Bottom quark

Infobox Particle
bgcolour =
name = Bottom quark


caption =
num_types =
composition = Elementary particle
family = Fermion
group = Quark
generation = Third
interaction =
particle =
antiparticle =
theorized =
discovered = Leon M. Lederman et al., 1977
symbol =
mass = 4,200 MeV/c² [ [http://pdg.lbl.gov/2007/reviews/quarks_q000.pdf PDG—Quark masses] ]
decay_time =
decay_particle =
electric_charge = −frac|1|3 e
color_charge =
spin = frac|1|2
num_spin_states =

The bottom quark is a third-generation quark with a charge of −frac|1|3"e". Although all quarks are described in a similar way by the quantum chromodynamics, the bottom quark's large mass (around 4,200 MeV (a bit more than 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), mesons containing the bottom quark are the easiest particles to use to investigate the phenomenon; such experiments are being performed at BaBar and Belle. The bottom quark is also notable because it is a product in almost all top quark decays, and would be a frequent decay product for the hypothetical Higgs boson if it is sufficiently light.

The bottom quark was discovered by the E288 experiment at Fermilab in 1977 when collisions produced bottomonium [http://www.symmetrymagazine.org/cms/?pid=1000195 Discovery of the Bottom Quark] . On its discovery, there were efforts to name it "beauty", paired along with "truth", but they came to be called "bottom" and "top" instead.

The bottom quark can decay into either an up or charm quark via the 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.
*SubatomicParticle|Charmed B and SubatomicParticle|Strange B mesons contain a bottom quark along with a charm quark or strange quark respectively.
*There are many bottomonium states, for example the SubatomicParticle|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. SubatomicParticle|Bottom Lambda0).

References

External links

* [http://www.fnal.gov/pub/inquiring/physics/discoveries/bottom_quark.html Bottom quark information on Fermilab website]
* [http://pdg.lbl.gov/2007/listings/q005.pdf Technical data from Particle Data Group (pdf)]