A quark is an elementary fermion that participates in the strong, weak and electromagnetic interactions, with the electromagnetic interaction applying because quarks carry fractional electric charge. Quarks are fundamental particles in the Standard Model and occur in six flavours called up, down, charm, strange, top and bottom.
Up type quarks, consisting of up, charm and top, have electric charge plus two thirds of the elementary charge, while down type quarks, consisting of down, strange and bottom, have charge minus one third. Quarks also carry colour charge and therefore participate in quantum chromodynamics, the quantum field theory of the strong interaction. Unlike electrons and other leptons, quarks are not observed as isolated freely propagating particles under ordinary conditions because of colour confinement. Instead, they occur inside composite hadrons. Baryons contain three quarks or three antiquarks, while mesons contain a quark and antiquark. The proton contains two up quarks and one down quark in its valence quark description, while the neutron contains one up quark and two down quarks.
Quark flavour identifies which type of quark is present and is associated with distinct mass and interaction properties. Up and down quarks are the lightest and are the principal valence constituents of ordinary protons and neutrons. Strange and charm quarks are heavier and occur in unstable hadrons produced in high energy processes. Bottom quarks are still heavier and appear in B mesons and other bottom containing hadrons. The top quark is by far the heaviest known elementary particle.
It is so short lived that it normally decays before it can become incorporated into a conventional hadron. Quark flavour can change through the weak interaction, subject to the mixing structure encoded by the Cabibbo Kobayashi Maskawa matrix. Strong and electromagnetic interactions generally preserve flavour, while weak interactions can change one quark flavour into another. These flavour distinctions are fundamental to understanding particle decays, hadron spectroscopy and the structure of the Standard Model.
Colour charge is a quantum property responsible for quarks participating in the strong interaction. It is conventionally described using three labels called red, green and blue, although these names do not refer to visible colours. Antiquarks carry corresponding anticolour states. Gluons carry colour charge themselves, unlike photons, and therefore gluons can interact with other gluons. This self interaction is a defining feature of quantum chromodynamics.
Physical hadrons are colour neutral overall, analogous in a mathematical sense to how combinations of colour charges can form a neutral state. The proton and neutron are colour neutral baryons containing three valence quarks, while mesons contain a quark and antiquark whose colour combinations form a neutral state. The colour degree of freedom is essential for calculating hadron properties and explaining why quarks cannot ordinarily be separated from one another as isolated particles.
Quark confinement is one of the most important features of strong interaction physics. At short distances and high momentum transfer, the strong interaction becomes weaker, a phenomenon called asymptotic freedom. At larger distances, the effective strong interaction does not simply weaken with separation in the way the electromagnetic force does. The energy stored in the colour field can increase as quarks are separated, eventually making it energetically favourable to produce new quark antiquark pairs rather than isolate a single quark.
The resulting process produces additional hadrons. This behaviour is observed experimentally through hadronisation and jet formation in high energy collisions. Quarks created in energetic collisions do not normally arrive at detectors as isolated quark tracks. Instead, they produce sprays of hadrons called jets. The statistical properties of these jets provide strong evidence for the underlying quark and gluon dynamics.
Quarks have fractional electric charges, but isolated fractional electric charges have not been observed as free stable particles in ordinary matter. The fractional values are entirely consistent within the Standard Model because quarks occur in combinations whose total electric charge is integral for familiar hadrons. The proton's valence quark charges sum to plus one elementary charge, while the neutron's valence charges sum to zero. Antiquarks have opposite electric and colour charges.
Quarks also carry baryon number, with each quark conventionally assigned baryon number one third and each antiquark minus one third. The Standard Model treats quarks as spin one half fermions, so they obey Fermi Dirac statistics and the Pauli exclusion principle. Their masses are not directly equivalent to the mass of a hadron because most hadron mass arises from the energy of strong interaction fields, quark kinetic energy and gluonic dynamics.
Quarks were not originally introduced merely as mathematical subdivisions of hadrons. The quark model was developed to organise the growing collection of strongly interacting particles discovered in the mid twentieth century. The model initially used up, down and strange quarks to classify many hadrons. Subsequent discoveries and theoretical developments established charm, bottom and top quarks, completing the six flavour pattern of the Standard Model.
The discovery of the top quark in particular provided strong evidence for the third quark generation and was associated with a mass far larger than that of the other known quarks. Deep inelastic scattering experiments at high energies provided evidence that protons contain pointlike constituents, supporting the parton and quark descriptions. Quantum chromodynamics then supplied the gauge theory explaining their strong interactions.
Quarks are therefore crucial to ordinary matter even though they are never encountered as isolated constituents in everyday conditions. The proton and neutron in atomic nuclei are composite quantum systems whose internal structure is dominated by quarks and gluons. Their properties, including mass, magnetic moments, spin and response to external probes, arise from complicated quantum chromodynamic dynamics. The simple valence quark picture is useful but incomplete because hadrons also contain gluons and transient quark antiquark contributions. Sea quarks contribute to the internal quantum structure, while gluons carry a substantial fraction of a hadron's momentum and are central to its dynamics. Modern experiments probe parton distribution functions, which describe how quarks and gluons share momentum and other quantum properties inside hadrons.
Quark physics has applications extending beyond particle classification. Understanding quark and gluon interactions is essential for predicting collision rates at particle accelerators, interpreting jets, calculating hadron properties and modelling conditions in the early universe. Heavy ion experiments investigate extremely hot and dense states of strongly interacting matter in which quarks and gluons can form a quark gluon plasma. This state is believed to resemble conditions that existed shortly after the hot early universe began expanding.
Lattice quantum chromodynamics uses numerical calculations on a discretised spacetime framework to study strongly coupled phenomena that are difficult to calculate analytically. Quarks also participate in weak decays that provide important information about fundamental parameters and matter antimatter asymmetries. The quark concept therefore links the microscopic structure of nucleons with some of the highest energy phenomena accessible experimentally.
