History of quarks

Half-length portrait of a white-haired man in his seventies talking. A painting of Beethoven is in the background.
Murray Gell-Mann at TED in 2007. Gell-Mann and George Zweig proposed the quark model in 1964.

The quark model was independently proposed by physicists Murray Gell-Mann[1] and George Zweig[2][3] in 1964.[4] The proposal came shortly after Gell-Mann's 1961 formulation of a particle classification system known as the Eightfold Way—or, in more technical terms, SU(3) flavor symmetry.[5] Physicist Yuval Ne'eman had independently developed a scheme similar to the Eightfold Way in the same year.[6][7]

At the time of the quark theory's inception, the "particle zoo" included, amongst other particles, a multitude of hadrons. Gell-Mann and Zweig posited that they were not elementary particles, but were instead composed of combinations of quarks and antiquarks. Their model involved three flavors of quarks—up, down, and strange—to which they ascribed properties such as spin and electric charge.[1][2][3] The initial reaction of the physics community to the proposal was mixed. There was particular contention about whether the quark was a physical entity or an abstraction used to explain concepts that were not properly understood at the time.[8]

In less than a year, extensions to the Gell-Mann–Zweig model were proposed. Sheldon Lee Glashow and James Bjorken predicted the existence of a fourth flavor of quark, which they called charm. The addition was proposed because it allowed for a better description of the weak interaction (the mechanism that allows quarks to decay), equalized the number of known quarks with the number of known leptons, and implied a mass formula that correctly reproduced the masses of the known mesons.[9]

In 1968, deep inelastic scattering experiments at the Stanford Linear Accelerator Center (SLAC) showed that the proton contained much smaller, point-like objects and was therefore not an elementary particle.[10][11][12] Physicists were reluctant to identify these objects with quarks at the time, instead calling them "partons"—a term coined by Richard Feynman.[13][14][15] The objects that were observed at the SLAC would later be identified as up and down quarks as the other flavors were discovered.[16] Nevertheless, "parton" remains in use as a collective term for the constituents of hadrons (quarks, antiquarks, and gluons).

The strange quark's existence was indirectly validated by the SLAC's scattering experiments: not only was it a necessary component of Gell-Mann and Zweig's three-quark model, but it provided an explanation for the kaon (K) and pion (π) hadrons discovered in cosmic rays in 1947.[17]

In a 1970 paper, Glashow, John Iliopoulos and Luciano Maiani presented further reasoning for the existence of the as-yet undiscovered charm quark.[18][19] The number of supposed quark flavors grew to the current six in 1973, when Makoto Kobayashi and Toshihide Maskawa noted that the experimental observation of CP violation[nb 1][20] could be explained if there were another pair of quarks.

Charm quarks were produced almost simultaneously by two teams in November 1974 (see November Revolution)—one at the SLAC under Burton Richter, and one at Brookhaven National Laboratory under Samuel Ting. The charm quarks were observed bound with charm antiquarks in mesons. The two parties had assigned the discovered meson two different symbols, J and ψ; thus, it became formally known as the J/ψ meson. The discovery finally convinced the physics community of the quark model's validity.[15]

In the following years a number of suggestions appeared for extending the quark model to six quarks. Of these, the 1975 paper by Haim Harari[21] was the first to coin the terms top and bottom for the additional quarks.[22]

In 1977, the bottom quark was observed by a team at Fermilab led by Leon Lederman.[23][24] This was a strong indicator of the top quark's existence: without the top quark, the bottom quark would have been without a partner. However, it was not until 1995 that the top quark was finally observed, also by the CDF[25] and DØ[26] teams at Fermilab.[4] It had a mass much greater than had been previously expected[27]—almost as great as a gold atom.[28]

Etymology

Gell-Mann originally named the quark after the sound made by ducks.[29] For some time, he was undecided on an actual spelling for the term he intended to coin, until he found the word quark in James Joyce's book Finnegans Wake: Template:Epigraph

Gell-Mann went into further detail regarding the name of the quark in his book, The Quark and the Jaguar:[30]

