initial state photons at the Tevatron

[Rikkert Frederix]

Hi guys,

Processes with initial state photons don't work for collisions including anti-protons. Both at LO and NLO. It's probably because in the auto_dsig*.f (at LO) and parton_lum_*.f (at NLO) there is a minus sign added to the PDF-codes for the anti-protons (as compare to the protons), hence, the photons get a PDF-code set to -7 for the anti-protons (and not 7, as it is for the protons). It works OK for gluons, since those get PDF-code equal to 0, which remains 0 when a minus sign is added for the anti-proton. I looked in the relevant code in expert_v4.py and expert_fks.py, respectively, but it's a bit of a mess and case-by-case switching, and I'm not sure if the best solution is to add another case there...

Cheers,
Rikkert

[Olivier Mattelaer (olivier-mattelaer)]

I see,

The easiest might be to fix this within the handling of the PDF it-self.
This might bit a bit tricky as well since we have the various PDF interface but this would make more sense.

What do you think?
I can propose a patch for LO and then you can review it and see if we do the same for NLO or if patching the export function makes more sense...

Cheers,

Olivier

[Olivier Mattelaer (olivier-mattelaer)]

See lp:~maddevelopers/mg5amcnlo/bug_1812828
for my proposition.

Cheers,

Olivier

[Rikkert Frederix (frederix)]

Hi Olivier,

Looks alright. However, why did you implement a slightly different solution for pdg2pdf_lhapdf.f as compared to pdg2pdf.f and pdg2pdf_lhapdf6.f. Shouldn't they be made the same (easier for future understanding of these codes).

I've made the same changes at the NLO. Please check and do the merge.

Cheers,
Rikkert

[Olivier Mattelaer (olivier-mattelaer)]

Hi,

The reason is that pdg2pdf_lhapdf.f
is for the support of lhapdf version 5. So I was a bit lazy and did the minimum in that file.
I actually think that such file should be remove in the 3.x branch

Cheers,

Olivier