Difference for photon-induced dilepton production b/w MG5 2.6/2.7.X vs 2.9.3

Asked by Kristin Lohwasser

Dear all,

we have noticed an about 10% change of cross section in a a > l+ l- going from MG5 2.7.X to 2.9.3 and a much more steeply falling off pT(ll) spectrum and would like to understand, where this is coming from.

We set fixed factorization scale (as needed for the newer MG5 version):
       'dsqrt_q2fact1' : 2.,
       'dsqrt_q2fact2' : 2.,
       'fixed_fac_scale' : 'T',

varying the scale from 2. -> 20. or even 200. does not change the results, so we are wondering, if you have any idea of what else has changed that could be causing this.

Thanks & Best
Kristin

Some T-channel width have been set to zero [new since 2.8.0]

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Olivier Mattelaer (olivier-mattelaer) said :
#1

Yes we have found the same issue at the begin of this month.
This should be fixed in the next Long Term Stable release (2.9.5) which should be release soon (when I'm back from holliday)
you can already test that version via the command
bzr branch lp:~maddevelopers/mg5amcnlo/LTS_dev

This version will also allow more control on the choice of the scale with the possibility to go back to dynamical scale

Cheers,

Olivier

> On 19 Aug 2021, at 21:05, Kristin Lohwasser <email address hidden> wrote:
>
> New question #698432 on MadGraph5_aMC@NLO:
> https://answers.launchpad.net/mg5amcnlo/+question/698432
>
> Dear all,
>
> we have noticed an about 10% change of cross section in a a > l+ l- going from MG5 2.7.X to 2.9.3 and a much more steeply falling off pT(ll) spectrum and would like to understand, where this is coming from.
>
> We set fixed factorization scale (as needed for the newer MG5 version):
> 'dsqrt_q2fact1' : 2.,
> 'dsqrt_q2fact2' : 2.,
> 'fixed_fac_scale' : 'T',
>
> varying the scale from 2. -> 20. or even 200. does not change the results, so we are wondering, if you have any idea of what else has changed that could be causing this.
>
> Thanks & Best
> Kristin
>
>
>
>
>
>
> Some T-channel width have been set to zero [new since 2.8.0]
>
> --
> You received this question notification because you are an answer
> contact for MadGraph5_aMC@NLO.

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