Bottom quark

The bottom quark or b quark, also known as the beauty quark, is a third-generation quark with a charge of −1/3 e.

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+0.04 −0.03 GeV/c^2 (MS scheme)[3] 4.65+0.03 −0.03 GeV/c^2 (1S scheme)[4]
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

All quarks are described in a similar way by electroweak 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.

Name and history

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 bottom quark was discovered in 1977 by the Fermilab E288 experiment team led by Leon M. Lederman, when collisions produced bottomonium.[2][7][8] Kobayashi and Maskawa won the 2008 Nobel Prize in Physics for their explanation of CP-violation.[9][10]

On its discovery, there were efforts to name the bottom quark "beauty", but "bottom" became the predominant usage, by analogy of "top" and "bottom" to "up" and "down".

Distinct character

The bottom quark's "bare" mass is around 4.18 GeV/c²[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 higher than those of charmed particles (~10⁻¹³ s), but lower than those of strange particles (from ~10⁻¹⁰ to ~10⁻⁸ s).[11]

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.

Hadrons containing bottom quarks

Main articles: list of baryons and list of mesons

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
• B-factory
• B meson

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 24 July 2009.
3. M. Tanabashi et al. (Particle Data Group) (2018). "Review of Particle Physics". Physical Review D. 98 (3): 030001. Bibcode:2018PhRvD..98c0001T. doi:10.1103/PhysRevD.98.030001.
4. 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. Archived from the original (PDF) on 12 May 2013. Retrieved 18 December 2012.
5. H. Harari (1975). "A new quark model for hadrons". Physics Letters B. 57 (3): 265–269. Bibcode:1975PhLB...57..265H. doi:10.1016/0370-2693(75)90072-6.
6. K.W. Staley (2004). The Evidence for the Top Quark. Cambridge University Press. pp. 31–33. ISBN 978-0-521-82710-2.
7. L.M. Lederman (2005). "Logbook: Bottom Quark". Symmetry Magazine. 2 (8). Archived from the original on 4 October 2006.
8. 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.
9. 2008 Physics Nobel Prize lecture by Makoto Kobayashi
10. 2008 Physics Nobel Prize lecture by Toshihide Maskawa
11. Nave, C. R. "Transformation of Quark Flavors by the Weak Interaction". HyperPhylsics.

Further reading

• 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. 424: 29–42. Bibcode:1998AIPC..424...29Y. doi:10.1063/1.55114. Retrieved 24 July 2009.

External links

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