[Rivet] Boosts in FinalStates/ParticleFinders

[Andy Buckley]

One of the things that we were asked to provide in Rivet recently was 
automatic frame transformation of projections, e.g. to return particles 
or jets computed in the beam centre-of-mass.

I've added some tools to the Beam projection to compute (I hope 
correctly...) the appropriate boost for that case, but it's not clear 
how to propagate it to the projections. I can't just make a 
BoostedFinalState, then pass that (maybe wrapped in another FinalState) 
to FastJets, for example, because then the tagging ghosts mode via an 
internal HeavyHadrons projection will be in a different frame from the 
jets they're being clustered into: not good.

Any ideas how we should go about doing this? We could maybe go down the 
rabbit-hole above, but I feel it gets very complex very quickly. Another 
way is to add an optional TransformationFinder projection argument to 
the ParticleFinder base class, so that the boost is computed 
event-by-event by that projection before being applied to the found 
particles. BeamBoostFinder would then be one of several implementations 
of TransformationFinder, and perhaps the existing but little-used 
DISKinematics projection would also be retrofitted that way.

This feels quite neat to me, but isn't quite as minor a bit of surgery 
as I'd hoped for. Any thoughts / much easier ways that I've not seen?

Cheers!
Andy

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