Quantum chromodynamics

Quantum chromodynamics (QCD) is the theoretical framework in particle physics that describes the strong interaction, the force that binds quarks together inside hadrons such as protons, neutrons, and pions. It is a non-abelian gauge theory based on the symmetry group SU(3), with gluons as the force carriers and a property called color charge as the analog of electric charge. QCD is a cornerstone of the Standard Model of particle physics, supported by extensive experimental evidence.

History

The theory emerged from the need to explain the growing number of hadrons discovered in the 1950s and 1960s. After hadrons were classified by properties like charge, isospin, and strangeness, the eightfold way organized them into groups. This led Murray Gell-Mann and George Zweig to propose in 1963 that hadrons are composed of smaller particles called quarks.

A key puzzle arose: certain baryons, such as the Ω⁻ hyperon and the Δ⁺⁺, seemed to contain three identical quarks with parallel spins, violating the Pauli exclusion principle. This was resolved independently in 1964–65 by Oscar Greenberg, and by Moo-Young Han and Yoichiro Nambu, who proposed that quarks carry an additional SU(3) gauge degree of freedom, later named color charge. Han and Nambu also suggested that an octet of vector gauge bosons—the gluons—mediate the interaction between these color charges.

QCD exhibits three defining properties. Color confinement means that separating two color charges requires increasing energy until a new quark–antiquark pair is spontaneously produced, forming hadrons instead of isolating a single color charge. This phenomenon, though not analytically proven, is well established by lattice QCD calculations and decades of experiments. Asymptotic freedom, discovered in 1973 by David Gross and Frank Wilczek, and independently by David Politzer, describes how the strong interaction weakens as the energy scale increases (and length scale decreases).

This work earned them the 2004 Nobel Prize in Physics. Chiral symmetry breaking, elucidated by Yoichiro Nambu in 1960 (for which he received the 2008 Nobel Prize), explains how hadrons acquire masses far larger than their constituent quarks, while pseudoscalar mesons remain exceptionally light. Lattice simulations have confirmed Nambu’s predictions.