gamma, p => lhapdf

Asked by Igal on 2020-05-29


I am using the DrellYanMatchingP.sin example and replace one of the proton beam by a photon beam and for this process: gamma, p => quark:antiq, p

And got this message

*** FATAL ERROR: The 'lhapdf' structure is intended only for protons and
*** pions, please use 'lhapdf_photon' for photon beams.

if I then use lhapdf_photon, I am getting this message

*** FATAL ERROR: This PDF set is not supported or not intended for photon beams.

Any ideas on what should be done to make it works?

Thanks, Igal.

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Juergen Reuter (j.r.reuter) said : #1

Hi Igal,
seems you are testing out all corner cases^^
There are several things to watch when dealing with photons in hadronic environments.
Writing a statement like
beams = gamma, p
beams = A, p
actually demands photon on proton collisions. When you then define a process like e.g. Drell-Yan, so
q qbar -> e+e-
then you need the quark content inside the photon, and then you have to take the full beams statement
beams = A, p => lhapdf_photon, lhapdf
$lhapdf_file = "cteq5l.LHgrid"
$lhapdf_photon_file = "GSG961.LHgrid"
However, as far as I know such a photon PDF (quark content inside the photon) only exists in
and that is something which e.g. exists in the GSG961.LHgrid PDF from LHAPDF5.
For LHAPDF6 I don't know actually.
The other thing is to get the photon content inside the proton, which you can get e.g. from LUXqed PDFs:
then you have a process like
process aaww = A, A => Wp, Wm
beams = p, p => lhapdf
$lhapdf_file = "LUXqed_plus_PDF4LHC15_nnlo_100"


Juergen Reuter (j.r.reuter) said : #2