In 1963, when I assigned the name "quark" to the fundamental constituents of the nucleon, I had the sound first, without the spelling, which could have been "kwork". Then, in one of my occasional perusals of Finnegans Wake, by James Joyce, I came across the word "quark" in the phrase "Three quarks for Muster Mark". Since "quark" (meaning, for one thing, the cry of the gull) was clearly intended to rhyme with "Mark", as well as "bark" and other such words, I had to find an excuse to pronounce it as "kwork". But the book represents the dream of a publican named Humphrey Chimpden Earwicker. Words in the text are typically drawn from several sources at once, like the "portmanteau" words in "Through the Looking-Glass". From time to time, phrases occur in the book that are partially determined by calls for drinks at the bar. I argued, therefore, that perhaps one of the multiple sources of the cry "Three quarks for Muster Mark" might be "Three quarts for Mister Mark", in which case the pronunciation "kwork" would not be totally unjustified. In any case, the number three fitted perfectly the way quarks occur in nature.

Zweig preferred the name ace for the particle he had theorized, but Gell-Mann's terminology came to prominence once the quark model had been commonly accepted.[31]

The quark flavors were given their names for a number of reasons. The up and down quarks are named after the up and down components of isospin, which they carry.[32] Strange quarks were given their name because they were discovered to be components of the strange particles discovered in cosmic rays years before the quark model was proposed; these particles were deemed "strange" because they had unusually long lifetimes.[33] Glashow, who coproposed charm quark with Bjorken, is quoted as saying, "We called our construct the 'charmed quark', for we were fascinated and pleased by the symmetry it brought to the subnuclear world."[34] The names "top" and "bottom", coined by Harari, were chosen because they are "logical partners for up and down quarks".[21][22][33] In the past, top and bottom quarks were sometimes referred to as "truth" and "beauty" respectively, but these names have mostly fallen out of use.[35]

