Heavy ion PDF in MG5_aMC_v2_6_7
Hello,
I would like to understand the implementation of heavy-ion photon flux in MG5_aMC_v2_6_7, as I am trying to benchmark the cross section of an elastic photon-fusion process, for example, a a > b b~, with a constrained invariant mass for the final state.
In Template/
double precision function get_ion_pdf(pdf, pdg, nb_proton, nb_neutron)
C******
C computing (heavy) ion contribution from proton PDF
C******
double precision pdf(-7:7)
double precision tmppdf(-2:2)
integer pdg
integer nb_proton
integer nb_neutron
double precision tmp1, tmp2
if (nb_proton.
return
endif
if (pdg.eq.
tmp1 = pdf(1)
tmp2 = pdf(2)
tmppdf(1) = nb_proton * tmp1 + nb_neutron * tmp2
tmppdf(2) = nb_proton * tmp2 + nb_neutron * tmp1
else if (pdg.eq.
tmp1 = pdf(-1)
tmp2 = pdf(-2)
tmppdf(-1) = nb_proton * tmp1 + nb_neutron * tmp2
tmppdf(-2) = nb_proton * tmp2 + nb_neutron * tmp1
else
endif
C set correct PDF normalisation
get_ion_pdf = get_ion_pdf * (nb_proton+
return
end
Hence the same process will have a higher cross section in heavy ion collisions than in pp collisions. However, when I launched Pb collisions with the following updates to the run card, MadGraph returned zero cross section.
set mmbb 100.
set mmbbmax 140.
set lpp1 2
set lpp2 2
set ebeam1 7000
set ebeam2 7000
set nb_proton1 82
set nb_neutron1 126
set nb_proton2 82
set nb_neutron2 126
In order to understand the reason, I kept everything the same but the number of nucleons, for which I tried a series of numbers between 2 and 50. I noticed that the cross section decreases as a function of the total nucleon number and vanishes at 50. This is clearly not what the corresponding photon flux is supposed to do, but I have no clue why this is the case.
Any help would be appreciated.
Cheers,
Wenyi
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