Fluctuations in the cross section resutls

Attached are the script and the result.

For the script (you can open it with the text editor), the maybe
relevant/important settings are:

import model sm-lepton_masses

set polbeam1 = -100

set ebeam2 0.

set no_parton_cut

set dynamical_scale_choice 4

For the results, you can see that using MSHT20qed or CT18qed photon PDFs
gives slightly different cross sections (because they are different
photon PDFs) but, interestingly and importantly, the fluctuations are at
the same neutrino energies and at the same level.

The calculations used Nevents=1e5. I've tried Nevents=1e6 and got the same
results.

So do you know what causes the fluctuations?

Thank you very much for your help!

On Fri, Jun 2, 2023 at 12:25 PM Qiji Xin <
<email address hidden>> wrote:

> New question #706878 on MadGraph5_aMC@NLO:
> https://answers.launchpad.net/mg5amcnlo/+question/706878
>
> Dear MadGraph developers,
>
> I am calculating ve p > e- w+ using madgraph with photon PDFs and find
> that the cross section results have fluctuations…
>
>
> --
> You received this question notification because you asked the question.
>

It seems that the helicity recycling optimization is too agressive for some energy.
So I would suggest to remove such optimization by adding
set hel_recycling False
to your script.

Cheers,

Olivier

Dear Olivier,

Thanks! Quick follow-up questions. What is the parameter "hel_recycling" for? And will it change my results if I set it to False?

It's an optimization parameter:
See 2102.00773
In principle it should not change the results, but in this particular case, it does (sometimes)

Cheers,

Olivier

Thanks Olivier Mattelaer, that solved my question.

Dear Olivier,

Another quick follow-up question. Do you know why helicity recycling causes problems in my case? Is it because the initial state is neutrino with "set polbeam1 = -100"? Or is it because the nucleon is at rest? Or else?

If it is the former reason, should I "set hel_recycling False" for all the neutrino scatterings?

Thanks!

> Do you know why helicity recycling causes problems in my case?

No I do not know that. Will look at that next week

Olivier

Thanks! Yes, please, if you have time. That could be important for using MadGraph to do neutrino fixed-target calculations.

Thanks Olivier Mattelaer, that solved my question.