Quark

A quark is an elementary particle in the Standard Model of particle physics. Quarks carry fractional electric charge and participate in the strong interaction. They occur in six types, called flavours: up, down, charm, strange, top, and bottom. Together with gluons, they constitute composite particles known as hadrons, including the proton and neutron. Quarks have not been observed as isolated, freely propagating particles; their observable behaviour is governed by colour confinement. (opendata.cern.ch)

Properties and flavours

Quarks are fermions with spin quantum number ½. Each flavour has a corresponding antiquark with the same mass but opposite electric charge and conjugate colour charge. Quarks have baryon number +⅓, while antiquarks have −⅓. These assignments account for the baryon numbers of particles assembled from them. (pdg.lbl.gov)

The six flavours are arranged in three generations. Charges are expressed below in units of the positive elementary charge, (e). (home.web.cern.ch)

Up and down quarks supply the valence content of ordinary protons and neutrons. The higher generations contain heavier quarks, which occur in unstable particles or are produced in energetic collisions. The generation pattern resembles that of leptons, the family containing the electron and neutrinos, although leptons do not carry colour charge. The Standard Model does not explain why there are exactly three generations or why their masses differ so substantially. (home.web.cern.ch)

Colour charge and confinement

The theory describing quarks’ strong interactions is quantum chromodynamics (QCD). Its force carriers are gluons. Quarks possess colour charge, conventionally labelled red, green, and blue; these names denote quantum properties, not visible colours. Observable hadrons are colour-neutral combinations. (opendata.cern.ch)

At short distances, or large momentum-transfer scales, QCD’s effective coupling becomes weaker. This property, asymptotic freedom, allows many high-energy processes to be calculated using perturbative methods. At larger distances, strong interactions become nonperturbative, and isolated colour-charged particles do not emerge. This behaviour is called colour confinement. Experiments and numerical calculations support confinement, although a general rigorous analytic proof remains lacking. (cds.cern.ch)

Increasing the separation of quarks does not simply liberate them. Energy supplied to the colour field can instead produce additional quark–antiquark pairs, yielding new colour-neutral hadrons. Consequently, energetic quarks usually appear experimentally through collections of hadrons rather than as individual detector tracks. The transition from quarks and gluons to hadrons is called hadronization. (cds.cern.ch)

The top quark is exceptional: it decays before hadronization can occur. Its properties are reconstructed from its decay products rather than from top-containing hadrons. This does not make the top quark an observable stable, isolated particle. (atlas-public.web.cern.ch)

Hadrons and ordinary matter

In the simplest quark classification, baryons contain three valence quarks, while mesons contain a valence quark and antiquark. A proton has valence content (uud), giving charge (+e); a neutron has (udd), giving charge zero. More complex combinations also exist, including states with tetraquark and pentaquark valence content. (pdg.lbl.gov)

Valence labels specify net flavour content, not a complete inventory of everything inside a hadron. A proton also contains gluons and a quantum-mechanical sea of quark–antiquark pairs. Its internal structure therefore cannot be represented fully as three small objects held together by static bonds. (cds.cern.ch)

Most of the mass of protons and neutrons arises from QCD dynamics rather than from the masses of their light valence quarks alone. The energy associated with their quark and gluon fields contributes to total mass through mass–energy equivalence. Thus, understanding ordinary matter requires both the elementary constituents and the interactions that organize them into composite particles. (alice-collaboration.web.cern.ch)

Historical development and evidence

Murray Gell-Mann and George Zweig independently proposed quark constituents in 1964 to explain patterns among the growing number of known hadrons. Gell-Mann introduced the name quark, adopting its spelling from James Joyce’s Finnegans Wake; Zweig called his proposed constituents aces. The original schemes involved up, down, and strange constituents with fractional electric charges. Initially, whether these entities were physical particles or chiefly mathematical classification devices was uncertain. (cds.cern.ch)

A major experimental advance came in 1968, when electron-scattering experiments at SLAC revealed pointlike constituents within nucleons. These deep inelastic scattering results, together with subsequent measurements, established evidence for the quark picture rather than a structureless proton. The interpretation developed through several complementary theoretical and experimental advances, not a single observation of a free quark. (slac.stanford.edu)

In 1974, teams at SLAC and Brookhaven National Laboratory independently discovered the J/ψ particle, providing evidence for charm. Bottom was discovered at Fermilab in 1977. On March 2, 1995, Fermilab’s CDF and DØ collaborations announced the discovery of top, completing the experimentally established six-flavour set. (fnal.gov)

Experimental investigation

Quark research examines both individual flavour properties and the collective structure of hadrons. Scattering measurements probe how quarks and gluons share a proton’s momentum and spin; hadron spectroscopy studies the particles their interactions produce. These approaches connect high-energy constituent descriptions with the composite states observed in experiments. (home.cern)