In particle physics, preons are "point-like" particles, conceived to be subcomponents of quarks and leptons. The word was coined by Jogesh Pati and Abdus Salam in 1974. Interest in preon models peaked in the 1980s but has slowed as the Standard Model of particle physics continues to describe the physics mostly successfully, and no direct experimental evidence for lepton and quark compositeness has been found.
Note that in the hadronic sector there are some intriguing open questions and some effects considered anomalies within the Standard Model. For example, four very important open questions are the proton spin puzzle, the EMC effect, the distributions of electric charges inside the nucleons as found by Hofstadter in 1956, and the ad hoc CKM matrix elements.
Before the Standard Model (SM) was developed in the 1970s (the key elements of the Standard Model known as quarks were proposed by Murray Gell-Mann and George Zweig in 1964), physicists observed hundreds of different kinds of particles in particle accelerators. These were organized into relationships on their physical properties in a largely ad-hoc system of hierarchies, not entirely unlike the way taxonomy grouped animals based on their physical features. Not surprisingly, the huge number of particles was referred to as the "particle zoo".
The Standard Model, which is now the prevailing model of particle physics, dramatically simplified this picture by showing that most of the observed particles were mesons, which are combinations of two quarks, or baryons which are combinations of three quarks, plus a handful of other particles. The particles being seen in the ever-more-powerful accelerators were, according to the theory, typically nothing more than combinations of these quarks.
Within the Standard Model, there are several different classes of particles. One of these, the quarks, has six different types, of which there are three varieties in each (dubbed "colors", red, green, and blue, giving rise to quantum chromodynamics). Additionally, there are six different types of what are known as leptons. Of these six leptons, there are three charged particles: the electron, muon, and tau. The neutrinos comprise the other three leptons, and for each neutrino there is a corresponding member from the other set of three leptons. In the Standard Model, there are also bosons, including the photons; W+, W−, and Z bosons; gluons and the Higgs boson; and an open space left for the graviton. Almost all of these particles come in "left-handed" and "right-handed" versions (see chirality). The quarks, leptons and W boson all have antiparticles with opposite electric charge.
The Standard Model also has a number of problems which have not been entirely solved. In particular, no successful theory of gravitation based on a particle theory has yet been proposed. Although the Model assumes the existence of a graviton, all attempts to produce a consistent theory based on them have failed. Additionally, mass remains a mystery in the Standard Model.
Kalman  observes that, according to the concept of atomism, fundamental building blocks of nature are indivisible bits of matter that are ungenerated and indestructible. Quarks are not truly indestructible, since some can decay into other quarks. Thus, on fundamental grounds, quarks are not themselves fundamental building blocks but must be composed of other, fundamental quantities—preons. Although the mass of each successive particle follows certain patterns, predictions of the rest mass of most particles cannot be made precisely, except for the masses of almost all baryons which have been recently described very well by the model of de Souza. The Higgs boson explains why particles show inertial mass (but does not explain rest mass).
The Standard Model also has problems predicting the large scale structure of the universe. For instance, the SM generally predicts equal amounts of matter and antimatter in the universe. A number of attempts have been made to "fix" this through a variety of mechanisms, but to date none have won widespread support. Likewise, basic adaptations of the Model suggest the presence of proton decay, which has not yet been observed.
Preon theory is motivated by a desire to replicate the achievements of the periodic table, and the later Standard Model which tamed the "particle zoo", by finding more fundamental answers to the huge number of arbitrary constants present in the Standard Model. It is one of several models to have been put forward in an attempt to provide a more fundamental explanation of the results in experimental and theoretical particle physics. The preon model has attracted comparatively little interest to date among the particle physics community.
Preon research is motivated by the desire to explain already known facts (retrodiction), which include
- To reduce the large number of particles, many that differ only in charge, to a smaller number of more fundamental particles. For example, the electron and positron are identical except for charge, and preon research is motivated by explaining that electrons and positrons are composed of similar preons with the relevant difference accounting for charge. The hope is to reproduce the reductionist strategy that has worked for the periodic table of elements.
- To explain the three generations of fermions.
- To calculate parameters that are currently unexplained by the Standard Model, such as particle masses, electric charges, and color charges, and reduce the number of experimental input parameters required by the Standard Model.
- To provide reasons for the very large differences in energy-masses observed in supposedly fundamental particles, from the electron neutrino to the top quark.
- To provide alternative explanations for the electro-weak symmetry breaking without invoking a Higgs field, which in turn possibly needs a supersymmetry to correct the theoretical problems involved with the Higgs field. Supersymmetry itself has theoretical problems.
- To account for neutrino oscillation and mass.
- The desire to make new nontrivial predictions, for example, to provide possible cold dark matter candidates.
- To explain why there exists only the observed variety of particle species and not something else and to reproduce only these observed particles (since the prediction of non-observed particles is one of the major theoretical problems, as, for example, with supersymmetry).
A number of physicists have attempted to develop a theory of "pre-quarks" (from which the name preon derives) in an effort to justify theoretically the many parts of the Standard Model that are known only through experimental data.
Other names which have been used for these proposed fundamental particles (or particles intermediate between the most fundamental particles and those observed in the Standard Model) include prequarks, subquarks, maons, alphons, quinks, rishons, tweedles, helons, haplons, Y-particles, and primons. Preon is the leading name in the physics community.
