produce lhe only for t-channel?
>
> please let me know if further information is needed for this question.
>
> Thanks in advance,
>
> Lata Panwar
>
> --
> You received this question notification because you are an answer
> contact for MadGraph5_aMC@NLO.

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