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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 most discussed of these fundamental models are string theory and its several variants. Another, newer approach is string-net condensation, which models elementary particles as excitations of collective motions of spacetime atoms (somewhat analogous to phonons in a crystal). 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 (postdiction), which include
Were string theory successful in its original objectives, preon theory research would not be necessary. String theory was supposed to account for the above issues in terms of string dynamics. The different particles of the standard model were accounted for as different frequencies (tension) of a planck-scale string, particle dynamics were explained in terms of the worldsheet diagrams, (the string theory equivalent of Feynman diagrams) and the three generations of fermions were explained in terms of strings "wrapping around" specific configuration of higher-dimensional moduli. The continuing failure of string theory to achieve the above objectives as a fundamental explanatory model of particle physics has reignited interest in investigating alternatives, including preon models. Relevant literature discussing shortcomings of string theory include: Peter Woit's Not Even Wrong, Lee Smolin's The Trouble with Physics, or Daniel Friedan's String theory is a complete scientific failure.
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, rishonss, tweedles, helons, haplons, and Y-particles. 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 Frizsch and Mandelbaum, a 1992 paper by D'Souza and Kalman, and a 1997 paper by Larson . None has gained wide acceptance in the physics world.
Each of the preon models identifies a set of far fewer fundamental particles than those of the Standard Model, explains rules governing how those fundamental particles operate, and shows how those proposed particles and rules can explain the Standard Model, often with predicted small discrepancies from the existing model, proposed new particles, and certain phenomena in the standard model that remain unexplained. 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 composite preons as what breaks the electro-weak symmetry, rather than a scalar Higgs fields. 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.
The vast bulk of recent theoretical research into the particle zoo has been string theory. It was thought string theory has completely supplanted preon research, and that one dimensional supersymmetric strings can reproduce all the particles of the standard model, and their superpartners, the MSSM, their properties, color, charge, parity, chirality, and energy-masses, obviating any need for preon research. To date, string theory has been unable to reproduce the standard model.
A search through Spires and Arxiv, show that approximately over 30,000 papers in string theory or supersymmetry since 1982, with several hundred new papers being published every month. In comparison, in 2006, since 2003, there have been about a dozen papers in preon theory listed as such in arxiv. String theories' continuing failure to reproduce the particle spectrum of the standard model has given some life for preon theories, and there have been recent papers on preon theory.
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".
As of 2006, Yershov, Fredriksson, and Bilson-Thompson have published a number of papers in Preon theory within the past 5 years. In addition to other paper referenced in this article, they include a 2003 paper by Fredriksson .
The term "preon" is the term of choice for Bilson-Thompson, Yershov, and Fredrickson, who use it both for the spin-1/2 fermions of leptons and quarks, and for spin-1 bosons.
Yershov's preon model
The 2003 papers by Yershov   are notable for being some of the only papers in the field to use the Preon model as a basis for providing specific numerical values from first principles for the masses of the particles described in the Standard Model. These papers suggest that the properties of the preons proposed flow from their topology.
Yershov's model does not predict the mass of the Higgs boson, as it predicts that it will not be found. Yershov's model deals with the mass paradox by proposing a huge binding energy (mass-defect) for his preons, which involves a new force that is at least 105 stronger than the strong nuclear force to bind preons together.
Yershov's model is patterned after the idea of naked singularities in general relativity, and closely resembles the geon (physics) of John Archibald Wheeler's research program in Geometrodynamics.
Electron structure in Yershov's theory was further elaborated on in 2006 .
The basic building block of this model is a primitive particle (preon) regarded as a source of a spherically-symmetric field defined on a 3D manifold with Klein-bottle topology. The symmetries of the field correspond to the tripolar (colour), bipolar (electric), and unipolar (gravitational) interactions. The strength of the tripolar field (in spherical coordinates) is modelled as Fs(ρ)=q0exp(-ρ-1) whereas the functional form for the electric field, Fe(ρ), is taken as the derivative of Fs(ρ), i.e., Fe(ρ)= q0 ρ-2exp(-ρ-1), where q0 is preon's charge. Except for its tripolarity, such a field is analogous to the Lennard-Jones fields used in molecular physics for modelling long-range attractive and short-range repulsive forces (the unipolar gravitational component is neglected). A trade-off between these forces leads to equilibrium particle configurations, the simplest of which are colour dipoles and tripoles composed of, respectively, two and three preons.
