Different cross sections for different mg5 v 2.5.5 and 2.6.7

Asked by Ana Maria Rodriguez Vera on 2020-11-19

I'm trying to calculate the cross section for the same Drell-Yan process using a model I had been working with in v 2.5.5. I decided to move to v 2.6.7 and I no longer get the same cross sections, the difference being 23 orders of magnitude! I also noticed I get a warning in version 2.6.7: WARNING: Particle 15 with small width detected (2.27e-12): See https://answers.launchpad.net/mg5amcnlo/+faq/3053 to learn the special handling of that case FAQ #3053 : Questions : MadGraph5_aMC@NLO
I tried changing the cut on the width as suggested by the recommendations in the FAQ in both versions and the cross sections are still displaying the same 23 order of magnitude difference.
Everything is set up in the same manner, and the lhe output file does not indicate any other different parameter.
Has anyone else experienced something like this?

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Last query:
2020-11-19
Last reply:
2020-11-19

This question was reopened

Hi,

width of "2.27e-12" are likely to be badly handle within 2.5.5 if you have a propagator associated to such particle due to numerical issue of the computation. Now I do not know your model/process and therefore I have no clue if this is your issue or not.

What you can do is to change your width and make a scan like
scan:[2.27e-12*10**i for i in range(0,10)]
(in both version)
and check that the associated cross-section scale as it should.

Cheers,

Olivier

> On 19 Nov 2020, at 06:45, Ana Maria Rodriguez Vera <email address hidden> wrote:
>
> New question #694078 on MadGraph5_aMC@NLO:
> https://answers.launchpad.net/mg5amcnlo/+question/694078
>
> I'm trying to calculate the cross section for the same Drell-Yan process using a model I had been working with in v 2.5.5. I decided to move to v 2.6.7 and I no longer get the same cross sections, the difference being 23 orders of magnitude! I also noticed I get a warning in version 2.6.7: WARNING: Particle 15 with small width detected (2.27e-12): See https://answers.launchpad.net/mg5amcnlo/+faq/3053 to learn the special handling of that case FAQ #3053 : Questions : MadGraph5_aMC@NLO
> I tried changing the cut on the width as suggested by the recommendations in the FAQ in both versions and the cross sections are still displaying the same 23 order of magnitude difference.
> Everything is set up in the same manner, and the lhe output file does not indicate any other different parameter.
> Has anyone else experienced something like this?
>
>
>
> --
> You received this question notification because you are an answer
> contact for MadGraph5_aMC@NLO.

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