MadGraph5_aMC@NLO

why I always get 0 when I try to calculate the cross section for muon decay by MadGraph5

Asked by qingyun wang

Hello,

I downloaded and installed Madgraph5 today. And I also finished the tutorial about p p > t t~. But when I try to calculate the cross section for muon decay mu- > vm e- ve~, I always got 0 width.
Anyone could tell me why is that??

Many thanks,

Qingyun

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

The muon mass is by default set to zero (this is the most optimal value for collider study). Consequently, the width is indeed zero.

You can compute this process by using a model where the muon mass is not zero.

for example:
import model sm-full
generate mu+ > e+ ve vm~
output
launch

> On Oct 10, 2016, at 02:43, qingyun wang <email address hidden> wrote:
>
> New question #402889 on MadGraph5_aMC@NLO:
> https://answers.launchpad.net/mg5amcnlo/+question/402889
>
> Hello,
>
> I downloaded and installed Madgraph5 today. And I also finished the tutorial about p p > t t~. But when I try to calculate the cross section for muon decay mu- > vm e- ve~, I always got 0 width.
> Anyone could tell me why is that??
>
> Many thanks,
>
> Qingyun
>
> --
> You received this question notification because you are an answer
> contact for MadGraph5_aMC@NLO.

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qingyun wang (qwang63) said : 		#2

Thanks Olivier Mattelaer, that solved my question.

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qingyun wang (qwang63) said : 		#3

Hello Olivier,
Thanks so much for your answer. it works very well.
But I still confused about one thing. I also try to modify the mass of muon by hand using: set mass 13 1.0e-1 , and I check the value in parameter card has been changed, but the result is till 0.
Do you know why is that?

Thanks,

Qingyun

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qingyun wang (qwang63) said : 		#4

Thanks Olivier Mattelaer, that solved my question.

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Olivier Mattelaer (olivier-mattelaer) said : 		#5

Hi,

when you use the command "set mass 13 1e-1"
you should have a warning raised:

WARNING: Note that this parameter seems to be ignore by MG.
MG will use instead the expression: : 0.0
You need to match this expression for external program (such pythia).

So as stated, the mass used by MadGraph is zero (and therefore the width is zero).
The reason for that behavior is that the mass is forced to be zero at the model level.

Cheers,

Olivier

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qingyun wang (qwang63) said : 		#6

Olivier,

Thanks for your help. I really appreciate it.

Qingyun