[Rivet] Rivet: Accessing intermediate particles

[David Hall]

Hello,

As a bit of background, I am using Sherpa to output a HepMC event file 
and then Rivet to perform an analysis on this file. I am studying the 
W+gamma* process, and need to identify the leptons that are coming from 
the gamma*. When parton showering is performed, additional leptons are 
produced and it becomes difficult to be certain which leptons are from 
the hard process.

I have heard that after the matrix element calculation, Sherpa outputs 
the four-momenta from the hard scatter as additional information in the 
HepMC file (perhaps as another GenVertex?). So I was thinking, if I had 
access to this information, I could try to match the 4-momenta of the 
final state particles to those from the hard process in order to 
identify the leptons I need.

I was wondering if there is anything in Rivet to provide access to this 
information? I can only find the FinalState particles currently. I 
understand the philosophy behind Rivet not providing access to the 
intermediate particles, but I believe that looking for the particles 
from the hard process in this way is not _so_ bad. :-)

Thanks,
David