MadGraph5_aMC@NLO

fail to reach target 100

Asked by rojalin padhan

Hello..

I am trying to generate following process using default model file(SM)

define pb = p b b~
generate pb pb > z, (z > ta+ ta-, (ta+ > l+ vl~ vl), (ta- > l- vl~ vl))

 I asked for 100 Events ,but it generated only one events and It shows " INFO: fail to reach target 100."

 I am using MG5_aMC_v2_6_3_2 , Also I tried the same in recent beta version but the same problem occurred. It seems to me that problem is in decay of tau but I am not getting .

Thank you very much in advance for any suggestion.

rojalin

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

What is the decay width of the tau in your model?

Cheers,

Olivier

> On 16 Mar 2019, at 19:08, rojalin padhan <email address hidden> wrote:
>
> New question #679257 on MadGraph5_aMC@NLO:
> https://answers.launchpad.net/mg5amcnlo/+question/679257
>
> Hello..
>
> I am trying to generate following process using default model file(SM)
>
> define pb = p b b~
> generate pb pb > z, (z > ta+ ta-, (ta+ > l+ vl~ vl), (ta- > l- vl~ vl))
>
> I asked for 100 Events ,but it generated only one events and It shows " INFO: fail to reach target 100."
>
> I am using MG5_aMC_v2_6_3_2 , Also I tried the same in recent beta version but the same problem occurred. It seems to me that problem is in decay of tau but I am not getting .
>
>
> Thank you very much in advance for any suggestion.
>
>
> rojalin
>
>
> --
> You received this question notification because you are an answer
> contact for MadGraph5_aMC@NLO.

Revision history for this message
rojalin padhan (rossy.mg) said : 		#2

It is set to zero.

Revision history for this message
Best Olivier Mattelaer (olivier-mattelaer) said : 		#3

Then the cross-section for that process is infinite. And we obviously have issue with infinite quantity.
If you want to do such computation, you need to use a model/benchmark where the width is different of zero.

Since the "normal" width of the tau is typically so small, this will actually lead to another issue (problem of numerical precision).
This issue should be fixed in later version of the code (by using a fake --larger-- width in the propagator and correct the cross-section accordingly). This feature is not present in 2.6.3.2. So I will advise to use 2.6.5.

Cheers,

Olivier

Revision history for this message
rojalin padhan (rossy.mg) said : 		#4

Thanks Olivier Mattelaer, that solved my question.