MadGraph5_aMC@NLO

NLO lepton collisions Higgs

Asked by Francisco Casalinho

Hello,

I'm trying to generate e+ e- > z h and e+ e- > e+ e- h events at NLO-QED but the program fails to give an output. (The program set the Z and Higgs width to zero )
What I can do?

Thanks,

Francisco

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

Concerning the width of the Z/H, for your syntax, you indeed have to set those width to zero to preserve gauge invariance.
If you want to have non zero width, you have to use the complex mass scheme formalism but in that case you cannot have Z/H in the final state.

For the rest, I'm not the NLO-QED expert, but they are various limitation for such type of computation.
In particular NLO+PS is not available.

Cheers,

Olivier

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Francisco Casalinho (francisco27) said : 		#2

Why can I not have Z or H in the final state?
( Is there any article about it?)

When I simulate the process, the MG5 imports the loop_qcd_qed_sm model but sets the Z and H width to 0. This is just because I generate a lepton-lepton collision process or for the hadron process, does this happens as well?

Is there a program where I can simulate this type of process that Oliver, or anyone else, knows about or recommends? Someone told me about MadJax but I don't know if that will help my problem.

Cheers,

Francisco

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Best Olivier Mattelaer (olivier-mattelaer) said : 		#3

Hi,

The point of the width is a physics limitation. So no code will be able to do it.

To said it one more times, if you use fix-width scheme you are breaking gauge invariance and NLO compuation will not be possible. They are two solutions
1) set the width to zero
2) use the complex mass scheme which restore gauge invariance even in presence of width but is not compatible with Narrow Width Approximation and does not make sense for ampliturde with amplitude with non zero width in the assomptatic state.

If you need more information on those, I would advise to search on inspire for paper on the complex mass scheme.

For your question, this indeed has nothing to do with the type of accelerator and the same apply for hadron process, additionally this is not only for Electroweak. For example, if you try to do p p > t t~ [QCD], you will also see that we set the top width to zero for the exact same reason.

Cheers,

Olivier

PS: MadJax is as Leading Order (as far as I know) and more related to machine learning and does not provide (again as far as I kow) event and/or cross-section computation. But I'm not an author of MadJax, so I can not really comment.

> On 26 Mar 2023, at 16:55, Francisco Casalinho <email address hidden> wrote:
>
> Question #705948 on MadGraph5_aMC@NLO changed:
> https://answers.launchpad.net/mg5amcnlo/+question/705948
>
> Francisco Casalinho posted a new comment:
> Why can I not have Z or H in the final state?
> ( Is there any article about it?)
>
> When I simulate the process, the MG5 imports the loop_qcd_qed_sm model but sets the Z and H width to 0. This is just because I generate a lepton-lepton collision process or for the hadron process, does this happens as well?
>
> Is there a program where I can simulate this type of process that Oliver, or anyone else, knows about or recommends? Someone told me about MadJax but I don't know if that will help my problem.
>
> Cheers,
>
> Francisco
>
> --
> You received this question notification because you are an answer
> contact for MadGraph5_aMC@NLO.

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Francisco Casalinho (francisco27) said : 		#4

Thanks Olivier Mattelaer, that solved my question.