Process p p > Z a, (Z > 2taus, 2taus > 2muons + 4v's) returns zero cross-section.

Which model are you using?

In the default model, the width of the tau is assumed to be zero which prevent such computation to be working.
In general
1) be very carefull with the tau total width since MG5aMC is using it but we are not able to compute it
2) be sure to use a recent version of the code since the width is so small that old version can face numerical unstabilites issue for the tau width (in recent version of the code, we use a fake large width to avoid that numerical issue and then use Narrow-width approximation to correct the cross-section accordingly.

Cheers,

Olivier

> On 15 Jun 2021, at 02:01, Maaz Ul Haq <email address hidden> wrote:
>
> New question #697533 on MadGraph5_aMC@NLO:
> https://answers.launchpad.net/mg5amcnlo/+question/697533
>
> Hello aMC@NLO team.
>
> I'm trying to generate events for the SM process p p > Z a, (Z > ta+ ta-, ta+ > mu+ vm vt~, ta- > mu- vm~ vt). When the run tries to create jobs, it runs into a zero cross-section error:
>
> Creating Jobs
> INFO: Refine results to 10000
> INFO: Generating 10000.0 unweighted events.
> INFO: Effective Luminosity 1.2e+103 pb^-1
> INFO: need to improve 0 channels
> Survey return zero cross section.
> Typical reasons are the following:
> 1) A massive s-channel particle has a width set to zero.
> 2) The pdf are zero for at least one of the initial state particles
> or you are using maxjetflavor=4 for initial state b:s.
> 3) The cuts are too strong.
> Please check/correct your param_card and/or your run_card.
>
> The single tau decay process p p > Z a, (Z > ta+ ta-, ta+ > mu+ vm vt~) works ok but it also returns a very tiny cross-section: 1.708e-07 +- 4.479e-10 pb.
>
> Is the way I'm decaying the tau here good with Madgraph?
>
> Thanks,
> Maaz
>
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