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Bottom quark

Bottom quark
Composition:	Elementary particle
Family:	Fermion
Group:	Quark
Generation:	Third
Discovered:	Leon M. Lederman et al., 1977
Mass:	4 GeV/c²
Electric charge:	-1/3 e
Spin:	½

Flavour in particle physics v • d • e
Flavour quantum numbers: Lepton number: L Baryon number: B Strangeness: S Charm: C Bottomness: B' Topness: T Isospin: I_z or I Weak isospin: T_z Electric charge: Q Combinations: Hypercharge: Y Y=2(Q-I_z) Weak hypercharge: Y_W Y_W=2(Q-T_z) Y_W=B−L Related topics: CPT symmetry CKM matrix CP symmetry Chirality

The bottom quark is a third-generation quark with a charge of -(1/3)e. Although all quarks are described in a similar way by the theory of quantum chromodynamics, the bottom quark's large mass (around 4 GeV, or four times the mass of a proton), combined with low values of the CKM matrix elements V_ub and V_cb, gives it a distinctive signature that makes it relatively easy to identify experimentally (using a technique called B-tagging). Because three generations of quark are required for CP violation (see CKM matrix for an explanation of why), mesons containing the bottom quark are the easiest particles to use to investigate the phenomenon; this is being done at the BaBar and Belle experiments. The bottom quark is also notable because it is a product in almost all top quark decays, and a frequent decay product for the hypothetical Higgs boson if it is sufficiently light.

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The bottom quark was discovered at Fermilab in 1977. On its discovery, a valiant effort was made by many particle physicists to name it "Beauty", paired along with "Truth". Poetic license gave way to pragmatics, and the quarks were named "top" and "bottom" instead.

The bottom quark can decay either into up or charm quark via weak interaction. Both these decays are suppressed by CKM matrix, making lifetimes of most bottom particles ( ~10^-12 s ) somewhat higher than those of charmed particles (several times 10^-13 s ), but lower than those of strange particles ( 10^-10 - 10^-8 s ).

Hadrons containing bottom quarks

Some of the hadrons containing bottom quarks include:

B mesons contain a bottom quark (or its antiparticle) and an up or down quark.
B_c and B_s mesons contain a bottom quark along with a charm quark or strange quark respectively.
There are many bottomonium states, for example the upsilon meson ( $\Upsilon\,$ ). These consist of a bottom quark and its antiparticle.
Bottom baryons have been observed, and are named in analogy with strange baryons (e.g. Λ_b⁰).