[Rivet] Rivet: Accessing intermediate particles

[Andy Buckley]

Hi David,

It's best not to rely on any particular definition of the "hard process"
as implemented in different generators: it's rather an arbitrary thing,
and in fact the distinction would not exist at all if it weren't for the
limitations of current calculations. The further down the line of
ME-enhanced parton showers we go, the less the concept makes sense in an
analysis... and Sherpa is probably the generator furthest down that line
of development!

Additionally, matching on momenta between the internal and final state
particles in an event record is *very* fragile: best not to go there.

In fact the parton showering will not emit more *leptons*, but hadronic
decays will. Fortunately the HepMC status codes can be used to
distinguish particles from decays from those produced in the rest of the
event: if you use an IdentifiedFinalState (or similar) in Rivet to find
all the final state leptons that you are interested in, then loop over
them and ass each one's HepMC GenParticle (via the genParticle() method)
what the status code of its parent was: if the parent status is 2, then
it's a lepton from a hadronic decay; if it's something else (0, 3, or >
10) then it's the sort of "prompt" lepton that you're interested in.

Note, however, that distinguishing leptons from the gamma* from those
produced in leptonic decay of the W is not physically meaningful if done
using truth record information about parentage... but a
reconstruction-type approach, using windowing mass(l+nu) around the W
mass would be robust and physical since it's based on real observable
quantities (the neutrino momentum may not be practically observable with
real detectors, but there's no reason not to use it in Rivet!)

Cheers, and I hope that helps,
Andy


On 29/06/12 11:02, David Hall wrote:
> 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
> 
> _______________________________________________
> Rivet mailing list
> Rivet at projects.hepforge.org
> http://www.hepforge.org/lists/listinfo/rivet
> 


-- 
Dr Andy Buckley, SUPA Advanced Research Fellow
Particle Physics Expt Group, University of Edinburgh

The University of Edinburgh is a charitable body, registered in
Scotland, with registration number SC005336.