MadGraph5_aMC@NLO

Narrow width versus broad width computations in madgraph

Asked by Disha Bhatia

Hello,

I have a particle, Z' of mass 5 TeV and width = 2 TeV for a particular choice of couplings.
In this limit the narrow width approximation fails.
But I wanted to know how madgraph evaluate the answers for broad width ?

Using MG5_aMC_v2_3_3,
I generated p p > Z', (Z' > lep lep), I got sigma = 0.0178 pb , where lep is for leptons
and with p p > Z' > lep lep, I got sigma = 0.0133 pb.

I used to think that the command p p > Z' > lep lep, will work for broad width,
but I see sigma obtained from p p > Z', (Z' > lep lep) and p p > Z' >lep lep are roughly the same.

However for such large widths there should have been large deviations for narrow width and broad width answers.

I wanted to know whether the command I am using for broad width p p > Z' > lep lep, is correct and if not how should I compute my answers for broad width using madgraph ?

Thanks,
Disha

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

Hi,

The syntax: p p > Z’, (Z’ > let let), include off-shell effects.
Please read for full explanation of the syntax and what it means in term of cut:
https://cp3.irmp.ucl.ac.be/projects/madgraph/attachment/wiki/Shanghai2015/Tutorial_shangai_basic_mg5.pdf

> I wanted to know whether the command I am using for broad width p p > Z’ > lep lep, is correct and if not how should I compute my answers for broad width using madgraph ?

Such syntax certainly miss interference term with other Feynman Diagram such interference. Those are likely to be important for the search of such large width particle.
So I guess that you have to include such interference in some way or another (or prove me wrong and show that such interference are negligible).
In conclusion, I would not believe your generation if you use either of those two syntax without at least comment on the interference.

Cheers,

Olivier

> On Jan 2, 2017, at 07:43, Disha Bhatia <email address hidden> wrote:
>
> New question #425012 on MadGraph5_aMC@NLO:
> https://answers.launchpad.net/mg5amcnlo/+question/425012
>
> Hello,
>
> I have a particle, Z' of mass 5 TeV and width = 2 TeV for a particular choice of couplings.
> In this limit the narrow width approximation fails.
> But I wanted to know how madgraph evaluate the answers for broad width ?
>
> Using MG5_aMC_v2_3_3,
> I generated p p > Z', (Z' > lep lep), I got sigma = 0.0178 pb , where lep is for leptons
> and with p p > Z' > lep lep, I got sigma = 0.0133 pb.
>
> I used to think that the command p p > Z' > lep lep, will work for broad width,
> but I see sigma obtained from p p > Z', (Z' > lep lep) and p p > Z' >lep lep are roughly the same.
>
> However for such large widths there should have been large deviations for narrow width and broad width answers.
>
> I wanted to know whether the command I am using for broad width p p > Z' > lep lep, is correct and if not how should I compute my answers for broad width using madgraph ?
>
> Thanks,
> Disha
>
>
> --
> You received this question notification because you are an answer
> contact for MadGraph5_aMC@NLO.

Revision history for this message
Disha Bhatia (dishabhatia1989) said : 		#2

Hi,

Thanks for your reply,
Yes I was missing interference effects and they are not negligible.

Revision history for this message
Disha Bhatia (dishabhatia1989) said : 		#3

Thanks Olivier Mattelaer, that solved my question.

Revision history for this message
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