B-tagging

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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).

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.

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. If particles can be identified that originate from a different place than where the b-quark was formed (e.g. the beam-beam collision point in a particle accelerator), this may indicate the 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, 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 is 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^0]), which allows the measurement of CP violation.

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