MadGraph5_aMC@NLO

Writing a neutron-proton collision in Madgraph

Asked by Francis Burk

To whom it may concern,

I am trying to write a collision between a neutron and proton. My input process is p p > e+ e-. I have generate the process with "set group_subprocesses False". Once I launched the process, I ran:

set ebeam1 6500

set ebeam2 6500

set nb_neutron1 1

set nb_proton1 0

set nb_neutron2 0

set nb_proton2 1

After running this process for nn, np, and pp collisions I have observed that the estimated cross sections are quite close to each other (830, 829, 828 pb, respectively). I worry that I may not have specified the pdf sets for the neutron and proton correctly. Only one pdf set is listed in the run card (nn23lo1). How do I ensure that I specified the correct pdfs for both the neutron and the proton beams? Thank you very much for the help!

Best regards,
Francis

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Solved by:
Olivier Mattelaer
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Best Olivier Mattelaer (olivier-mattelaer) said : 		#1

Hi,

The definition nb_neutron/nb_proton are used to rescale a proton PDF to something else.
They are therefore no need to specify a dedicated PDF for the neutron beam since in this mode you are expected to provide a proton PDF.

Now given your process which has a natural scale at MZ, you are likely not dominated by valence quark but by sea quark which should be the same for proton and neutron case. So I do not think that your result are problematic.

Cheers,

Olivier

> On 17 Apr 2024, at 18:20, Francis Burk <email address hidden> wrote:
>
> New question #809228 on MadGraph5_aMC@NLO:
> https://answers.launchpad.net/mg5amcnlo/+question/809228
>
> To whom it may concern,
>
> I am trying to write a collision between a neutron and proton. My input process is p p > e+ e-. I have generate the process with "set group_subprocesses False". Once I launched the process, I ran:
>
> set ebeam1 6500
>
> set ebeam2 6500
>
> set nb_neutron1 1
>
> set nb_proton1 0
>
> set nb_neutron2 0
>
> set nb_proton2 1
>
> After running this process for nn, np, and pp collisions I have observed that the estimated cross sections are quite close to each other (830, 829, 828 pb, respectively). I worry that I may not have specified the pdf sets for the neutron and proton correctly. Only one pdf set is listed in the run card (nn23lo1). How do I ensure that I specified the correct pdfs for both the neutron and the proton beams? Thank you very much for the help!
>
> Best regards,
> Francis
>
>
> --
> You received this question notification because you are an answer
> contact for MadGraph5_aMC@NLO.

Revision history for this message
Francis Burk (burkfm) said : 		#2

Thanks Olivier Mattelaer, that solved my question.