Quarks are elementary fermions that participate in the strong interaction and carry colour charge. Six quark flavours are known, called up, down, charm, strange, top and bottom. Quarks have spin one half and therefore obey Fermi Dirac statistics. They also carry fractional electric charges, with up type quarks having positive two thirds of the elementary charge and down type quarks having negative one third of the elementary charge. Quarks combine into colour neutral composite states called hadrons. Their interactions are described by Quantum Chromodynamics, in which gluons carry the strong interaction between colour charged particles and also interact with one another.
The quark model emerged during the 1960s from attempts to organise the rapidly expanding collection of observed hadrons. Murray Gell Mann and George Zweig independently proposed constituent particles that could explain hadron classification patterns. The term quark was introduced by Gell Mann, who took the unusual word from literature. Initially, quarks were theoretical constituents whose physical interpretation was debated, but deep inelastic scattering experiments at SLAC in the late 1960s provided strong evidence that protons contain pointlike constituents with properties consistent with quarks. The later development of Quantum Chromodynamics established a detailed theory of their strong interactions and confinement.
Quarks differ in flavour, mass and electric charge but share spin one half and colour charge. Up and down quarks are the lightest and form most ordinary atomic nuclei through their participation in protons and neutrons. Strange and charm quarks occur in heavier hadrons, while bottom and top quarks are substantially heavier. The top quark is particularly unusual because it decays so rapidly that it does not form ordinary long lived hadrons before decaying.
Quark flavour is changed by weak interactions, while strong and electromagnetic interactions preserve flavour in the relevant Standard Model processes. The masses assigned to quarks also depend on the renormalisation scheme and energy scale in quantum field theory, making simple statements about quark mass more subtle than treating each quark as an isolated classical object.
Quarks are not observed freely under ordinary conditions because of colour confinement. Instead, they occur within hadrons. Baryons contain three valence quarks, while mesons contain a quark and antiquark in the simplest classification. The proton has two up quarks and one down quark, while the neutron has one up quark and two down quarks in terms of their valence quark content. These descriptions do not imply that a proton contains only three quarks at all times. Quantum Chromodynamics predicts a complex sea of virtual quark antiquark pairs and gluons, and the internal structure of hadrons depends on scale and the quantum state being probed.
Quarks interact through all three Standard Model interactions that can apply to charged matter, namely the strong, electromagnetic and weak interactions. Their colour charge makes them subject to Quantum Chromodynamics. Their fractional electric charge allows electromagnetic interactions. Weak interactions can change quark flavour and are responsible for processes such as beta decay at the underlying quark level. Quark flavour transitions are described by the Cabibbo Kobayashi Maskawa matrix, which determines how weak interaction eigenstates relate to mass eigenstates. This framework also provides one established source of CP violation, although it is insufficient by itself to explain the observed matter antimatter asymmetry of the universe.
Quarks are central to collider physics because high energy quark and gluon interactions generate observable jets. A high energy quark produced in a collision cannot normally travel freely to a detector. Instead, the colour field evolves through processes collectively known as parton showering and hadronisation, producing many colour neutral hadrons whose trajectories and energy deposits form a jet. Physicists reconstruct jet properties to infer information about the underlying quark or gluon process. Measurements of jet production, scaling behaviour and deep inelastic scattering have provided extensive experimental support for Quantum Chromodynamics and the partonic structure of matter.
Quark masses span a very large range. The top quark is roughly comparable in mass to a heavy atomic nucleus despite being elementary, while up and down quarks have masses much smaller than the proton mass. The difference between quark masses and hadron masses illustrates the importance of strong interaction energy. Most of the mass of ordinary nucleons arises from QCD dynamics rather than simply adding the rest masses of their valence quarks. Heavy quarks are particularly useful probes because their masses introduce energy scales that permit specialised theoretical calculations and provide distinctive experimental signatures.
A common misconception is that the proton is simply a permanent object made from three tiny balls called quarks. The valence quark picture is useful for explaining its net flavour quantum numbers, but the full proton state is a dynamic quantum field configuration containing gluons, sea quarks and antiquarks as well as valence contributions. Another misconception is that fractional electric charges should be directly observable in isolation. Free isolated quarks have not been observed because of confinement. Quarks are therefore best understood as elementary quantum fields whose observable consequences appear through colour neutral hadrons, scattering patterns, decay processes and high energy jets.
