parton c.m. energy

This should be a question to chatgpt:
Here is the answer from copilot:

Calculating Parton-Level c.m. Energy: The c.m. energy of the partons is given by:

\hat s= x1x2s

where ( x_1 ) and ( x_2 ) are the momentum fractions of the partons from each proton, and ( s ) is the square of the total c.m. energy of the proton-proton system. The values of ( x_1 ) and ( x_2 ) are sampled according to the PDFs during the event generation process2.

PS: the value of x_1 and x_2 are not fixed... so it varies event by event

Dear Olivier,
Thank you for your quick response. I have another question regarding the article. It describes the following:

For the process pp→Nljj, we obtain the “inclusive” cross-section by applying a minimal jet pT cut of pT>(pT)min​ to avoid the collinear singularity, whereas the infrared part is approximated by the inclusive cross section of the process pγ→Nlj, where the photon comes from a proton. The latter process was calculated using the EPA with the improved Weizsäcker-Williams formula [20] for a fixed factorization scale of μF=(pT)min​. For concreteness, we used the equivalent photon distribution functions as implemented in MADGRAPH5 [22], whereas the quark and gluon distribution functions of the proton were taken from CTEQ6L [23].

Based on this, I have set the following parameters:

# Collider type and energy
# lpp: 0=No PDF, 1=proton, -1=antiproton,
# 2=elastic photon of proton/ion beam
# +/-3=PDF of electron/positron beam
# +/-4=PDF of muon/antimuon beam
*********************************************************************
1 = lpp1 ! beam 1 type
2 = lpp2 ! beam 2 type
6500.0 = ebeam1 ! beam 1 total energy in GeV
6500.0 = ebeam2 ! beam 2 total energy in GeV
*********************************************************************
# PDF CHOICE: this automatically fixes alpha_s and its evolution
# pdlabel: lhapdf=LHAPDF (installation needed) [1412.7420]
# iww=Improved Weizsaecker-Williams Approx.[hep-ph/9310350]
# eva=Effective W/Z/A Approx. [2111.02442]
# edff=EDFF in gamma-UPC [eq.(11) in 2207.03012]
# chff=ChFF in gamma-UPC [eq.(13) in 2207.03012]
# none=No PDF, same as lhapdf with lppx=0
*********************************************************************
cteq6_l = pdlabel1 ! PDF type for beam #1
edff = pdlabel2 ! PDF type for beam #2
230000 = lhaid ! if pdlabel=lhapdf, this is the lhapdf number

However, I keep encountering the error: "InvalidRunCard: lpp1 '1' not compatible with pdlabel1 'edff'."
Despite setting pdlabel1 to cteq6_l, I am still getting this error. Could you please help me understand why this is happening?

Well looks like you did a lot of mistake here.
Your process should not be p p > ...
if you want to use the EWA but rather
p a > ...
or
a p > ...

If you want to use
EPA with the improved Weizsäcker-Williams formula
you need to set the pdlabel to "iww" (for the photon side) to iww and not to something else.

Cheers,

Olivier