MadGraph5_aMC@NLO

photon-induced diagrams when running at NLO QCD with loop_qcd_qed_sm_Gmu models

Asked by simone amoroso

Dear madgraph authors,

When running Drell-Yan production at NLO QCD using the loop_qcd_qed_sm_Gmu model using the commands below

import model loop_qcd_qed_sm_Gmu
generate p p > e+ e- [QCD]

I see that photon-induced diagrams get included, despite them typically considered as an NLO EW contribution.
In fact if I run using other loop models (loop_sm) the same diagrams are NOT included.

Is there a reason to include the PI contribution at NLO QCD or is it a bug?
And would it be possible to run at NLO QCD with the loop_qcd_qed_sm_Gmu but excluding those diagrams?

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

Hi,

The point of the model "loop_qcd_qed_sm_Gmu" is to be a model for EW loop/NLO computation.
So indeed we do setup our code automatically for EW computation when such model is loaded.
You can obviously do QCD only computation with such model but all our default will not be setup for that mode and therefore you need to change some setup to overwrite them.

So to remove photon-induced processes, you need to do
import model loop_qcd_qed_sm_Gmu
define p = p /a
define j = p
generate p p > e+ e- [QCD]

Cheers,

Olivier

PS: also check the run_card carefully to check that all setting are also making sense (like the choice of the PDF of the cuts, ...)

Revision history for this message
simone amoroso (amsimone) said : 		#2

Thanks Olivier,

This explains. May be useful to flag this with a message, as I am not sure this is the expect behavior.

Revision history for this message
Olivier Mattelaer (olivier-mattelaer) said : 		#3

Hi,

I have checked that when you import the model you do have this message (in blue color/bold to attract attention)

> Pass the definition of 'j' and 'p' to 5 flavour scheme, including the photon.

So not sure which message you want to add in top of that.

Cheers,

Olivier

> On 13 Mar 2024, at 17:15, simone amoroso <email address hidden> wrote:
>
> Question #709567 on MadGraph5_aMC@NLO changed:
> https://answers.launchpad.net/mg5amcnlo/+question/709567
>
> Status: Answered => Solved
>
> simone amoroso confirmed that the question is solved:
> Thanks Olivier,
>
> This explains. May be useful to flag this with a message, as I am not
> sure this is the expect behavior.
>
> --
> You received this question notification because you are an answer
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