Use of input beam energy distribution for photon-heavy nucleus collision

Asked by Arka Santra on 2020-09-14

Hello,
  I am trying to simulate a photon - lead nucleus collision using MadGraph UFO model. This is the command I use:

generate a nuc > ax nuc

where nuc is the lead nucleus (having adequate form factor defined in my model), ax is an Axion Like Particle (defined in my UFO model). This generation is successful when I provide specific energies to the input beams:

set run_card ebeam1 2.2
set ebeam2 0.

for incoming photon energy of 2.2 GeV and lead nucleus at rest.

But in a realistic experiment, the incoming photon beam will not have exactly 2.2 GeV of energy - there will be a distribution of incoming photon beam energy. In a proton-proton collision, this is handled by the PDF provided. But for this photon-nucleus collision scenario, how I can inform MadGraph that the incoming photon beam energy is not constant? We have a text file of the incoming photon beam energy distribution (kind of a lookup table). Is there a nice way to tell MadGraph to use that photon energy distribution?

Thank you,
Arka

Question information

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Last query:
2020-09-14
Last reply:
2020-09-14

Hi,

This is highly non trivial.
The issue is not the encoding of this kind of PDF (which is simple) but the modification to the phase-space integrator to correctly handle such type of PDF.

Indeed the phase-space integration strategy is not the same is this is a very thin gaussian or if this has a very strong low energy tail. In the first case a dedicated phase-space integrator (with multi-channel method) is required to solve such case. (Which is your case I guess).

I know that they are work in progress in that direction for lepton collider (which has similar issue) but this is not yet release (and I do not know the exact status).
In the past, I implemented a very basic strategy to handle this.
(the user was providing unweighted events for the beam configuration --of each beam--)
and I was using it agnostically of the matrix-element configuration, I never release it since while working on some specific case, it was failing on many others.

Cheers,

Olivier

> On 14 Sep 2020, at 18:35, Arka Santra <email address hidden> wrote:
>
> New question #692905 on MadGraph5_aMC@NLO:
> https://answers.launchpad.net/mg5amcnlo/+question/692905
>
> Hello,
> I am trying to simulate a photon - lead nucleus collision using MadGraph UFO model. This is the command I use:
>
> generate a nuc > ax nuc
>
> where nuc is the lead nucleus (having adequate form factor defined in my model), ax is an Axion Like Particle (defined in my UFO model). This generation is successful when I provide specific energies to the input beams:
>
> set run_card ebeam1 2.2
> set ebeam2 0.
>
> for incoming photon energy of 2.2 GeV and lead nucleus at rest.
>
>
> But in a realistic experiment, the incoming photon beam will not have exactly 2.2 GeV of energy - there will be a distribution of incoming photon beam energy. In a proton-proton collision, this is handled by the PDF provided. But for this photon-nucleus collision scenario, how I can inform MadGraph that the incoming photon beam energy is not constant? We have a text file of the incoming photon beam energy distribution (kind of a lookup table). Is there a nice way to tell MadGraph to use that photon energy distribution?
>
>
> Thank you,
> Arka
>
>
> --
> You received this question notification because you are an answer
> contact for MadGraph5_aMC@NLO.

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