Bottom quark

Bottom quark

Composition	Elementary particle
Statistics	Fermionic
Generation	Third
Interactions	Strong, Weak, Electromagnetic force, Gravity
Symbol	b
Antiparticle	Bottom antiquark ( b )
Theorized	Makoto Kobayashi and Toshihide Maskawa (1973)[1]
Discovered	Leon M. Lederman et al. (1977)[2]
Mass	4.18+0.03 −0.03 GeV/c^2 (MS scheme)[3] 4.65+0.03 −0.03 GeV/c^2 (1S scheme)[3]
Decays into	Charm quark, or up quark
Electric charge	−1/3 e
Color charge	Yes
Spin	1/2
Weak isospin	LH: −1/2, RH: 0
Weak hypercharge	LH: 1/3, RH: −2/3

The bottom quark or b quark, also known as the beauty quark, is a third-generation quark with a charge of −1/3 e. Although all quarks are described in a similar way by quantum chromodynamics, the bottom quark's large bare mass (around 4.2 GeV/c²,^[3] 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 the BaBar, Belle and LHCb experiments.

The bottom quark is also notable because it is a product in almost all top quark decays, and is a frequent decay product for the Higgs boson. The bottom quark was theorized in 1973 by physicists Makoto Kobayashi and Toshihide Maskawa to explain CP violation.^[1] The name "bottom" was introduced in 1975 by Haim Harari.^[4][5] The bottom quark was discovered in 1977 by the Fermilab E288 experiment team led by Leon M. Lederman, when collisions produced bottomonium.^[2][6][7] Kobayashi and Maskawa won the 2008 Nobel Prize in Physics for their explanation of CP-violation.^[8][9] On its discovery, there were efforts to name the bottom quark "beauty", but "bottom" became the predominant usage.

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

See also

• Quark model

References

1. M. Kobayashi; T. Maskawa (1973). "CP-Violation in the Renormalizable Theory of Weak Interaction". Progress of Theoretical Physics. 49 (2): 652–657. Bibcode:1973PThPh..49..652K. doi:10.1143/PTP.49.652.
2. "Discoveries at Fermilab – Discovery of the Bottom Quark" (Press release). Fermilab. 7 August 1977. Retrieved 2009-07-24.
3. J. Beringer (Particle Data Group); et al. (2012). "PDGLive Particle Summary 'Quarks (u, d, s, c, b, t, b′, t′, Free)'" (PDF). Particle Data Group. Retrieved 2012-12-18.
4. H. Harari (1975). "A new quark model for hadrons". Physics Letters B. 57 (3): 265. Bibcode:1975PhLB...57..265H. doi:10.1016/0370-2693(75)90072-6.
5. K.W. Staley (2004). The Evidence for the Top Quark. Cambridge University Press. pp. 31–33. ISBN 978-0-521-82710-2.
6. L.M. Lederman (2005). "Logbook: Bottom Quark". Symmetry Magazine. 2 (8). Archived from the original on 4 October 2006.
7. S.W. Herb; Hom, D.; Lederman, L.; Sens, J.; Snyder, H.; Yoh, J.; Appel, J.; Brown, B.; Brown, C.; Innes, W.; Ueno, K.; Yamanouchi, T.; Ito, A.; Jöstlein, H.; Kaplan, D.; Kephart, R.; et al. (1977). "Observation of a Dimuon Resonance at 9.5 GeV in 400-GeV Proton-Nucleus Collisions". Physical Review Letters. 39 (5): 252. Bibcode:1977PhRvL..39..252H. doi:10.1103/PhysRevLett.39.252.
8. 2008 Physics Nobel Prize lecture by Makoto Kobayashi
9. 2008 Physics Nobel Prize lecture by Toshihide Maskawa

Further reading

• L. Lederman (1978). "The Upsilon Particle". Scientific American. 239 (4): 72. doi:10.1038/scientificamerican1078-72.
• R. Nave. "Quarks". HyperPhysics. Georgia State University, Department of Physics and Astronomy. Retrieved 2008-06-29.
• A. Pickering (1984). Constructing Quarks. University of Chicago Press. pp. 114–125. ISBN 0-226-66799-5.
• 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. Fermilab. Retrieved 2009-07-24.

External links

• History of the discovery of the bottom quark / Upsilon meson