[Rivet] Use proton-lead MC generators with Rivet

[Leif Lönnblad]

Hi Hans,

There is actually a flag for rivet to --ignore-beams, but it only 
prevents checks that the different analyses included requires the same 
beams. It doesn't seem unreasonable to me if the checking of beams in 
the code you pointed to was switched off if --ignore-beams is given, ie.

     if ( !_ignoreBeams ) {
     // Ensure that beam details match those from the first event
       const PdgIdPair beams = Rivet::beamIds(ge);
       const double sqrts = Rivet::sqrtS(ge);
       if (!compatible(beams, _beams) || !fuzzyEquals(sqrts, sqrtS())) {
         cerr << "Event beams mismatch: "
              << PID::toBeamsString(beams) << " @ " << sqrts/GeV
              << " GeV" << " vs. first beams "
              << this->beams() << " @ " << this->sqrtS()/GeV
              << " GeV" << endl;
         exit(1);
       }
     }

Of course, individual analyses may still fail if they require specific 
beams.


/Leif

On 2017-02-28 12:10, Hans Jozef H Van Haevermaet wrote:
> 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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