Null width particles

Hi,

A massive particle with a zero width present in S-channel will create a divergence in the matrix-element, and therefore the cross-section will be infinite.
This is clear if you realise that in the narrow width approximation, your cross-section is proportional to the inverse of the width.

So this is something that can not be handle.
Additionally have a massive particle which decay imply that such particle has a non zero width. This is therefore inconsistent.

Cheers,

Olivier

> On 6 Apr 2017, at 10:47, Maria Giulia Ratti <email address hidden> wrote:
>
> New question #612910 on MadGraph5_aMC@NLO:
> https://answers.launchpad.net/mg5amcnlo/+question/612910
>
> How are unstable particles with null width treated in MadGraph? Can they decay?
>
> --
> You received this question notification because you are an answer
> contact for MadGraph5_aMC@NLO.

Thanks for your reply. My concern was that I was able to generate events with a squark decaying to neutralino and light quark, even when the mass of the neutralino is higher than the mass of the squark.
But I figured that the most important diagram in this case is with the mediator in t-channel. So I guess these are not pahotlogical events..?

Thanks again,
Maria Giulia

Hi,

> Thanks for your reply. My concern was that I was able to generate events with a squark decaying to neutralino and light quark, even when the mass of the neutralino is higher than the mass of the squark.

In that case, you are never going to hit the pole and should not be sensitive to the width (depending of the mass gap obviously).

> But I figured that the most important diagram in this case is with the mediator in t-channel.

Here again you are not sensitive to the width.

Cheers,

Olivier

> On 6 Apr 2017, at 22:09, Maria Giulia Ratti <email address hidden> wrote:
>
> Question #612910 on MadGraph5_aMC@NLO changed:
> https://answers.launchpad.net/mg5amcnlo/+question/612910
>
> Maria Giulia Ratti posted a new comment:
> Thanks for your reply. My concern was that I was able to generate events with a squark decaying to neutralino and light quark, even when the mass of the neutralino is higher than the mass of the squark.
> But I figured that the most important diagram in this case is with the mediator in t-channel. So I guess these are not pahotlogical events..?
>
> Thanks again,
> Maria Giulia
>
> --
> You received this question notification because you are an answer
> contact for MadGraph5_aMC@NLO.