The dipoles are deficient in one colour, whereas the tripole fields are colourless at infinity and colour-polarised nearby. This polarisation allows axial coupling of tripoles into strings. Due to the Z3-symmetry of the tripoles, they are consecutively 120°-rotated within a string of like-charged tripoles. Given the cyclic property of the group, a three-component string is likely to close in a loop, which would minimise its potential energy.
The tripoles in such a system retain their rotational and translational degrees of freedom (around and along their common ring-closed axis), while maintaining their relative 120°-orientation with respect to each other. The colour-currents formed due to the motions of the looped colour charges are helices with either clockwise or anticlockwise winding. Helicity is found to be one of the important properties of these structures.
A string of tripole-antitripole pairs has a similar symmetry, which allows its closure in a neutral loop containing six such pairs (twelve tripoles). By their properties, the three- and twelve-tripole loops can be identified with the electron and its neutrino, respectively. Different combinations of these loops (involving tripoles as well) lead to a variety of structures identical by their properties to the observed variety of elementary particles.
Stable configurations of preons in this model obey kinematic constraints of a topological nature corresponding to the minima of their potential energy. Therefore, the number of constituents in each configuration is well-defined and the set of the generated configurations is unique, which leads to the explanation of the origin of the observed spectrum of particle species and of their properties, including masses, without invoking any free parameters.
The masses of these structures are assumed to arise as the energies of the preon motions inside of the structures, combined with the preon binding energies (mass defects), which are known as two standard mass-generating mechanisms for composite systems. In a first approximation, the masses of the charged structures could be derived from the number of their constituent preons by setting for simplicity the energy of each preon to unity.
The fields of opposite polarities (or colours) cancel each other if two unlike-charged (unlike-coloured) preons get close to each other or overlap, which would also nullify the mass of the system. In this case the energy would be stored in the form of the mass defect (also dampening the oscillatory motions of the components). In fact, in the neutral systems, such as the electron neutrino, the preons' centres do not coincide exactly, therefore the complete cancellation of fields (and masses) in such systems does not occur, resulting in non-vanishing neutrino masses. The mass of a neutral loop (neutrino) is completely recovered when an electric field source (e.g., a tripole) is enclosed by the loop or when this neutral loop is enclosed by a larger charged loop (such as the electron).
For more complicated structures (like the second and third generation fermions regarded in this model as clusters of simpler structures belonging to the first generation) the computation of masses is unlikely to be simple because of possible multiple resonances in the clusters. However, an empirical (non-physical) summation rule was proposed to show that, at least in principle, these masses are computable and that the variety of these structures does, indeed, match the observed variety of particles.
Criticisms specific to Yershov's model
Yershov's model has been described in terms of classical general relativity, rather than quantum mechanics. By contrast, the standard model is framed entirely in the language of Quantum Field Theory. It is unclear whether Yershov's model can take quantum field theory into account, and successfully address t'Hooft's anomaly matching constraints.
A theory that can post-dict particle masses and half-lives would be a major breakthrough, however, and Yeshov's model purports to do this.
Some physicists are unconvinced with Yershov's mass formula for elementary particles in his research papers on preons on the grounds that it is highly arbitrary, both in the derivation of his formula, and the manner in which he assigns values to his preons.. Proposing unobserved "preon binding forces" to account for the preon mass paradox may result in predictions in conflict with observation. The non-quantum nature of this preon model has attracted little, if any, interest, among particle physicists and there appears to be no citations to his research in this literature.
Loop quantum gravity and Bilson-Thompson Preon theory
In a 2005 paper , Sundance Bilson-Thompson proposed a model which adapted the Harari Rishon Model to preon-like objects which were extended ribbons, rather than point-like particles. This provided a possible explanation for why ordering of the subcomponents matters (giving rise to colour charge) whereas in the older point-particle preon model (Rishon Model), this feature must be treated as an ad hoc assumption. Bilson-Thompson refers to his extended ribbons as "helons", and his model as the Helon Model.
This model leads to an interpretation of electric charge as a topological quantity (twists carried on the individual ribbons).
