Vanishing Cross Section (massless b-quark?)

Hi Eric,

> I am simulating a process for a model I created. I am finding that the cross-section is 10^{-50} pb. Is that equivalent to 0?

No 10^-50 should be interpreted as different of zero (if the integration error is smaller than that obviously).
The limit that we have is around 10^{-100}.

> I cannot find an error with the model (doesn't mean there isnt one). One possible cause of the small cross-section could be in MadGraph. The interaction vertex contains the derivative of the bottom-quark. Thus, the matrix element is proportional to the bottom-quark mass. When calculating matrix elements, is the bottom quark taken to be massless?

This depend of the model, If the b-mass is different of zero in the model and the param_card, then all mass effect are kept.
I would suggest to increase the mass of the b to see how this affect your cross-section.

Cheers,

Olivier

On May 4, 2014, at 3:11 PM, Eric Kuflik <email address hidden> wrote:

> New question #248138 on MadGraph5_aMC@NLO:
> https://answers.launchpad.net/mg5amcnlo/+question/248138
>
> I am simulating a process for a model I created. I am finding that the cross-section is 10^{-50} pb. Is that equivalent to 0?
>
> I cannot find an error with the model (doesn't mean there isnt one). One possible cause of the small cross-section could be in MadGraph. The interaction vertex contains the derivative of the bottom-quark. Thus, the matrix element is proportional to the bottom-quark mass. When calculating matrix elements, is the bottom quark taken to be massless?
>
> Eric
>
> --
> You received this question notification because you are an answer
> contact for MadGraph5_aMC@NLO.