[Rivet] ParticleFinder/FinalState compound & constituent particle reporting

[Andy Buckley]

Hi all,

I realised while putting finishing touches on the Rivet 2.5 smearing 
mechanisms that we need a standardised interface for distinctly 
returning "compound particles" created by a projection, and for 
returning the raw constituent particles from which they are created.

In many cases these are the same thing. For example, a FinalState's 
particles() method returns all raw particles matching the cuts, and 
there are no compound particles. But WFinder, ZFinder, DressedLeptons, 
and arguably things like TauFinder are able to return both compound 
particles -- the Ws, Zs, and dressed leptons -- and the raw particles 
from which they are built.

At the moment, the particles() method always returns the raw ones, and 
there are special methods like WFinder::bosons() and 
DressedLeptons::dressedLeptons() to get the compounds. But for smearing 
we are really interested in the "most compound" objects returned by a 
projection, so I can use a SmearedParticles projection to wrap either an 
IdentifiedFinalState of raw electrons, or a DressedLeptons wrapper of 
the same. At the moment if I make a 
SmearedParticles(DressedLeptons(IdentifiedFinalState)), then the 
electron smearing will be applied equally to raw electrons and raw 
photons, and all those smeared raw particles will be returned -- oops.

I don't think this is something in need of fixing for the 2.5.0beta1 
(hopefully to go out today?), but it should be done for 2.5.0 proper. I 
suggest adding rawParticles() and compoundParticles() methods to 
ParticleFinder and making sure that each derived class returns the 
appropriate collection. Or, since I find it rather odd that 
WFinder::particles(), DressedLeptons::particles(), etc. do *not* 
currently return Ws, dressed leptons, etc., we could change those 
methods to do so, and just add rawParticles() as the standard hook for 
retrieving objects that come direct from HepMC -- and adapt the 
veto-on-FS / remainingFinalState mechanisms to use rawParticles() rather 
than particles(). I very much doubt anyone is using that behaviour 
directly in user code. I prefer the latter for several reasons, but this 
is the sort of thing that should be discussed. Feedback, please!!

Andy

-- 
Dr Andy Buckley, Lecturer / Royal Society University Research Fellow
Particle Physics Expt Group, University of Glasgow