QCD

Quantum chromodynamics (QCD) is the fundamental theory of the strong interaction, describing how quarks and gluons interact through color charge. It is a non-Abelian gauge theory with symmetry group SU(3). Quarks, the matter fields, come in six flavors (up, down, charm, strange, top, bottom) and carry one of three color charges. Gluons are the gauge bosons that mediate the force and themselves carry color, resulting in self-interactions among gluons.

Two hallmark properties define QCD: asymptotic freedom, whereby the interaction becomes weaker at high energies or short distances, enabling perturbative calculations; and confinement, the empirical fact that color-charged particles are not observed in isolation, instead forming color-neutral hadrons such as mesons (quark–antiquark pairs) and baryons (three quarks). The strength of the interaction runs with energy, described by the QCD beta function and the scale parameter Λ_QCD.

The theory is formulated by a gauge-invariant Lagrangian, including quark fields coupled to gluons via the covariant derivative and the gluon self-interaction through the field strength tensor. At high energies, perturbation theory provides precise predictions; at low energies, nonperturbative methods such as lattice QCD are used to study hadron spectra, decays, and confinement.

Phenomenology and experiments: deep inelastic scattering revealed quarks inside nucleons; jet production in high-energy collisions reflects quark and gluon fragmentation. Lattice QCD computations yield hadron masses and other observables, consistent with experimental results. QCD is a central component of the Standard Model, tested across a broad range of energies and processes.