A few question about the photon PDF in MadGraph

Hi,

> 1) According to https://answers.launchpad.net/mg5amcnlo/+question/403755, I think the default PDF set of has the photon PDF, right? If so, does it have both elastic and inelastic photon PDF?

The default is NNPDF2.3 Yes it contains both elastic and inelastic.

> If it has both, are there any PDF set has only inelastic photon PDF?

I think that CTEQ pdf might be what you are looking for. But please read their paper to be sure.

> 2) To use the photon PDF, do I need to include an additional photon in the proton, i.e., "define p a g u d c s u~ d~ c~ s~".

yes

> 3) Even if I use the default definition of the proton, it will still generate diagrams that have photon propagators to the quarks, in addition to the diagrams that have Z propagators to the quarks. So is it possible that the former diagrams actually already included the photon PDF thing implicitly?

Those correspond to photon above the factorisation scale.

Cheers,

Olivier

> On 11 Mar 2019, at 05:37, Qiji Xin <email address hidden> wrote:
>
> New question #679103 on MadGraph5_aMC@NLO:
> https://answers.launchpad.net/mg5amcnlo/+question/679103
>
> Dear MadGraph experts,
>
> I have a few question about the photon PDF in MadGraph.
>
> 1) According to https://answers.launchpad.net/mg5amcnlo/+question/403755, I think the default PDF set of has the photon PDF, right? If so, does it have both elastic and inelastic photon PDF?
> If it has both, are there any PDF set has only inelastic photon PDF?
>
> 2) To use the photon PDF, do I need to include an additional photon in the proton, i.e., "define p a g u d c s u~ d~ c~ s~".
>
> 3) Even if I use the default definition of the proton, it will still generate diagrams that have photon propagators to the quarks, in addition to the diagrams that have Z propagators to the quarks. So is it possible that the former diagrams actually already included the photon PDF thing implicitly?
>
> Thank you so much!!
>
> --
> You received this question notification because you are an answer
> contact for MadGraph5_aMC@NLO.

Hi Olivier,

Thanks a lot for your answer! A follow-up question,
From your answer to 2) and 3), do you mean that there is no overlap (or double counting) counting between the photon PDF and the quark (PDF) that a photon propagator connect to? In other words, no overlap between photon and quark component of the proton?

If so I need to include both component to account for all the physics I need to consider.

Thanks!!

Hi,

This is a good question.
I would recomend to read the description of the pdf set that you are using to be sure to understand what they include and the associated hyppothesis.
My understanding is that is should be the same "ambiguity" between a quark line emitting a gluon and having a gluon inside the PDF. This is handle by using a scale (in this case the factorisation scale) which is responsible for a theoretical uncertainty. In order to reduce such uncertainties you need to go NLO precision (or higher) where you can reduce such issue.

Cheers,

Olivier

> On 11 Mar 2019, at 17:12, Qiji Xin <email address hidden> wrote:
>
> Question #679103 on MadGraph5_aMC@NLO changed:
> https://answers.launchpad.net/mg5amcnlo/+question/679103
>
> Status: Answered => Open
>
> Qiji Xin is still having a problem:
> Hi Olivier,
>
> Thanks a lot for your answer! A follow-up question,
>> From your answer to 2) and 3), do you mean that there is no overlap (or double counting) counting between the photon PDF and the quark (PDF) that a photon propagator connect to? In other words, no overlap between photon and quark component of the proton?
>
> If so I need to include both component to account for all the physics I
> need to consider.
>
> Thanks!!
>
> --
> You received this question notification because you are an answer
> contact for MadGraph5_aMC@NLO.

Dear Olivier,

Thanks so much for your help!

Best,
Qiji