In a subsequent paper  from 2006 Bilson-Thompson, Fotini Markopolou, and Lee Smolin suggested that in any of a class of quantum gravity theories similar to loop quantum gravity (LQG) in which spacetime comes in discrete chunks, excitations of spacetime itself may play the role of preons, and give rise to the standard model of particle physics as an emergent property of the quantum gravity theory.
Consequently, Bilson-Thompson et al proposed that loop quantum gravity could reproduce the standard model. In this scenario the four forces would already be unified. The first generation of fermions (leptons and quarks) with correct charge and parity properties have been modelled using preons constituted of braids of spacetime as the building blocks.
Bilson-Thompson's original paper suggested that the higher-generation fermions could be represented by more complicated braidings, although explicit constructions of these structures were not given. The electric charge, colour, and parity properties of such fermions would arise in the same way as for the first generation.
Utilization of quantum computing concepts made it possible to demonstrate that the particles are able to survive quantum fluctuations.
The ribbon-like structures of Bilson-Thompson's model have been described as "pieces of spacetime ribbon-tape", in that they may be made of the same structure that makes up spacetime itself. . While Bilson-Thompson's papers do offer an explanation on how to get fermions and spin-1 bosons, he does not show a braiding that would account for the Higgs boson.
In a 2006 paper , L. Freidel, J. Kowalski-Glikman, A. Starodubtsev suggest that elementary particles are Wilson lines of gravitational field, which implies that the properties of elementary particles, such as mass, energy, and spin, can be described by LQG's Wilson loops, and particle dynamics can be modeled on breaks in these Wilson loops, adding theoretical support to Bilson-Thompson's preon proposals.
The spin foam formalism (closely related to loop quantum gravity) allows for the derivation of certain other particles of the standard model, the spin-1 bosons, such as photons and gluons,  and gravitons ,  from loop quantum gravity's fundamental principles, and independent of Bilson-Thompson's braiding scheme for fermions. However, as of 2006, there is not a derivation of the helon model from spin foam formalism, as described by braiding. The helon model does not offer a braiding that would account for a Higgs, but does not rule out the possibility of a Higgs boson as a composite object. Bilson-Thompson himself observes that since the particles with higher masses have (generally) more complicated internal "structure"—including twists which are identified as electric charges—it is possible that this internal structure gives rise to inertial mass through some unspecified mechanism. (The massless photon is not twisted in Bilson-Thompson's preon scheme.) As of 2006, it remains to be seen whether the derivation of the photon from the spin foam formalism in  can be matched with Bilson-Thompson's braiding of three untwisted ribbons , or perhaps, there are multiple ways to derive photons from the spin foam formalism.
When the term "preon" was first coined, it was used to describe pointlike subparticles that describe spin-1/2 fermions that include leptons and quarks. Such sub-quark pointlike particles would suffer from the mass paradox described below. It is observed that Bilson-Thompson's ribbon structures are not actually "classical" pointlike preons, as defined in the introduction to this article. Bilson-Thompson uses the term "preon" in the sense of more fundamental "subparticles" of quarks, leptons, and gauge bosons, and to maintain continuity in terminology with the larger physics community. His braiding also accounts for spin-1 bosons.
One way to understand Bilson-Thompson's "preon" theory, in contrast to older particle preon theories, is that elementary particles such as electrons are described in terms of a wave function, and the state-sum of coherent phase spin foam is also described in terms of a wave function. It is hoped that it might be possible to derive a wave function from the spin foam formalism that matches the wave function used to describe elementary particles such as photons and electrons. Coupling matter to loop quantum gravity remains an active area of research .
Bilson-Thompson has recently updated his paper dated October 27, 2006,  and notes that his model, while not preon in the strict sense of the term, nevertheless is a preon-inspired model, and is open to the possibility that other more-fundamental theories, such as M-Theory, may account for his topological diagrams, as well as the Higgs boson and gravity. The theoretical objections that apply to classic preon models do not necessarily apply to his preon-inspired model, as it is not the particles themselves, but the relations between his preons (braiding) that give rise to the properties of particles. In this newer version of his paper, he has added a new section, section IV, called "unresolved issues" and acknowledges that open issues include mass, spin, Cabibbo mixing, and grounding in a more fundamental theory. He states that grounding preons in M-theory is a possibility, as well as loop quantum gravity.
