b-tagging is a method of jet flavor tagging used in modern

particle physics Particle physics or high-energy physics is the study of Elementary particle, fundamental particles and fundamental interaction, forces that constitute matter and radiation. The field also studies combinations of elementary particles up to the s ...

experiments. It is the identification (or "tagging") of jets originating from

bottom quark The bottom quark, beauty quark, or b quark, is an elementary particle of the third generation. It is a heavy quark with a charge of − ''e''. All quarks are described in a similar way by electroweak interaction and quantum chromodynamic ...

s (or b quarks, hence the name).

Importance

b-tagging is important because: * The physics of bottom quarks is quite interesting; in particular, it sheds light on

CP violation In particle physics, CP violation is a violation of CP-symmetry (or charge conjugation parity symmetry): the combination of C-symmetry (charge conjugation symmetry) and P-symmetry ( parity symmetry). CP-symmetry states that the laws of physics s ...

. * Some important high-mass particles (both recently discovered and hypothetical) decay into bottom quarks.

Top quark The top quark, sometimes also referred to as the truth quark, (symbol: t) is the most massive of all observed elementary particles. It derives its mass from its coupling to the Higgs field. This coupling is very close to unity; in the Standard ...

s very nearly always do so, and the

Higgs boson The Higgs boson, sometimes called the Higgs particle, is an elementary particle in the Standard Model of particle physics produced by the excited state, quantum excitation of the Higgs field, one of the field (physics), fields in particl ...

is expected to decay into bottom quarks more than any other particle given its mass has been observed to be about 125 GeV. Identifying bottom quarks helps to identify the decays of these particles.

Methods

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

Hadron In particle physics, a hadron is a composite subatomic particle made of two or more quarks held together by the strong nuclear force. Pronounced , the name is derived . They are analogous to molecules, which are held together by the electri ...

s containing bottom 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 place different to where the bottom quark was formed (e.g. the beam–beam collision point in a

particle accelerator A particle accelerator is a machine that uses electromagnetic fields to propel electric charge, charged particles to very high speeds and energies to contain them in well-defined particle beam, beams. Small accelerators are used for fundamental ...

), and thus indicating the likely presence of a b-jet. * The bottom quark is much more massive than anything it decays into. Thus its decay products tend to have higher transverse momentum (

momentum In Newtonian mechanics, momentum (: momenta or momentums; more specifically linear momentum or translational momentum) is the product of the mass and velocity of an object. It is a vector quantity, possessing a magnitude and a direction. ...

perpendicular In geometry, two geometric objects are perpendicular if they intersect at right angles, i.e. at an angle of 90 degrees or π/2 radians. The condition of perpendicularity may be represented graphically using the '' perpendicular symbol'', � ...

to the original direction of the bottom quark, and therefore of the b-jet). This causes b-jets to be wider, have higher multiplicities (numbers of constituent particles) and

invariant mass The invariant mass, rest mass, intrinsic mass, proper mass, or in the case of bound systems simply mass, is the portion of the total mass of an object or system of objects that is independent of the overall motion of the system. More precisely, ...

es, and also to contain low-energy

lepton In particle physics, a lepton is an elementary particle of half-integer spin (Spin (physics), spin ) that does not undergo strong interactions. Two main classes of leptons exist: electric charge, charged leptons (also known as the electron-li ...

s with momentum perpendicular to the jet. These two features can be measured, and jets that have them are more likely to be b-jets. * Opposite-side algorithms have been used at the

LHCb The LHCb (Large Hadron Collider beauty) experiment is a particle physics detector collecting data at the Large Hadron Collider at CERN. LHCb specializes in the measurements of the parameters of CP violation in the interactions of b- and c-hadro ...

to tag the flavor in pairs of b quarks using the decay products of B-hadrons to infer the flavor of B-mesons. 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. Monte Carlo simulations are used to develop and evaluate the performance of tagging algorithms. Experiments making precise measurements of

B meson In particle physics, B mesons are mesons composed of a bottom antiquark and either an up (), down (), strange () or charm quark (). The combination of a bottom antiquark and a top quark is not thought to be possible because of the top quark' ...

s (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 ( – oscillation), which allows the measurement of

References

{{DEFAULTSORT:B-Tagging Experimental particle physics Hadrons B_physics

Importance

Methods

See also

References