Preon

In particle physics, preons are postulated "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 some proposed models were ruled out by collider experiments and no model was able to predict a new experimental result.

Background: The urge to simplify the Standard Model

Before the Standard Model (SM) was developed in the 1970s (the key elements of the standard model known as quarks were proposed by Gell-Mann and 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 types of particles. One of these, the quarks, has six different kinds, of which there are three varieties in each (dubbed "colors", red, green, and blue, giving rise to QCD: 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 tauon. 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 the photons, W⁺, W⁻, and Z particles, gluons, and a few open spaces left for the graviton and Higgs boson, which have not yet been discovered. Almost all of these particles come in "left-handed" and "right-handed" versions (see " chirality").

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. Although the mass of each successive particle follows certain patterns, predictions of the rest mass of most particles can not be made precisely. The Higgs boson is assumed to "solve" this problem, but to date the Higgs mechanism remains unproven.

The Model also has problems predicting the large scale structure of the universe. For instance, the Model generally predicts equal amounts of matter and anti-matter in the universe, something that is observably not the case. 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.

Preon theory 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.

Theoretical particle physics considerations to research preon theory

Preon research is motivated by the desire to explain already existing 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.
*The second and third generation fermions are supposedly fundamental, yet they have higher masses than those of the first generation, and the quarks are unstable and decay into their first generation counterparts. Historically, the instability and radioactivity of some chemical elements were explained in terms of isotopes. By analogy this suggests a more fundamental structure for at least some fermions. [http://arxiv.org/abs/hep-ph/0411313]
*To give prediction for parameters that are otherwise 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 explain the number of generations of fermions.
*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, or to predict that the Large Hadron Collider will not observe a Higgs boson or superpartners.
*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).

History: Pre-quark theories

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", and "Y-particles" [http://uk.arxiv.org/abs/physics/0609185] . "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 [http://ccdb3fs.kek.jp/cgi-bin/img_index?7905333] and Shupe, a 1981 paper by Frizsch and Mandelbaum, a 1992 paper by D'Souza and Kalman, and a 1997 paper by Larson [http://www.dipmat.unipg.it/~bartocci/fis/larson2.htm] . None has gained wide acceptance in the physics world.

Each of the preon models postulates a set of far 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 Harari Rishon Model illustrates some of the typical efforts in the field.

Many of the Preon models theorize that the apparent imbalance of matter and anti-matter in the universe is in fact illusory, with large quantities of preon level anti-matter 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 predicts that the Higgs boson does not exist. In the above cited paper, Fredricksson acknowledges the mass paradox represents a problem in his accounting for neutrino mass; however, he proposes a specific arrangement of preons in his model, which he calls the X-quark, which his theory suggests could be a stable good cold, dark matter candidate.

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".

Theoretical objections to preon theories

The mass paradox

Heisenberg's uncertainty principle states that ΔxΔp ≥ ħ/2 and thus anything confined to a box smaller than Δx would have a momentum of uncertainty proportionally greater. Some candidate preon models propose particles smaller than the elementary particles they make up, therefore, the momentum of uncertainty Δp should be greater than the particles themselves.

One preon model started as an internal paper at the Collider Detector at Fermilab (CDF) around 1994. The paper was written after the occurrence of an unexpected and inexplicable excess of jets with energies above 200 GeV were detected in the 1992—1993 running period.

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, 50,000 times larger than the rest mass of an up-quark and 400,000 times larger than the rest mass of an electron.

Thus, 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.

Chirality and the 't Hooft anomaly-matching constraints

Any candidate preon theory must address particle chirality and the 't Hooft anomaly-matching constraints, and would ideally be more parsimonious in theoretical structure than the Standard Model itself.

Possible manner of experimental falsification

Often, 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.

For example, should the LHC observe a Higgs boson, or superpartners, or both, the observation would be in conflict with the predictions of many preon models with respect to the existence of the Higgs boson.

By contrast, should a Higgs boson not appear in the increasingly constrained circumstances where the Standard Model predict that it will be found, the preon theory would receive a significant theoretical boost, while many competing theories would be falsified.

Rishon model

See additional discussion about preons in the article on the Rishon Model.

Preons in popular culture

* In the 1948 reprint/redit of his 1930 novel "Skylark Three", E. E. Smith postulated a series of 'subelectrons of the first and second type' with the latter being material properties that corresponded to gravitation. 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 elementary particles.
* In the 1931 Olaf Stapledon science fiction novel "Last and First Men", a history of the future development of the human race for the next one billion years, the highly advanced future civilizations of the "Fifth Men" and the "Eighteenth Men" are powered by what Stapledon calls "sub-etheric energy", i.e., the systematic interaction and mutual annihilation of subatomic particles.
* In the novel version of the 1982 motion picture "", 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).

ee also

*Preon star
*Preon-degenerate matter
*Harari Rishon Model

References

* Pati, J. C.; Salam, A. (1974); [http://prola.aps.org/abstract/PRD/v10/i1/p275_1 "Lepton number as the fourth "color"] , Phys. Rev. D10, 275-289

External links

* [http://www.nature.com/news/2007/071130/full/news.2007.292.html Splitting the quark] , Nature, "30 November 2007"