In particle physics, preons are hypothetical point particles, conceived of as sub-components 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 physics mostly successfully, and no direct experimental evidence for lepton and quark compositeness has been found. Preons come in four varieties: plus, anti-plus, zero, and anti-zero. W bosons have six preons, and quarks and leptons have only three.
In the hadronic sector, some effects are considered anomalies within the Standard Model. For example, 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.
When the term "preon" was coined, it was primarily to explain the two families of spin- fermions: quarks and leptons. More recent preon models also account for spin-1 bosons, and are still called "preons". 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 combine and interact. 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.
Goals of preon models
Preon research is motivated by the desire to: Reduce the large number of particles, many that differ only in charge, to a smaller number of more fundamental particles. For example, the down quark and up quark are nearly identical except for charge, and a slight mass difference; preon research is motivated by explaining that quarks are composed of similar preons, with incremental differences accounting for charge. The hope is to reproduce the reductionist strategy that has worked for the periodic table of elements and the quark model of mesons and baryons. Explain the reason for there being exactly three generations of fermions. Calculate parameters that are currently unexplained by the Standard Model, such as the masses of S.M. fundamental fermions, their electric charges, and color charges; in effect, reduce the number of model-required experimental input parameters from the number required by the Standard Model. Provide reasons for the very large range of mass-energy observed in supposedly fundamental particles, from the electron neutrino to the top quark. Provide alternative explanations for the electro-weak symmetry breaking without invoking a Higgs field, which itself possibly needs a supersymmetry to correct the theoretical problems involved with the Higgs field; (further, the supersymmetric theories proposed so far have theoretical and observational problems of their own). Account for neutrino oscillation and apparently unique mass mechanism. Make new, non-repetitive predictions, such as providing cold dark matter candidates. Explain why there exists only the observed variety of particle species, and give a model with reasons for producing only these observed particles (since the prediction of non-observed particles is a problem with many current models, such as supersymmetry).
Background
Before the Standard Model 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.
Comparisons of quarks, leptons, and bosons
Within the Standard Model, there are several classes of particles. One of these, the quarks, has six types, of which there are three varieties in each (dubbed "colors", red, green, and blue, giving rise to quantum chromodynamics).