Pron

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Preons (English Preons ; sometimes also referred to as pre-quarks or sub-quarks ) are hypothetical point-like particles in elementary particle physics , which, in extension of the quark structure of the Standard Model, are viewed as "building blocks" of leptons or quarks .

The concept comes from Abdus Salam and Jogesh Pati (1974) and is often related to extended gauge theories ( Technicolor ). So far (as of 2016) there is no experimental evidence of the structure of quarks and leptons from smaller particles.

Bulk

Behind the idea of ​​the existence of preons is the question of “even more elementary” building blocks of matter than leptons and quarks. One aim of the pron models is also to explain unsolved questions of the Standard Model, such as why there are three generations or how the masses of the particles come about and, in general, the definition of the parameters of the Standard Model. In many Präonenmodellen is Higgs boson also composed.

Today no substructure of quarks or leptons has been found up to distances from . This creates major problems with regard to the acceptance of preon models. The limit of for confinement lengths of the preons leads, according to the uncertainty principle, to enormous energies of the enclosed particles (order of magnitude 100 G eV ). But an electron only has a mass of 0.5 MeV and the light quarks only a few MeV, not to mention the neutrino with almost vanishing mass. The energy of the preons as part of the particles of the Standard Model would probably have to be much higher, because their mass must also be high as they have not yet been observed in the LHC (with energies in the TeV range). A dynamic theory of preons would have to explain why the systems made up of preons have such small masses, and that for a large number of possible bound states. The problem does not arise when considering the composition of hadrons from quarks, here the enclosed quarks also have high energies (in the order of 100 MeV), but the mass of the entire system is comparable (a proton has a mass of around 1 GeV). The same consideration also makes it unlikely to receive the higher generations as excited states of the first generation in pre-son models, since the excitation energies would also have orders of magnitude in the region of 100 GeV and would therefore be much too high (factor 1000). According to Harari, a previously unknown symmetry is necessary to explain this problem (such as an unbroken chiral symmetry).

Rishon model

An example is the Rishon model by Haim Harari and (independently) Michael A. Shupe.

In the Rishon model there are two fundamental fermions, the T with a third-digit positive electric charge and the V, which is electrically neutral. Rishon means primarily or originally in Hebrew, T stands for Tohu ( unformed , "desolate") and V for Vohu (Bohu), emptiness , from the Hebrew Genesis (see Tohuwabohu ).

Leptons and quarks consist of three rishons or anti-rishons, but no rishons can appear together with antirishons. That gives 16 possibilities (without antiparticles) and especially the quarks, antiquarks (each with three colors) and leptons of the first of the three generations of the standard model:

  • TTT = anti-electron
  • VVV = electron neutrino
  • TTV, TVT, VTT = Up-Quark (with three colors)
  • TVV, VTV and VVT = Down-Antiquark (with three colors)

and accordingly the antiparticles from the anti-rishons. The T can have one of three colors, the V one of three anti-colors, so that the leptons (TTT, VVV and antiparticles) can be made colorless, but the particles that contain V as well as T cannot. Because of the assignment of electrical charge to the rishons, there is also a relationship between color and electrical charge, which is not explained in the standard model (particles with a whole-number electrical charge are color-neutral, those with a third-number electrical charge are not).

Under certain conditions, pre-son models make it possible to explain the existence of three generations of particles in the standard model. In the simplest case, the particles of the second and third generation would be excited states of the first generation, but from the same components. In detail, however, there are difficulties here and, for example, in the Rishon model this does not work. Harari and Seiberg therefore pursued the idea of ​​explaining the higher generations by the addition of a compound particle made up of a preon and an anti-preon. Similarly, a Higgs-like scalar particle could also be added to describe the differences between the generations.

Other models

Another model would be the original model by Pati and Salam from 1974 (expanded by them in various forms with John Strathdee). In such and similar models one could introduce prons that indicate the generations (called somons), those for color (four chromons, corresponding to the three color degrees of freedom and one for neutrality), and electrical charge (flavons, for example one with an electrical charge +1/2, one -1/2). A particle is made up of one flavone, one chromone and one somon. There are a total of 24 combinations for the quarks and leptons of the three generations of the standard model. For the correct assignment, however, one generally has to refrain from restricting quantum numbers such as electrical charge to only one type of pron (and, for example, distributing electrical charge between flavons and chromons). Variants come from Hidezumi Terazawa, Yoichi Chikachige and Keiichi Akama (University of Tokyo) and Oscar Wallace Greenberg and Joseph Sucher (University of Maryland) , among others .

literature

  • Harari, The structure of quarks and leptons, Scientific American, Volume 248, 1983, Volume 4, pp. 56-68
  • IA D'Souza, CS Kalman: Preons: Models of Leptons, Quarks and Gauge Bosons as Composite Objects. World Scientific 1992. ISBN 978-981-02-1019-9

Individual evidence

  1. ↑ Pati, Salam: Lepton number as the fourth “color”, Physical Review D 10, 1974, pp. 275-289, Erratum: Physical Review D 11, 1975, p. 703
  2. ↑ Preon models aroused new interest at times when the CDF collaboration at Fermilab (which analyzed their data after the discovery of the top quark) in 1996 believed they had found indications of a sub-quark structure (F. Abe et al., Measurement of dijet angular distributions at CDF, Phys. Rev. Lett., Volume 55, 1996, pp. 5336-5341, abstract ), but this could not be confirmed by other experiments.
  3. ↑ For example, this is the limit that results from the very good agreement between calculations of quantum electrodynamics and the magnetic moment of the electron assumed to be punctiform
  4. ^ Harari, Scientific American, April 1983, p. 67
  5. ↑ Harari, A schematic model of quarks and leptons, Physics Letters B 86, 1979, pp. 83-86
  6. ↑ Harari, Nathan Seiberg : The Rishon Model, Nuclear Physics B 204, 1982, p. 141
  7. ^ Harari, The Structure of Quarks and Leptons, Scientific American, Volume 248, April 1983, p. 56
  8. ↑ Shupe: A composite model of leptons and quarks, Physics Letters B 86, 1979, pp. 87-92
  9. ↑ Representation based on Harari, Scientific American, 1983. Harari calls such models the actual preon models and the general case prequark (including his own Rishon model).
  10. ↑ Terazawa, Chikachige, Akama: A unified model of the Nambu-Jona-Lasinio-type for all elementary particle forces, Phys. Rev. D. Vol. 15, 1977, pp. 480-487
  11. ↑ Terazawa, Subquark model of leptons and quarks, Phys. Rev. D, Volume 22, 1980, pp. 184-199
  12. ↑ Terazawa, Algebra of subquark charges, Progr. Theor. Phys., Vol. 64, 1980, pp. 1763-1771
  13. ↑ Greenberg, Sucher, A quantum structuredynamic model of quarks, leptons, weak vector bosons and Higgs mesons, Phys. Lett. B, Vol. 99, 1981, pp. 339-343
  14. ↑ Greenberg came in 1975 (University of Maryland Preprint) after Pati, Salam, Strathdee, Are quarks composite?, Phys. Lett. B, Volume 59, 1975, pp. 265-268, independent of Pati and Salam on Préon Ideas.