Unexpected result for total decay widths near a threshold

Asked by Santiago Tanco on 2021-06-09

Dear MadGraph team,

We would like to understand some results we are obtaining for the total width of a sterile neutrino within the context of a new physics model. This sterile neutrino couples to the W/Z bosons and SM leptons, therefore the dominant decay modes are 1>3 or 1>2, depending on the sterile neutrino mass allowing for the W/Z to be on-shell or not.

When we plot the total width of the sterile neutrino as a function of its mass, we expect to observe bumps at the W and Z masses, as 2-body decays become available. Instead we are obtaining dips at both values.

We were wondering how reliable are MadGraph/MadWidth results near this threshold, i.e. when 3-body decay processes are replaced by 2-body decays.

Thanks in advance,
Santiago

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Olivier Mattelaer (olivier-mattelaer) said :
#1

No idea, this is a regime where the NWA should not be valid and this computation assumes that hypothesis.
So this is not too surprising to me if you have non physical result in that regime.

Cheers,

Olivier

> On 9 Jun 2021, at 22:55, Santiago Tanco <email address hidden> wrote:
>
> New question #697469 on MadGraph5_aMC@NLO:
> https://answers.launchpad.net/mg5amcnlo/+question/697469
>
> Dear MadGraph team,
>
> We would like to understand some results we are obtaining for the total width of a sterile neutrino within the context of a new physics model. This sterile neutrino couples to the W/Z bosons and SM leptons, therefore the dominant decay modes are 1>3 or 1>2, depending on the sterile neutrino mass allowing for the W/Z to be on-shell or not.
>
> When we plot the total width of the sterile neutrino as a function of its mass, we expect to observe bumps at the W and Z masses, as 2-body decays become available. Instead we are obtaining dips at both values.
>
> We were wondering how reliable are MadGraph/MadWidth results near this threshold, i.e. when 3-body decay processes are replaced by 2-body decays.
>
> Thanks in advance,
> Santiago
>
> --
> You received this question notification because you are an answer
> contact for MadGraph5_aMC@NLO.

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