Four orders of magnitud difference in cross section for process with decay and without the decay

Hi,

I guess that the total width of the stop is not correct in your first computation.

Cheers,

Olivier

On 07 Oct 2015, at 16:22, alefisico <email address hidden> wrote:

> New question #272166 on MadGraph5_aMC@NLO:
> https://answers.launchpad.net/mg5amcnlo/+question/272166
>
> Hi
>
> I want to understand the reason of a difference in the integrated weight (pb) reported in madgraph for different "versions" of the same process.
> The main process that I want to generate is a p p > stop stop + 0,1,2 j, stop > q q. An RPV process using the RPVMSSM model.
>
> Now, if I generate
> 1) p p > stop stop, stop > q q the integrated weight that I got is 2.14E+07, but if I generate
> 2) p p > stop stop + 0,1,2 j, where j = u d s c b g, this weight is 5.91E+03, four orders of magnitud less.
>
> Because of computational problems I NEED to generate 2) in madgraph and then I will manage the stop decays in pythia8. But also, the other issue is that because of the high cross section at low RPV stop masses, I will need to generate a huge number of events and I want to lower that number by applying an HT cut at madgraph level. But now I am not sure how to calculate the efficiency of this selection.
> Any ideas?
>
> thanks
>
> --
> You received this question notification because you are an answer
> contact for MadGraph5_aMC@NLO.

Hi Olivier

thanks for the quick reply. I am using the same param and run cards for both process. The width that I am using is 0.001
But also, just to test I varied that number from 0.001 to 1 and the number didn't change considerably.

cheers,

Hi,

> I am using the same param and run cards for both process.

Yes but for the second process you are not dependent of the width, while you are for the first.

> But also, just to test I varied that number from 0.001 to 1 and the number didn’t change considerably.

If you are in narrow width approximation (which should be the case for those two method to be valid)
then your cross-section should be proportional to 1/width**2.
So it should vary a lot from one to the other.
If it is not the case, then you are not in NWA and you can not use either method one or two.

Cheers,

Olivier

On 07 Oct 2015, at 16:43, alefisico <email address hidden> wrote:

> Question #272166 on MadGraph5_aMC@NLO changed:
> https://answers.launchpad.net/mg5amcnlo/+question/272166
>
> Status: Answered => Open
>
> alefisico is still having a problem:
> Hi Olivier
>
> thanks for the quick reply. I am using the same param and run cards for both process. The width that I am using is 0.001
> But also, just to test I varied that number from 0.001 to 1 and the number didn't change considerably.
>
> cheers,
>
> --
> You received this question notification because you are an answer
> contact for MadGraph5_aMC@NLO.

Hi

yes you are right, the change is minimal for 2) but for 1) for instance (after I applied HT=500):
a) width 0.001 the integrated weight = 0.3471E+06
b) width 0.1 the integrated weight = 0.3450E+02

so now, how can I correctly determine the efficiency of my selection?

thanks again for your help.

Thanks Olivier Mattelaer, that solved my question.