Abstract
A large fraction of the CMS physics program relies on the identification (tagging) of jets containing the decay of a B hadron (b jets). The b jets can be discriminated from jets produced by the hadronization of light quarks based on characteristic properties of B hadrons, such as the long lifetime. An overview of the large variety of b-tagging algorithms and the measurement of their performance with data collected in 2011 and 2012 are presented in this paper. A special focus lies on new methods of b-tagging in jet substructure. Searches for new physics often focus on boosted final states characterized by particles with large transverse momenta, resulting in decay products of heavy particles tending to be collimated and reconstructed as a single jet, known as fat jet. In this case, the reconstruction of the fat jet substructure is necessary to identify the particle initiating the fat jet. The substructure reconstruction can significantly be improved by the identification of b jets.
Highlights
Identification of jets arising from bottom-quark hadronisation and decay (b-tagging [1][2]) is used in many physics analyses to perform precise measurements of the standard model (SM) and for new particle searches
New physics signatures with b jets in the final states are expected at high mass, where the b quarks might end up in boosted topologies with overlapping jets from top-quark or Higgs boson decays, making b-tagging more challenging
The Compact Muon Solenoid (CMS)[3] detector recorded protonproton collisions occurring at the LHC during the 2012 data taking
Summary
The hadronization of a b quark produces a B hadron which propagates a measurable distance before decaying. Such behavior leads to special properties of the arising b jet, like the presence of an inner displaced secondary vertex with a flying distance higher than its resolution. Tracks coming from a secondary vertex have a large impact parameter that can be used to identify b jets. In 20% of cases, a b jet will contain a lepton coming from the semi-leptonic decay of the B hadron. These features are used to build taggers, yielding a single discriminator value for each jet. Three operation points, corresponding to a fixed misidentification probability (P) for light partons, are defined: ”Loose” (P=10%), ”Medium” (P=1%) and Tight (P=0.1%)
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