Chromodynamics

Chromodynamics, more commonly called quantum chromodynamics (QCD), is the sector of the Standard Model that describes the interactions of quarks and gluons via the exchange of color charge. It is a non-Abelian gauge theory based on the SU(3) color symmetry. Quarks come in six flavors and carry one of three color charges, whereas gluons are the gauge bosons carrying color and anticolor combinations, enabling self-interaction.

Key properties include asymptotic freedom at high energies, meaning coupling becomes weak at short distances, and confinement at low energies, preventing isolated color charges. The QCD Lagrangian contains quark fields and gluon fields, interacting through the covariant derivative and the field strength tensor. The strength of the interaction is governed by the running coupling constant alpha_s, which decreases with energy scale due to quantum corrections.

Predictions of QCD have been confirmed in deep inelastic scattering, jet formation in e+e- annihilation, scaling violations, hadron spectra, and lattice QCD simulations reproducing hadron masses. Non-perturbative phenomena, such as confinement and chiral symmetry breaking, require lattice QCD calculations. In high-temperature or high-density conditions, quark-gluon plasma is predicted and studied experimentally in heavy-ion collisions.

Chromodynamics is the theoretical framework that underpins the description of strong interactions, complementing electroweak theory. It remains an active area of research, including precision determinations of alpha_s, studies of hadron structure, and investigations into the behavior of matter under extreme conditions.