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 −⁠1/3⁠ e.

All quarks are described in a similar way by electroweak interaction and quantum chromodynamics, but the bottom quark has exceptionally low rates of transition to lower-mass quarks. The bottom quark is also notable because it is a product in almost all top quark decays, and is a frequent decay product of the Higgs boson.

The bottom quark was first described theoretically in 1973 by physicists Makoto Kobayashi and Toshihide Maskawa to explain CP violation.^[1] The name "bottom" was introduced in 1975 by Haim Harari.^[5][6]

The evidence for the bottom quark was first obtained in 1977 by the Fermilab E288 experiment team led by Leon M. Lederman, when proton-nucleon collisions produced bottomonium decaying to pairs of muons.^[2][7][8] The discovery was confirmed about a year later by the PLUTO and DASP2 Collaborations at the electron-positron collider DORIS at DESY.^[9][10] It was reported at the time that DESY scientists were in favor of the name "beauty", while the American scientists tended towards "bottom".^[10]

Kobayashi and Maskawa won the 2008 Nobel Prize in Physics for their explanation of CP-violation.^[11][12]

While the name "beauty" is sometimes used, "bottom" became the predominant usage by analogy of "top" and "bottom" to "up" and "down".^{[citation needed]}

The bottom quark's "bare" mass is around 4.18 GeV/c^2[3] – a bit more than four times the mass of a proton, and many orders of magnitude larger than common "light" quarks.

Although it almost exclusively transitions from or to a top quark, the bottom quark can decay into either an up quark or charm quark via the weak interaction. CKM matrix elements V_ub and V_cb specify the rates, where both these decays are suppressed, making lifetimes of most bottom particles (~10⁻¹² s) somewhat longer than those of charmed particles (~10⁻¹³ s), but shorter than those of strange particles (from ~10⁻¹⁰ to ~10⁻⁸ s).^[13]

The combination of high mass and low transition rate gives experimental collision byproducts containing a bottom quark a distinctive signature that makes them relatively easy to identify using a technique called "B-tagging". For that reason, mesons containing the bottom quark are exceptionally long-lived for their mass, and are the easiest particles to use to investigate CP violation. Such experiments are being performed at the BaBar, Belle and LHCb experiments.

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
  ϒ
  meson and χ_b(3P), the first particle discovered in LHC. These consist of a bottom quark and its antiparticle.
• Bottom baryons have been observed, and are named in analogy with strange baryons (e.g.
  Λ⁰
  _b).

• Quark model
• B-factory
• B meson

• L. Lederman (1978). "The Upsilon Particle". Scientific American. 239 (4): 72–81. Bibcode:1978SciAm.239d..72L. doi:10.1038/scientificamerican1078-72.
• R. Nave. "Quarks". HyperPhysics. Georgia State University, Department of Physics and Astronomy. Retrieved 29 June 2008.
• A. Pickering (1984). Constructing Quarks. University of Chicago Press. pp. 114–125. ISBN 978-0-226-66799-7.
• J. Yoh (1997). The Discovery of the b Quark at Fermilab in 1977: The Experiment Coordinator's Story (PDF). Proceedings of Twenty Beautiful Years of Bottom Physics. AIP Conference Proceedings. Vol. 424. pp. 29–42. Bibcode:1998AIPC..424...29Y. doi:10.1063/1.55114.
• Stone, Sheldon (1994). B Decays (2nd ed.). Syracuse University: World Scientific. doi:10.1142/1441. ISBN 978-981-02-0708-3. OCLC 636743000.

• History of the discovery of the bottom quark / Upsilon meson Archived 24 August 2017 at the Wayback Machine