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  • Top quark mass from Tevatron Electroweak Working Group2
  • Other quark masses from Particle Data Group3; these masses are given in the MS-bar scheme.
  • The quantum numbers of the top and bottom quarks are sometimes known as truth and beauty respectively, as an alternative to topness and bottomness.

Antiquarks

The additive quantum numbers of antiquarks are equal in magnitude and opposite in sign to those of the quarks. CPT symmetry forces them to have the same spin and mass as the corresponding quark. Tests of CPT symmetry cannot be performed directly on quarks and antiquarks, due to confinement, but can be performed on hadrons. Notation of antiquarks follows that of antimatter in general: An up quark is denoted by , and an anti-up quark is denoted by .

Substructure

Some extensions of the Standard Model begin with the assumption that quarks and leptons have substructure. In other words, these models assume that the elementary particles of the Standard Model are in fact composite particles, made of some other elementary constituents. Such an assumption is open to experimental tests, and these theories are severely constrained by data. At present there is no evidence for such substructure. For more details see the article on preons.

History

The notion of quarks evolved out of a classification of hadrons developed independently in 1961 by Murray Gell-Mann and Kazuhiko Nishijima, which nowadays goes by the name of the quark model. The scheme grouped together particles with isospin and strangeness using a unitary symmetry derived from current algebra, which we today recognise as part of the approximate chiral symmetry of QCD. This is a global flavor SU(3) symmetry, which should not be confused with the gauge symmetry of QCD.

In this scheme the lightest mesons (spin-0) and baryons (spin-½) are grouped together into octets, 8, of flavor symmetry. A classification of the spin-3/2 baryons into the representation 10 yielded a prediction of a new particle, Ω, the discovery of which in 1964 led to wide acceptance of the model. The missing representation 3 was identified with quarks.

This scheme was called the eightfold way by Gell-Mann, a clever conflation of the octets of the model with the eightfold way of Buddhism. He also chose the name quark and attributed it to the sentence “Three quarks for Muster Mark” in James Joyce's Finnegans Wake.4 The negative results of quark search experiments caused Gell-Mann to hold that quarks were mathematical fiction.

Analysis of certain properties of high energy reactions of hadrons led Richard Feynman to postulate substructures of hadrons, which he called partons (since they form part of hadrons). A scaling of deep inelastic scattering cross sections derived from current algebra by James Bjorken received an explanation in terms of partons. When Bjorken scaling was verified in an experiment in 1969, it was immediately realized that partons and quarks could be the same thing. With the proof of asymptotic freedom in QCD in 1973 by David Gross, Frank Wilczek, and David Politzer, the connection was firmly established.

The charm quark was postulated by Sheldon Glashow, Iliopoulos, and Maiani in 1970 to prevent unphysical flavor changes in weak decays which would otherwise occur in the standard model. The discovery in 1975 of the meson, which came to be called the J/ψ, led to the recognition that it was made of a charm quark and its antiquark.

The existence of a third generation of quarks was predicted in 1973 by Makoto Kobayashi and Toshihide Maskawa who realized that the observed violation of CP symmetry by neutral kaons could not be accommodated into the Standard Model with two generations of quarks. The bottom quark was discovered in 1977 and the top quark in 1996 at the Tevatron collider in Fermilab.

See also

Notes

  1. ↑ Summary of Top Mass Results - March 2007. Retrieved September 14, 2007.
  2. ↑ Tevatron Electroweak Working Group. Retrieved March 5, 2008.
  3. ↑ Particle Data Group. Retrieved March 5, 2008.
  4. ↑ Quark. American Heritage® Dictionary. Retrieved September 14, 2007.

References

  • Gell-Mann, Murray. 1964. A Schematic Model of Baryons and Mesons. Retrieved September 14, 2007.
  • Gribbin, John. 1998. Richard Feynman: A Life in Science. New York, NY: Plume (Penguin). ISBN 0452276314
  • Griffiths, David J. 1987. Introduction to Elementary Particles. New York, NY: Wiley. ISBN 0-471-60386-4
  • Halzen, Francis, and Alan D. Martin. 1984. Quarks and Leptons: An Introductory Course in Modern Particle Physics. New York, NY: Wiley. ISBN 0471887412
  • Povh, Bogdan. 1995. Particles and Nuclei: An Introduction to the Physical Concepts. Berlin, Germany: Springer-Verlag. ISBN 0-387-59439-6
  • Observation of the top quark at Fermilab. Retrieved September 14, 2007.
  • Particle Data Group on quarks. Retrieved September 14, 2007.

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