Photon from proton in equivalent photon approximation

Hi,

In the run_ card you have lpp1 and lpp2.
To have the fully elastic one, you have to set both on “2”.
The semi-elastic one being one on “1” and the second on “2” (plus the symmetric)

Now for PDF question, this is actually a mess since this depends of the PDF set.
Some PDF do not include the photon contribution at all, some of them only include the inelastic component, and some of them include both elastic and inelastic component.
So depending of the PDF, (1,1) can include all contribution (inelastic, semi-elastic and fully elastic) and in that case, one can not split the various contribution.
(i.e. if you want to separate all the various component, you need to use a PDF which only include the inelastic part)

Cheers,

Olivier

> On 10 May 2017, at 02:48, daniele barducci <email address hidden> wrote:
>
> Question #631933 on MadGraph5_aMC@NLO changed:
> https://answers.launchpad.net/mg5amcnlo/+question/631933
>
> Description changed to:
> Dear Madgraph Team,
>
> I am interested in studying photon collision at the LHC and I saw that
> MadGraph implements the equivalent photon approximation (EPA) framework
> which can be chosen by selecting "2=photon from proton" in the run_card.
> This is based on the formulation of https://arxiv.org/pdf/hep-
> ph/9310350.pdf
>
> Is not however clear to me the following issue. Within the EPA one can
> have both an elastic contribution (i.e. when the photon do not
> dissociate and in principle can be tagged using very forward detectors)
> and (semi)-inelastic processes, i.e. when one or both protons
> dissociate, as explained for example in https://arxiv.org/pdf/hep-
> ex/0009065.pdf in page 2, where the form factors are given for both
> processes. My question is then the following. By choosing "2=photon from
> proton", are both processes taken into account, or just the elastic one
> is?
>
> Moreover, in case one chooses thee option "1=photon from proton", the
> photon is extracted as a proton constituent (massless and on-shell with
> Q2=0) with a photon PDF. Is there any kind of overlap with the choice
> "2=photon from proton"(both elastic and inelastic) for some choices of
> the PDFs sets, or they are completely independent?
>
> Thanks a lot
>
> Daniele
>
> --
> You received this question notification because you are an answer
> contact for MadGraph5_aMC@NLO.

Hi,

In the run_ card you have lpp1 and lpp2.
To have the fully elastic one, you have to set both on “2”.
The semi-elastic one being one on “1” and the second on “2” (plus the symmetric)

Now for PDF question, this is actually a mess since this depends of the PDF set.
Some PDF do not include the photon contribution at all, some of them only include the inelastic component, and some of them include both elastic and inelastic component.
So depending of the PDF, (1,1) can include all contribution (inelastic, semi-elastic and fully elastic) and in that case, one can not split the various contribution.
(i.e. if you want to separate all the various component, you need to use a PDF which only include the inelastic part)

Cheers,

Olivier

> On 10 May 2017, at 02:48, daniele barducci <email address hidden> wrote:
>
> Question #631933 on MadGraph5_aMC@NLO changed:
> https://answers.launchpad.net/mg5amcnlo/+question/631933
>
> Description changed to:
> Dear Madgraph Team,
>
> I am interested in studying photon collision at the LHC and I saw that
> MadGraph implements the equivalent photon approximation (EPA) framework
> which can be chosen by selecting "2=photon from proton" in the run_card.
> This is based on the formulation of https://arxiv.org/pdf/hep-
> ph/9310350.pdf
>
> Is not however clear to me the following issue. Within the EPA one can
> have both an elastic contribution (i.e. when the photon do not
> dissociate and in principle can be tagged using very forward detectors)
> and (semi)-inelastic processes, i.e. when one or both protons
> dissociate, as explained for example in https://arxiv.org/pdf/hep-
> ex/0009065.pdf in page 2, where the form factors are given for both
> processes. My question is then the following. By choosing "2=photon from
> proton", are both processes taken into account, or just the elastic one
> is?
>
> Moreover, in case one chooses thee option "1=photon from proton", the
> photon is extracted as a proton constituent (massless and on-shell with
> Q2=0) with a photon PDF. Is there any kind of overlap with the choice
> "2=photon from proton"(both elastic and inelastic) for some choices of
> the PDFs sets, or they are completely independent?
>
> Thanks a lot
>
> Daniele
>
> --
> You received this question notification because you are an answer
> contact for MadGraph5_aMC@NLO.

Dear Olivier,

Thanks for your reply which clarifies a part of the problem, while the rest is probably due to the not correct understanding from my side of the physics problem. However I try to write it here, so maybe it could be useful for someone else.

I actually understood that also the inelastic part could be modeled with the EPA approximation, see again https://arxiv.org/pdf/hep-ex/0009065.pdf eq.1 where FM and FE are given both for the elastic case and for the inelastic one (in the second case they are integrals on the F2 proton form factors), and that this inelastic contribution from quasi real photon (Q2!=0) is physically different from the contribution that one obtains when uses the parton distribution function of the photon itself (Q2=0). This is why I was asking if the switch 2 in the MG run_card was taking into account this inelastic contribution (that I thought different from the case when one extracts the photon with a PDF).

However from your reply it seems that I misunderstood the problem and that the inelastic contribution that can be written with the EPA approximation can also be written in terms of PDFs, and that the EPA approximation in MadGraph contains only the elastic part, while the inelastic is fully taken into account by setting the switch = 1

I hope this understanding is correct

Thanks also for the clarification regarding how the different PDFs sets treat the inelastic contribution

Daniele