Reconstruction of b Tag Information in CMSSW 1.2

The B objects are essentially made by a jet, a collection of tracks associated (in some way) to the jet, and a discriminator which is the actual result of the tagging. Each algorithm may also produce some "extended" information relative to the algorithm used to produce the object.

The reconstruction chain is very simple:

  • Jets and tracks are reconstructed by standard producers
  • A JetTrackAssociator associates tracks to jets (at the moment there is only one cone-based implementation, and a slightly smarter one taking into account effective tracks propagation is underway)
  • The output of the associator is used as input for the producers
  • If needed, the primary vertex is used
  • Some algorithms, like the SoftLepton and SecondaryVertex, require additional inputs which are in turn read from the Event, such as the reconstructed leptons and secondary vertices.


Several different producers exist or are being written for B tagging...:

  • Track Counting
  • Soft Lepton
  • Track Probability (work in progress)
  • Combined Secondary Vertex (work in progress)
...and Tau tagging:
  • Isolation Cone
  • Impact Parameter
  • Mass Tag (work in progress)

Each producer creates two collections: a JetTagCollection and an extended xxxTagInfoCollection.


For this tutorial on reconstruction, in your CMSSW 1.2 working area you can do:

cmsRun test_btag.cfg

The relevant fragments of this configuration files are:

    # Tracks to jets association
    include "RecoBTau/JetTracksAssociator/data/jetTracksAssociator.cfi"

    # Track counting and SoftLepton b tagging
    include "RecoBTag/TrackCounting/data/trackCounting.cff"
    include "RecoBTag/SoftLepton/data/softLepton.cff"
    # module execution
    path btag = { jetTracksAssociator, trackCountingJetTags, softLeptonJetTags }

All this configuration files share the same basic structure, requiring at least two modules

  • a jet-tracks associator
  • a tagging algorithm, for example the TrackCounting B tagging algorithm

The default configuration for these producers is defined in their respective .cfi files, which expand to

    module jetTracksAssociator = JetTracksAssociator {
      InputTag tracks = ctfWithMaterialTracks
      InputTag jets   = iterativeCone5CMS.CaloJets
      double coneSize = 0.5
    module trackCountingJetTags = TrackCounting {
      string jetTracks = "jetTracksAssociator"
      string primaryVertex = "offlinePrimaryVerticesFromCTFTracks"
      PSet AlgorithmPSet =  {
         int32  NthTrack = 2
         int32  ImpactParamterType = 0
         int32  MinimumNumberOfPixelHits = 2
         int32  MinimumNumberOfHits = 8
         double MaximumTransverseImpactParameter = 0.2
         double MinimumTransverseMomentum = 1.0
         double MaximumDecayLength = 5.0
         double MaximumChiSquared = 5.0
         double MaximumLongitudinalImpactParameter = 170.0
         double MaximumDistanceToJetAxis = 0.07

-- AndreaBocci 2006

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Topic revision: r2 - 2007-11-13 - IanTomalin

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