Bottom quark

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

Bottom quark
Compositionelementary particle
Statisticsfermionic
Familyquark
Generationthird
Interactionsstrong, weak, electromagnetic, gravity
Symbol
b
Antiparticlebottom antiquark (
b
)
TheorizedMakoto Kobayashi and Toshihide Maskawa (1973)[1]
DiscoveredLeon M. Lederman et al. (1977)[2]
Mass4.18+0.04
−0.03 GeV/c2 (MS scheme)[3]
4.65+0.03
−0.03 GeV/c2 (1S scheme)[4]
Decays intocharm quark or
up quark
Electric charge−1/3 e
Color chargeyes
Spin1/2
Weak isospinLH: −1/2, RH: 0
Weak hyperchargeLH: 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]

While the name "beauty" is sometimes used, "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 Vub and Vcb 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).[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 χb(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

References
  1. Kobayashi, M.; Maskawa, T. (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. hdl:2433/66179.
  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. Harari, H. (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. Staley, K. W. (2004). The Evidence for the Top Quark. Cambridge University Press. pp. 31–33. ISBN 978-0-521-82710-2.
  7. Lederman, L. M. (2005). "Logbook: Bottom Quark". Symmetry Magazine. 2 (8). Archived from the original on 4 October 2006.
  8. Herb, S. W.; 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. OSTI 1155396.
  9. 2008 Physics Nobel Prize lecture by Makoto Kobayashi
  10. 2008 Physics Nobel Prize lecture by Toshihide Maskawa
  11. Nave, C.R. (ed.). "Transformation of Quark Flavors by the Weak Interaction". Department of Physics and Astronomy. HyperPhysics. Atlanta, GA: Georgia State University.

Further reading
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