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Bottom quark

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Bottom quark
CompositionElementary particle
StatisticsFermionic
FamilyQuark
GenerationThird
InteractionsStrong, Weak, Electromagnetic force, Gravity
Symbol
b
AntiparticleBottom antiquark (
b
)
TheorizedMakoto Kobayashi and Toshihide Maskawa (1973)[1]
DiscoveredLeon M. Lederman et al. (1977)[2]
Mass4.19+0.18
−0.06 GeV/c2 (MS scheme)[3]
4.67+0.18
−0.06 GeV/c2 (1S scheme)[3]
Decays intoCharm quark, up quark
Electric charge−1⁄3 e
Color chargeYes
Spin1⁄2
Weak isospinLH: −1⁄2, RH: 0
Weak hyperchargeLH: 1⁄3, RH: −2⁄3

The bottom 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 the quantum chromodynamics, the bottom quark's large bare mass (around 4200 MeV/c2,[3] a bit more than four times the mass of a proton), combined with low values of the CKM matrix elements Vub and Vcb, 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 and Belle experiments. 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 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 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

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    . 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. ^ a b 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. ^ a b "Discoveries at Fermilab - Discovery of the Bottom Quark" (Press release). Fermilab. 7 August 1997. Retrieved 24 July 2009.
  3. ^ a b c K. Nakamura et al. (Particle Data Group) (2011). "PDGLive Particle Summary 'Quarks (u, d, s, c, b, t, b', t', Free)'" (PDF). Particle Data Group. Retrieved 8 August 2011.
  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 9780521827102.
  6. ^ L.M. Lederman (2005). "Logbook: Bottom Quark". Symmetry Magazine. 2 (8).
  7. ^ S.W. Herb; 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. {{cite journal}}: Explicit use of et al. in: |author= (help)
  8. ^ 2008 Physics Nobel Prize lecture by Makoto Kobayashi
  9. ^ 2008 Physics Nobel Prize lecture by Toshihide Maskawa

Further reading

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