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Bottom quark
From Wikipedia, the free encyclopedia
| 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 |
Flavour quantum numbers:
Combinations:
Related topics: |
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 Vub and Vcb, 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.
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 ( ~10-15 s ), but lower than those of strange particles ( 10-10 - 10-8 s ).
Some of the hadrons containing bottom quarks include:
- B mesons contain a bottom quark (or its antiparticle) and an up or down quark.
- Bc and Bs mesons contain a bottom quark along with a charm quark or strange quark respectively.
- There are many bottomonium states, for example the upsilon meson (
). These consist of a bottom quark and its antiparticle. - Bottom baryons have been observed, and are named in analogy with strange baryons (e.g. Λb0).
- Bottom quark information on Fermilab website
- Logbook showing bottom quark discovery
- Technical data from Particle Data Group (pdf) (Last accessed April 21, 2006).
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| Fermions: Quarks: (Up · Down · Strange · Charm · Bottom · Top) | Leptons: (Electron · Muon · Tau · Neutrinos) | |
| Gauge bosons: Photon | W and Z bosons | Gluons | |
| Not yet observed: Higgs boson | Graviton | Other hypothetical particles | |