Color confinement

Color confinement is the phenomenon in quantum chromodynamics that prevents quarks and gluons from appearing as isolated free particles at ordinary energies. Instead, they occur in color-neutral bound states such as protons and mesons.

Quantum chromodynamics: The quantum field theory describing quarks, gluons, and the strong interaction. Its non-Abelian gauge dynamics produce the interactions that confine color charge.
Quark: A fundamental fermion that carries color charge and forms components of hadrons. Quarks are the confined constituents whose separation raises the strong-interaction energy.
Deep inelastic scattering: A high-energy scattering process that probes the internal structure of protons and other hadrons. It revealed pointlike partons inside hadrons, though none emerge as free particles.
Wilson loop: A gauge-invariant quantity formed by tracing the gauge field around a closed path. Its large-loop behavior offers a precise criterion for confinement in idealized gauge theories.
Color charge: The charge carried by quarks and gluons that couples them to the strong interaction. Confinement applies to this charge, not to electric charge.
Hadron: A composite particle made of quarks bound by the strong interaction. Hadrons are the observable bound states in which confined quarks occur.
Parton shower: A cascade of quark and gluon emissions produced as an energetic parton evolves toward lower energies. The cascade ends in hadronization rather than a beam of isolated colored particles.
Area law: The scaling of a large Wilson loop’s expectation value with the area it encloses. An area law signals a linearly rising static potential in pure gauge theory.
Gluon: The massless gauge boson that mediates the strong interaction between color-charged particles. Gluon self-interactions help make the force between color charges grow with distance.
Baryon: A hadron composed primarily of three quarks, or three antiquarks for an antibaryon. Protons and neutrons illustrate confinement in three-quark color-neutral states.

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