tagging

B-tagging

b-tagging is an example of a jet flavor tagging method used in modern high energy particle physics experiments. It is the identification ("tagging") of jets originating from bottom quarks (b-quarks).

Importance

b-tagging is important because:

  • The physics of b-quarks is quite interesting; in particular, it sheds light on CP violation.
  • Some important high-mass particles (both recently-discovered and hypothetical) decay into b-quarks. Top quarks very nearly always do so, and the Higgs boson is expected to decay into b-quarks more than any other particle if it is sufficiently light. Identifying b-quarks helps identify the decays of these particles.

Methods

The methods for b-tagging are based on the unique features of b-jets. These include:

  • Hadrons containing b-quarks have sufficient lifetime that they travel some distance before decaying. On the other hand, their lifetimes are not so high as those of light quark hadrons, so they decay inside the detector rather than escape. The advent of precision silicon detectors within particle detectors has made it possible to identify particles that originate from a different place where the b-quark was formed (e.g. the beam-beam collision point in a particle accelerator), and thus indicating the likely presence of a b-jet.
  • The b-quark is much more massive than anything it decays into. Thus its decay products tend to have higher momentum perpendicular to the direction the b-quark (and therefore the b-jet) was going. This causes b-jets to be wider, have higher multiplicities (numbers of constituent particles) and invariant masses, and also to contain low-energy leptons with momentum perpendicular to the jet. These two features can be measured, and jets that have them are more likely to be b-jets.

None of the methods of identifying b-jets are foolproof, and modern particle physics experiments must devote significant time to studying how often they successfully identify b-jets and how often they misidentify other jets.

Experiments making precise measurements of B-mesons (i.e. mesons containing b-quarks) also try to identify the particular initial B-meson within the jet. This is done in order to observe the oscillation of one meson into another (e.g. B^0-overline{B}^0), which allows the measurement of CP violation.

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