Quark
A quark is, in the Standard Model, one of the six fundamental constituents from which protons, neutrons and the other hadrons are built. Quarks are the only particles assigned fractional electric charge — +2/3 or −1/3 of the elementary charge — and the only ones carrying the "colour" charge of the strong interaction.
The idea was introduced independently in 1964 by Murray Gell-Mann and George Zweig as a way of organising the confusing profusion of hadrons discovered in the 1950s: if baryons were triplets and mesons quark–antiquark pairs, the observed families fell into simple patterns. What began as a bookkeeping device was given physical weight by the deep inelastic scattering experiments at SLAC in 1968–69, which found that electrons fired at protons scattered as though from hard point-like centres inside them. Six flavours are now recognised — up, down, strange, charm, bottom and top — the last of them observed at Fermilab in 1995. The theory of their interaction, quantum chromodynamics, was completed with the discovery of asymptotic freedom in 1973: the force between quarks weakens at short range and grows at long range, so that pulling two apart costs enough energy to create a new quark–antiquark pair instead. This is confinement, and it is the reason no free quark has ever been observed.
Two things about that account are genuinely unsettled in mainstream physics itself, and are worth stating plainly. Confinement has never been derived analytically from quantum chromodynamics; establishing it rigorously remains an open mathematical problem. And the quark masses are not predicted by the theory but measured and inserted, while the great bulk of a proton's mass is not the rest mass of its quarks at all but the energy of the field binding them.
On this wiki
Almost nothing in the archive collected here accepts the quark. The objection is rarely to the arithmetic of the hadron classification; it is to confinement, which most contributors read as a rule constructed specifically to explain why the required evidence is missing. That argument is set out in detail on the Standard Model page, and pressed hardest by Alexander Unzicker in Bankrupting Physics and The Higgs Fake, where he argues that much of modern particle physics has been invention rather than discovery.
The alternatives fall into a few recognisable groups.
The ring modellers of Common Sense Science replace quark structure with circulating charge. David L Bergman's The Real Proton (2000) sets the charged ring model directly against the quark model and the quark sea, and Fine-Structure Properties of the Electron, Proton and Neutron (2006) reports the properties of all three particles calculated to about five significant figures from ring geometry alone. Physical Models for Elementary Particles, Atoms and Nuclei (1997) states the programme; Glen C Collins's Physical Models for Sub-Atomic Particles and Atomic Structure (2002) applies it to nuclear structure.
The sub-quark compositionalists accept that the proton has parts but deny that quarks are them. Rati Ram Sharma argues in Unified Theory Composition-Structure of Electron, Proton & Neutron (2010) that no particle of the Standard Model actually satisfies the definition of an element, since quarks and leptons are themselves compressible and assembleable; he composes them instead from cosminos — positrino and negatrino — and holds that mutual repulsion among like-charged cosminos makes quarks "fragile and non-existent as intact units", so that colour is not a fundamental charge. Christopher G Provatidis's A Possible Theory for Particle Composition of Matter Based on Only Three Functional Quantum Particles (2011) rebuilds the subatomic inventory from three particles and three interactions.
The pair modellers dispense with substructure of that kind altogether. Ernest J Sternglass built matter from tightly bound, near-light-speed electron–positron pairs, explicitly without quarks. Karl Teppo's The Secret Life of the Neutron (2009) inverts the usual hierarchy, arguing that the neutron itself is the fundamental particle and that the search for constituents has been misled by the "expectation that such a particle must be small."
Thomas N Lockyer's vector particle physics and Osvaldo Domann's work each give structural accounts of hadrons outside the quark model. A partial exception in the archive is William R Hohenberger, whose Integrating Vector Particle Physics with the Dodecahedron Quark Ball and the Octahedral Hexagonal Fractal (2013) keeps a quark-like geometry — a dodecahedral "quark ball" — but derives it from electromagnetic field structure rather than from a gauge theory, and ties it to Lockyer's electron cube and Don Briddell's Field Structure Theory.
Charles William Lucas's The Trouble with Modern Physics and the Solution Based on Logic and Metatheory (2016) makes the general case: that the standard model of elementary particles, along with relativity and Copenhagen quantum mechanics, should be replaced by a single completed electrodynamics.