[Rivet] Use proton-lead MC generators with Rivet

[Hans Jozef H Van Haevermaet]

Dear Rivet authors,

I am a member of CMS (forward and small-x community in fact), and I’m trying to get Rivet running with proton-lead events coming from the EPOS MC generator.
It turns out however, that this is not possible (?).
When executing Rivet, it stops because it detected that the incoming beams are different for each event:
Event beams mismatch: [PROTON, PROTON] @ 1.87656 GeV vs. first beams [NEUTRON @ 2511.57 GeV, NEUTRON @ 2511.57 GeV] @ 1.87914 GeV

This in turn, is caused by the fact that the EPOS generator writes out the interacting nucleon (p or n) in the HEPMC file, and not the particle id code of lead itself.

Could you let us know if it is possible for Rivet to get compatible with these kind of situations?
One direct option (for me) is to comment out this function:
https://rivet.hepforge.org/code/dev/AnalysisHandler_8cc_source.html
At line 95.

But that can lead to unwanted behaviour in other cases, I guess. Maybe one can include a parameter to turn this check on/off? So that it is the user’s responsibility? i.e. by default the check is done, but the user can turn it off manually if needed.

It would be very good to solve this incompatibility, since in CMS we try to get Rivet routines from all our analyses, and it would be nice to be able to extend this to proton-lead, or even lead-lead measurements. As far as I can see on the web, I did not find any Heavy Ion analysis implemented in Rivet?

Thanks!
Best Regards,
Hans


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