Wikipedia's review (to which I made some significant contributions a long time ago, many of which have since been superseded) is under the article for
Preons.
Some More Recent Preon Models
Yershov
One of the better efforts in the current century (although it didn't get a lot of attention) was by V.N. Yershov. This attempt was notable for accounting for all three generations and producing good numerical agreement with measured masses as of the date of publication. It also has a partial work around to the energy scale of compositeness question (since the masses of composites of a preons are more than just the sum of the parts plus binding energy with a partial shielding mechanism).
This said, it isn't a complete solution to the limitations of the PDG exclusions and the mass fits aren't as good now as they were in 2003.
The First Paper V.N. Yershov, "Fermions as topological objects" (2003)
A preon-based composite model of fermions is discussed. The preon is regarded as a topological object with three degrees of freedom in a dual (3+1)-dimensional manifold. It is shown that dualism of this manifold gives rise to a set of preon structures, which resemble three families of fermions. The number of preons in each structure is readily associated with its mass. Although just a sketch, our model predicts masses of fermions to an accuracy of about 10-6 without using experimental input parameters.
The Second Paper V.N.Yershov, "Neutrino masses and the structure of the weak gauge boson" (2003).
It is supposed that the electron neutrino mass is related to the structures and masses of the W± and Z0 bosons. Using a composite model of fermions (described elsewhere), it is shown that the massless neutrino is not consistent with the high values of the experimental masses of W± and Z0. Consistency can be achieved on the assumption that the electron-neutrino has a mass of about 4.5 meV. Masses of the muon- and tau-neutrinos are also estimated.
Sundance Bilson-Thompson
Another notable effort was some of the original preon theorists including Sundance Bilson-Thompson , Jonathan Hackett , Louis Kauffman , and Yidun Wan, "
Emergent Braided Matter of Quantum Geometry" (2011), that abstract of which states:
We review and present a few new results of the program of emergent matter as braid excitations of quantum geometry that is represented by braided ribbon networks, which are a generalisation of the spin networks proposed by Penrose and those in models of background independent quantum gravity theories, such as Loop Quantum Gravity and Spin Foam models.
This program has been developed in two parallel but complimentary schemes, namely the trivalent and tetravalent schemes. The former studies the trivalent braids on trivalent braided ribbon networks, while the latter investigate the tetravalent braids on tetravalent braided ribbon networks.
Both schemes have been fruitful. The trivalent scheme has been quite successful at establishing a correspondence between the trivalent braids and Standard Model particles, whereas the tetravalent scheme has naturally substantiated a rich, dynamical theory of interactions and propagation of tetravalent braids, which is ruled by topological conservation laws. Some recent advances in the program indicate that the two schemes may converge to yield a fundamental theory of matter in quantum spacetime.
Hidezumi Terazawa
Yet another notable effort by Hidezumi Terazawa in "Masses of Fundamental Particles" tried to
compute fundamental particle masses from a preon model. A version of this model was published at 5(5)
Journal of Modern Physics 205 (2014).
W. Królikowski
Another notable attempt involved papers by W. Królikowski
in 1980 and
in 1987 on preon models, and a composite Higgs model
in 1992 and
in 1998,
in 2000 (
also here).
He garners attention, in part, because a (not explicitly preon based)
theoretical estimate he made of the top quark mass and CP violating phase of the CKM matrix in 1990 (the formula was refined in 2004) predicted a top quark mass of about 171 GeV and a CP violating phase of 72-75 degrees. Thus, the top quark mass and CKM matrix mixing angle predictions from 21 years ago by Mr. Krowlikowski in this paper were within about 2 standard deviations of current experimental values, despite quite uncertain inputs from other masses that he had to use to make the prediction at the time.
Harald Fritzsch
An
interesting preon paper was also published by Harald Fritzsch in October 2012 in the journal "Modern Physics Letters A."
Other Relevant Observational Evidence
The non-discovery of either electromagnetically neutral color charged fermions, or leptoquarks, to date, which are generic expectations of many ways you could attempt to create a preon theory, also significantly constrains the parameters and nature of any preon theory.
Another pro-preon observation that has been made is that the neutrino mixing matrix (aka the PMNS matrix) is
very similar to the hybrid electron orbital matrixes of chemistry. The natural suggestion, of course, is that they arise via some preon-like internal structure comparable to an atom, although the experimental evidence would suggest that any such structure must be very small and very tightly bound.
Non-Point-Like Fundamental Particle Models
Between preon models, that assume that the so called fundamental particles are really composites of smaller pieces, and point particle models that are inconsistent with general relativity, are extended particle models, that have non-point-like particles that occupy a non-point-like volume, although the line between the two is fine one.
A recent example of such a model is Chih-Hsun Lin, Jurgen Ulbricht, Jian Wu, Jiawei Zhao,
"Experimental and Theoretical Evidence for Extended Particle Models" (2010). The abstract, in part, for the 147 page paper with 41 figures reads as follows:
We review the experimental searches on those interactions where the fundamental particles could exhibit a non point-like behavior. In particular we focus on the QED reaction measuring the differential cross sections for the process $ \EEGG $ at energies from sqrt{s} =55 GeV to 207 GeV using the data collected with the VENUS, TOPAZ, ALEPH, DELPHI L3 and OPAL from 1989 to 2003.
The global fit to the data is 5 standard deviations away from the standard model expectation for the hypothesis of an excited state of the electron, corresponding to the cut-off scale Lambda =12.5 TeV.
Assuming that this cut-off scale restricts the characteristic size of QED interaction to 15.7x10^{-18} cm, we perform an effort to assign in a semi-mechanical way all available properties of fundamental particles to a hypothetical classical object. Such object can be modeled as a classical gyroscope consisted of a non rotating inner massive kernel surrounded by an outer rotating massive layer equipped with charged sorted in a way to match the charge contents for different interactions.
The model size of an electron agrees with 1.86x10^{-17} cm with the experiment. The introduction of a particle like structure related to gravity allows to estimate the inner mass kernel of an electron to 1.7x10^{-19} cm and the mass of a scaler to 154 GeV. The extension of the model to electrical charged particle-like structure in nonlinear electrodynamics coupled to General Relativity confirms the model in the global geometrical structure of mass and field distribution.
Some of the same authors have explored the same ideas in papers
here (2009),
here (2003),
here (2001), and
here (1999). The 2009 paper is much shorter and has a nice survey of the research in the field, with this particular approach related closely to non-linear electrodynamics coupled to gravity (NED-GR) theories and the Born-Infeld Lagrangian, discussed, for example,
here (2010) and
here (2009). Most of the literature related to this focuses on modeling atypical hypothetical types of black holes. Ultimately, the matter goes to fundamental issues of quantum gravity discused, for example, in
this paper (2000) that systematically explores different possible couplings of the electromagnetic field and gravity.
Conclusion
This said, it is fair to say that the preon possibility has been largely discarded from mainstream theoretical consideration by now after it heyday in the 1970s and 1980s, in part, due to further experimental data that has been collected since the early proposals were made then.