  1. ↑ 1.0 1.1 M. Gell-Mann (1964). "A Schematic Model of Baryons and Mesons". Physics Letters 8 (3): 214–215. doi:10.1016/S0031-9163(64)92001-3.
  2. ↑ 2.0 2.1 G. Zweig (1964). "An SU(3) Model for Strong Interaction Symmetry and its Breaking". CERN Report No.8182/TH.401. http://cdsweb.cern.ch/record/352337/files/CM-P00042883.pdf.
  3. ↑ 3.0 3.1 G. Zweig (1964). "An SU(3) Model for Strong Interaction Symmetry and its Breaking: II". CERN Report No.8419/TH.412. http://lib-www.lanl.gov/la-pubs/00323548.pdf.
  4. ↑ 4.0 4.1
  5. ↑ M. Gell-Mann (2000) [1964]. "The Eightfold Way: A theory of strong interaction symmetry". In M. Gell-Manm, Y. Ne'emann. The Eightfold Way. Westview Press. p. 11. ISBN 0-7382-0299-1.
    Original: M. Gell-Mann (1961). "The Eightfold Way: A theory of strong interaction symmetry". Synchroton Laboratory Report CTSL-20 (California Institute of Technology).
  6. ↑ Y. Ne'emann (2000) [1964]. "Derivation of strong interactions from gauge invariance". In M. Gell-Manm, Y. Ne'emann. The Eightfold Way. Westview Press. ISBN 0-7382-0299-1.
    Original Y. Ne'emann (1961). "Derivation of strong interactions from gauge invariance". Nuclear Physics 26: 222. doi:10.1016/0029-5582(61)90134-1.
  7. ↑ Companion to the History of Modern Science. Taylor & Francis. 1996. p. 673. ISBN 0415145783.
  8. ↑ A. Pickering (1984). Constructing Quarks. University of Chicago Press. pp. 114–125. ISBN 0226667995.
  9. ↑ B.J. Bjorken, S.L. Glashow (1964). "Elementary Particles and SU(4)". Physics Letters 11 (3): 255–257. doi:10.1016/0031-9163(64)90433-0.
  10. ↑
  11. ↑
  12. ↑ J.I. Friedman. "The Road to the Nobel Prize". Hue University. Retrieved 2008-09-29.
  13. ↑ R.P. Feynman (1969). "Very High-Energy Collisions of Hadrons". Physical Review Letters 23 (24): 1415–1417. doi:10.1103/PhysRevLett.23.1415.
  14. ↑ S. Kretzer et al. (2004). "CTEQ6 Parton Distributions with Heavy Quark Mass Effects". Physical Review D 69 (11): 114005. doi:10.1103/PhysRevD.69.114005. Template:ArXiv.
  15. ↑ 15.0 15.1 D.J. Griffiths (1987). Introduction to Elementary Particles. John Wiley & Sons. p. 42. ISBN 0-471-60386-4.
  16. ↑ M.E. Peskin, D.V. Schroeder (1995). An introduction to quantum field theory. Addison–Wesley. p. 556. ISBN 0-201-50397-2.
  17. ↑ V.V. Ezhela (1996). Particle physics. Springer. p. 2. ISBN 1563966425.
  18. ↑ S.L. Glashow, J. Iliopoulos, L. Maiani (1970). "Weak Interactions with Lepton–Hadron Symmetry". Physical Review D 2 (7): 1285–1292. doi:10.1103/PhysRevD.2.1285.
  19. ↑ D.J. Griffiths (1987). Introduction to Elementary Particles. John Wiley & Sons. p. 44. ISBN 0-471-60386-4.
  20. ↑ M. Kobayashi, T. Maskawa (1973). "CP-Violation in the Renormalizable Theory of Weak Interaction". Progress of Theoretical Physics 49 (2): 652–657. doi:10.1143/PTP.49.652. http://ptp.ipap.jp/link?PTP/49/652/pdf.
  21. ↑ 21.0 21.1 H. Harari (1975). "A new quark model for hadrons". Physics Letters B 57B: 265. doi:10.1016/0370-2693(75)90072-6.
  22. ↑ 22.0 22.1 K.W. Staley (2004). The Evidence for the Top Quark. Cambridge University Press. pp. 31–33. ISBN 9780521827102. http://books.google.com/?id=K7z2oUBzB_wC.
  23. ↑ S.W. Herb et al. (1997). "Observation of a Dimuon Resonance at 9.5 GeV in 400-GeV Proton-Nucleus Collisions". Physical Review Letters 39: 252. doi:10.1103/PhysRevLett.39.252.
  24. ↑ M. Bartusiak (1994). A Positron named Priscilla. National Academies Press. p. 245. ISBN 0309048931.
  25. ↑ F. Abe et al. (CDF Collaboration) (1995). "Observation of Top Quark Production in pp Collisions with the Collider Detector at Fermilab". Physical Review Letters 74: 2626–2631. doi:10.1103/PhysRevLett.74.2626.
  26. ↑ S. Abachi et al. (DØ Collaboration) (1995). "Search for High Mass Top Quark Production in pp Collisions at Template:Radical = 1.8 TeV". Physical Review Letters 74: 2422–2426. doi:10.1103/PhysRevLett.74.2422.
  27. ↑ K.W. Staley (2004). The Evidence for the Top Quark. Cambridge University Press. p. 144. ISBN 0521827108. http://books.google.com/?id=K7z2oUBzB_wC.
  28. ↑ "New Precision Measurement of Top Quark Mass". Brookhaven National Laboratory News. Retrieved 2008-09-24.
  29. ↑ J. Gribbin, M. Gribbin (1997). Richard Feynman: A Life in Science. Penguin Books. p. 194. ISBN 0-452-27631-4.
  30. ↑ M. Gell-Mann (1995). The Quark and the Jaguar: Adventures in the Simple and the Complex. Henry Holt and Co. p. 180. ISBN 978-0805072532.
  31. ↑ J. Gleick (1992). Genius: Richard Feynman and modern physics. Little Brown and Company. p. 390. ISBN 0-316-903167.
  32. ↑ J.J. Sakurai (1994). S.F Tuan. ed. Modern Quantum Mechanics (Revised ed.). Addison–Wesley. p. 376. ISBN 0-201-53929-2.
  33. ↑ 33.0 33.1 D.H. Perkins (2000). Introduction to high energy physics. Cambridge University Press. p. 8. ISBN 0521621968.
  34. ↑ M. Riordan (1987). The Hunting of the Quark: A True Story of Modern Physics. Simon & Schuster. p. 210. ISBN 9780671504663.
  35. ↑ F. Close (2006). The New Cosmic Onion. CRC Press. p. 133. ISBN 1584887982.
  1. ↑ CP violation is a phenomenon which causes weak interactions to behave differently when left and right are swapped (P symmetry) and particles are replaced with their corresponding antiparticles (C symmetry).
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