Backporting MC jet flavour definition from CMSSW 9 to CMSSW 8

Intro

This adds in the new jet flavour definition in CMSSW 9 into CMSSW 8. It updates the partonFlavor() attribute of a pat::Jet collection.

In this new definition, you recluster each jet's constituents along with all genpartons as “ghosts”, then pick the highest pT genparton that gets clustered, irrespective of status.
This way it can work for both Pythia and Herwig (in the former it often corresponds to status==23 particle anyway for MG+Pythia8).

Reference PRs:

https://github.com/cms-sw/cmssw/pull/19048
https://github.com/cms-sw/cmssw/pull/20281/
https://github.com/cms-sw/cmssw/pull/20840/

https://indico.cern.ch/event/649196/contributions/2641282/attachments/1483715/2302215/JetFlavour.pdf

In terms of modules, this is how the partonFlavour gets calculated normally:

  • A HadronAndPartonSelector EDProducer produces collections of partons, leptons, bHadrons, and cHadrons from some input collection (often genParticles) according to some selection criteria. This is the critical part that determines which gen particles can be used for the flavour definition.
  • A JetFlavourClustering EDProducer takes in these collections, along with some jet collection, and does the ghost reclustering. The jet<>flavour info is stored as a JetFlavourInfoMatchingCollection, which is really a edm::AssociationVector<edm::RefToBaseProd<reco::Jet>,std::vector<reco::JetFlavourInfo> > (i.e. a map between a reference to a Jet, and a JetFlavourInfo object)
This is normally done on reco::Jet when making the MiniAOD, and gets stored when the pat::Jets are made.

However, if you just want to run over MiniAOD you will need another module to update the pat::Jets (which I have written).

Instructions

The full PRs update both how the definition is calculated, as well as configs for making MiniAOD. We don't need that, so I have only picked the necessary algorithmic parts.

This was done on top of CMSSW 8_0_28, but should work across 80X releases.

To merge the changes, simply do:

git cms-merge-topic raggleton:Backport_JetFlavour

For reference:

Algorithmic changes: https://github.com/raggleton/cmssw/commit/8a04d7bdd690b815f9f65b74ef45cf30c0e5a9f4

pat::Jet updater: https://github.com/raggleton/cmssw/commit/f0d5542b24478f9f6196440131766cbc7a0793e0

-- RobinAggleton - 2017-12-08

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Topic revision: r3 - 2017-12-11 - RobinAggleton
 
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