Preon

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).

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.

Kalman[4] asserts 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.[5]

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.

Several models have been proposed in an attempt to provide a more fundamental explanation of the results in experimental and theoretical particle physics, using names such as "parton" or "preon" for the hypothetical basic particle constituents.

Preon theory is motivated by a desire to replicate in particle physics the achievements of the periodic table in Chemistry, which reduced 94 naturally occurring elements to combinations of just three building-blocks (proton, neutron, electron). Likewise, the Standard Model later organized the "particle zoo" of hadrons by reducing several dozen particles to combinations at a more fundamental level of (at first) just three quarks, consequently reducing the huge number of arbitrary constants in mid-twentieth-century particle physics prior to the Standard Model and quantum chromodynamics.

However, the particular preon model discussed below has attracted comparatively little interest among the particle physics community to date, in part because no evidence has been obtained so far in collider experiments to show that the fermions of the Standard Model are composite.

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,[6] alphons, quinks, rishons, tweedles, helons, haplons, Y-particles,[7] and primons.[8] 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.[9] Other attempts include a 1977 paper by Terazawa, Chikashige and Akama,[10] similar, but independent, 1979 papers by Ne'eman,[11] Harari,[12] and Shupe,[13] a 1981 paper by Fritzsch and Mandelbaum,[14] and a 1992 book by D'Souza and Kalman.[1] None of these have gained wide acceptance in the physics world. However, in a recent work[15] 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.[16]

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.[17] 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, the Fredriksson preon model and the de Souza model predict that the Standard Model Higgs boson does not exist.

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 ${\displaystyle \operatorname {\Delta } x\cdot \operatorname {\Delta } p\geq {\tfrac {1}{2}}\hbar }$ and thus anything confined to a box smaller than ${\displaystyle \operatorname {\Delta } x}$ 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 ${\displaystyle \operatorname {\Delta } p}$ should be greater than the particles themselves.

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.

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, now that the LHC's observation of a Higgs boson is confirmed, the observation contradicts the predictions of many preon models that did not include it.

Preon theories require quarks and leptons to have a finite size. It is possible that the Large Hadron Collider will observe this after it is upgraded to higher energies.