Newer paper is present here  which describes the dynamics of braiding though pachner moves.
Recently, Levin and Wen have suggested string-net condensation as an alternative approach to incorporating standard model physics within LQG's spin networks.
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?
Yershov's model, referenced above, proposes that when both particles and anti-particles of the proposed tripoles in the theory are present, such as in the model's proposed neutrino composition, the mass of the constituent parts "cancels out", but can appear again when the configuration of the tripoles is changed. Thus, Yershov's model proposes that particle mass is partially stored in the form of a mass defect (binding energy) among his preons, which helps account for the mass paradox.
The Sundance Approach
The Sundance preon model may avoid this by denying that preons are pointlike particles confined in a box less than 10−18 m, and instead positing that preons are extended 2-dimensional ribbon-like structures, not necessarily smaller than the elementary particles they compose, not necessarily confined in a small box as point particles preon models propose, and not necessarily "particle-like", but more like glitches and topological folds of spacetime that exist in threefold bound states that interact as though they were point particles when braided in groups of three as a bound state, with other particle properties such as mass and pointlike interaction arising as an emergent property, so that their momentum uncertainty would be on the same order as the elementary particles themselves.
The String Theory Approach
String theory posits one-dimensional strings on the order of the Planck scale as giving rise to all the particles of the Standard Model, which would appear to also have the mass paradox problem. String theorist Lubos Motl has offered explanations as to how string theory gets around the mass paradox , and consents to have his explanation shared here. The center-of-mass position X0 and the total momentum of a string, P0, behave just like for pointlike particles. They don't commute and follow the uncertainty principle. A certain X0 means uncertain P0 and vice versa, the product being above ħ/2.
Besides the zero modes (center-of-mass degrees of freedom), every string has infinitely many internal degrees of freedom. That's like an atom with many electrons, but you have infinitely many arranged along a string. The relative motion of pieces of string gives energy—expressed as the usual sum of the kinetic and potential contributions. And because they're relativistic strings, energy also means mass via E=mc².
The result is that the minimal size of the string—in the lowest-energy or lowest-mass states—is a compromise in which the kinetic terms from the internal degrees of freedom contribute the same as the potential terms, just like for X and P in the harmonic oscillator, attempting to minimize the energy while satisfying the uncertainty relation for the internal X,P degrees of freedom. This minimum occurs if the typical size of the string is comparable to a typical distance scale derivable from the tension of the string, the string length, that is conventionally believed to be close to the Planck scale 10−35 meters (a bit longer than that).
The actual numerical coefficient of the string is actually logarithmically divergent but this fact doesn't affect any finite-energy experiments.
The question has the same answer in string theory just like for ordinary particles because it is the zero modes that matter here. The internal degrees of freedom are only relevant for experimental uncertainty considerations if you probe the internal structure of a string, and indeed, you will always find out that the "radius" of it seems to be of order the string length.
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 their proposed preons compose the particle zoo, which may make the theory even more complicated than the Standard Model, or 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, which predict the Higgs boson does not exist, or are unable to derive a combination of preons which would give rise to a Higgs Boson.
In contrast, should a Higgs boson not appear in the increasingly constrained circumstances where the leading proponents of the Standard Model predict that it will be found, preon theory would receive a significant theoretical boost, while many competing theories would be falsified.
String theory and preon theory
String theory proposes that a one dimensional string on the order of a Planck scale has a tension, and the oscillations in the strings give rise directly to all the particles of the standard model and their super partners, in interaction with the proper compactified 6 or 7 dimensional Calabi-Yau manifold and SUSY breaking. To date, string theory has been no more successful than preon theory in achieving this goal. John Baez and Lubos Motl have discussed the possibility  that should preon theory prove successful, it may be possible to formulate a version of string theory that gives rise to a successful model of preons.
There have been recent research papers that have proposed preon models that are made of superstrings in Arxiv ,  or supersymmetry . Composite strings also emerge in the above mentioned Yershov's 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 thirty 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.
See additional discussion in the article on the Rishon Model.
|This article is licensed under the GNU Free Documentation License. It uses material from the Wikipedia article "Preon". A list of authors is available in Wikipedia.|