[Rivet] trimmed jets and constituents

[James Monk]

Hi,

As per my previous mail on ghosts (which apparently caused some amusement), I am working with trimmed jets and need access to constituents.  The ghosts thing works ok with the patch I provided, but I also need to know the Rivet::Particles that are the constituents of the trimmed jets.  Following some existing Rivet analyses, I am defining a trimmer thus:

	fastjet::filter _trimmer(fastjet::Filter(fastjet::JetDefinition(fastjet::kt_algorithm, 0.2), fastjet::SelectorPtFractionMin(0.05)));
	fastjet::PseudoJet trimmedJet = _trimmer(unTrimmedJet);

where unTrimmedJet is a Rivet:Jet found previously by the FastJet projection.

The resulting trimmed PseudoJet has constituents, but they are themselves PseudoJets, not Rivet::Jets, so the link with the input particles is broken and I can’t look up things like barcodes, pdg_ids etc.  I could probably hack something together based on the PseudoJet.user_index and the order of the PseudoJets and Rivet::Particle constituents in the un-trimmed jet (their order *should* be the same), but that’s not very friendly for most users, and not completely robust.  Given the increasing prevalence of trimmed jets, I think it would be worthwhile if either the FastJet projection could take a trimmer on construction and apply it to all jets, or have a method to return a trimmed version of a given jet with full particle info.  The latter would be more computationally-efficient as only the one or two jets of interest could be trimmed.

What do you think?

cheers,

James