For example, the proton spin puzzle, the EMC effect, the distributions of electric charges inside the nucleons, as found by Robert Hofstadter in 1956,[2][3] and the ad hoc CKM matrix elements.
When the term "preon" was coined, it was primarily to explain the two families of spin-1/2 fermions: quarks and leptons.
Preon research is motivated by the desire to: 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.
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).
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.
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.
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).
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.
Efforts to develop a substructure date at least as far back as 1974 with a paper by Pati and Salam in Physical Review.
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 "point like" down to distance scales of less than 10−18 m (or 1⁄1000 of a proton diameter).
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?
One way of resolving this paradox is to postulate a large binding force between preons that cancels their mass-energies.
[citation needed] Preon theories require quarks and leptons to have a finite size.