Efforts to develop a substructure date at least as far back as 1974 with a paper by Pati and Salam in Physical Review. Other attempts include a 1977 paper by Terazawa, Chikashige and Akama, similar, but independent, 1979 papers by Ne'eman, Harari, and Shupe, a 1981 paper by Fritzsch and Mandelbaum, and a 1992 book by D'Souza and Kalman. None of these has gained wide acceptance in the physics world. However, in a recent work de Souza has shown that his model describes well all weak decays of hadrons according to selection rules dictated by a quantum number derived from his compositeness model. In his model leptons are elementary particles and each quark is composed of two primons, and thus, all quarks are described by four primons. Therefore, there is no need for the Standard Model Higgs boson and each quark mass is derived from the interaction between each pair of primons by means of three Higgs-like bosons. In his 1989 Nobel Prize acceptance lecture, Hans Dehmelt described a most fundamental elementary particle, with definable properties, which he called the cosmon, as the likely end result of a long but finite chain of increasingly more elementary particles.
Each of the preon models postulates a set of fewer fundamental particles than those of the Standard Model, together with the rules governing how those fundamental particles operate. Based on these rules, the preon models try to explain the Standard Model, often predicting small discrepancies with this model and generating new particles and certain phenomena, which do not belong to the Standard Model. The Rishon model illustrates some of the typical efforts in the field.
Many of the preon models theorize that the apparent imbalance of matter and antimatter in the universe is in fact illusory, with large quantities of preon level antimatter confined within more complex structures.
Many preon models either do not account for the Higgs boson or rule it out, and propose that electro-weak symmetry is broken not by a scalar Higgs field but by composite preons. For example, Fredriksson preon theory does not need the Higgs boson, and explains the electro-weak breaking as the rearrangement of preons, rather than a Higgs-mediated field. In fact, Fredriksson preon model and de Souza model predict that the Standard Model Higgs boson does not exist.
When the term "preon" was coined, it was primarily to explain the two families of spin-1/2 fermions: leptons and quarks. More-recent preon models also account for spin-1 bosons, and are still called "preons".
The rishon model (RM) is the earliest effort to develop a preon model to explain the phenomenon appearing in the Standard Model (SM) of particle physics. It was first developed by Haim Harari and Michael A. Shupe (independently of each other), and later expanded by Harari and his then-student Nathan Seiberg.
The model has two kinds of fundamental particles called rishons (which means "primary" in Hebrew). They are T ("Third" since it has an electric charge of ⅓ e, or Tohu which means "unformed" in Hebrew Genesis) and V ("Vanishes", since it is electrically neutral, or Vohu. [Bohu means "void" in the Hebrew Tanakh (the Old Testament), though bohu may be pronouced as vohu by modern Israelis when the "b" is preceded by a vowel and thus lacks dagesh.] All leptons and all flavours of quarks are three-rishon ordered triplets. These groups of three rishons have spin-½.
The mass paradox
One preon model started as an internal paper at the Collider Detector at Fermilab (CDF) around 1994. The paper was written after an unexpected and inexplicable excess of jets with energies above 200 GeV were detected in the 1992–1993 running period. However, scattering experiments have shown that quarks and leptons are "pointlike" down to distance scales of less than 10−18 m (or 1/1000 of a proton diameter). The momentum uncertainty of a preon (of whatever mass) confined to a box of this size is about 200 GeV/c, 50,000 times larger than the rest mass of an up-quark and 400,000 times larger than the rest mass of an electron.
Heisenberg's uncertainty principle states that ΔxΔp ≥ ħ/2 and thus anything confined to a box smaller than Δx would have a momentum uncertainty proportionally greater. Thus, the preon model proposed particles smaller than the elementary particles they make up, since the momentum uncertainty Δp should be greater than the particles themselves. And so the preon model represents a mass paradox: How could quarks or electrons be made of smaller particles that would have many orders of magnitude greater mass-energies arising from their enormous momenta? This paradox is resolved by postulating a large binding force between preons cancelling their mass-energies.
Any candidate preon theory must address particle chirality[why?] and the 't Hooft Chiral anomaly constraints[why?], and would ideally have simpler theoretical structure than the Standard Model itself.[why?]
Conflicts with observed physics
Preon models propose additional unobserved forces or dynamics to account for the observed properties of elementary particles, which may have implications in conflict with observation.
Preon theories require that quarks and electrons should have a finite size. It is possible that the Large Hadron Collider will observe this when raised to higher energies.
- In the 1948 reprint/edit of his 1930 novel Skylark Three, E. E. Smith postulated a series of 'subelectrons of the first and second type' with the latter being fundamental particles that were associated with the gravitation force. While this may not have been an element of the original novel (the scientific basis of some of the other novels in the series was revised extensively due to the additional eighteen years of scientific development), even the edited publication may be the first, or one of the first, mentions of the possibility that electrons are not fundamental particles.
- In the novelized version of the 1982 motion picture Star Trek II: The Wrath of Khan, written by Vonda McIntyre, two of Dr. Carol Marcus' Genesis project team, Vance Madison and Delwyn March, have studied sub-elementary particles they've named "boojums" and "snarks", in a field they jokingly call "kindergarten physics" because it is lower than "elementary" (analogy to school levels).
- James P. Hogan's novel Voyage from Yesteryear discussed preons (called tweedles), the physics of which became central to the plot. Hogan's "tweedle" physics was patently derived from the Rishon model.
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- Have We Hit Bottom Yet?- an article about preons and minuteness