• "Track Counting" algorithm: More... Close This is a very simple tag, exploiting the long lifetime of B hadrons. It calculates the signed impact parameter significance of all good tracks, and orders them by decreasing significance. Its b tag discriminator is defined as the significance of the N'th track. It comes in two variations for N = 2 (high efficiency) or N = 3 (high purity).
  • "Jet Probability" algorithm: More... Close This is a more sophisticated algorithm, also exploiting the long lifetime of B hadrons. Its b tag discriminator is equal to the negative logarithm of the confidence level that all the tracks in the jet are consistent with originating from the primary vertex. This confidence level is calculated from the signed impact parameter significances of all good tracks. It reads the resolution function on these from a database (DB). Indeed, we have two versions of this tagger: JetProbabilityBJetTags and JetBProbabilityBJetTags - the latter uses only the four most displaced tracks, matching the typical reconstructed multiplicity of a B decay vertex.
  • "Soft Muon" and "Soft Electron" algorithms: More... Close These two algorithms tag b jets by searching for the lepton from a semi-leptonic B decay, which typically has a large Pt_rel with respect to the jet axis. Their b tag discriminators are the output of neural nets based on the leptons Pt_rel, impact parameter significance and a few other variables. For each of these taggers, we want to have a simple one (cutting basically only on the presence and pT of the lepton), and a complex one using also jet quantities to compute a MVA analysis. This is already the case for Muons; Electron taggers need refinement and are not really usable up to 2.1.X.
  • "Simple Secondary Vertex" algorithms: More... Close These class of algorithms reconstructs the B decay vertex using an adaptive vertex finder, and then uses variables related to it, such as decay length significance to calculate its b tag discriminator. It has been found to be more robust to Tracker misalignment than the other lifetime-based tags. CMSSW releases <= 35X contain one version of the algorithm (simpleSecondaryVertexBJetTags). Starting from 36X two versions will be provided: simpleSecondaryVertexHighEffBJetTags (equivalent to the previous version) and simpleSecondaryVertexHighPurBJetTags (with increased purity due to a cut on the track multiplicity at the secondary vertex)
  • "Combined Secondary Vertex" algorithm: More... Close This sophisticated and complex tag exploits all known variables, which can distinguish b from non-b jets. Its goal is to provide optimal b tag performance, by combining information about impact parameter significance, the secondary vertex and jet kinematics. (Currently lepton information is not included). . The variables are combined using a likelihood ratio technique to compute the b tag discriminator. A variant of this tagger combines the variables using the Multivariant Analysis (MVA) tool.
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Topic revision: r3 - 2010-06-06 - JyothsnaK
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