[Rivet] Rivet analysis for CMS UE in DY

[Frank Siegert]

Hi Hendrik, Albert,

> I had a look yesterday and today, and I'm giving up. Can you please ask
> the author to check the following things:
>
> - const FinalState& fs = zfinder.constituentsFinalState();
>   => There is no "constituentsFinalState()" in the ZFinder,
>   so the code doesn't compile.
>
> - if (zfinder.particles().size() != 1) vetoEvent;
>   Typically, a Z will decay into two fermions, so this cut
>   will veto *all* events.
>
> - double _Zmass = zfinder.particles()[0].momentum().mass()/GeV;
>   This is not the Z mass, but the mass of one of the muons the
>   Z decayed to. Same for _Zpt and _Zphi.

All of these are related to an API change of the ZFinder when going
from Rivet 1.5 to 1.6 a bit over a year ago. I can deal with the
transition of that if it's the only problem that is left in the
analysis.

Cheers,
Frank