Gluons

Gluons are the gauge bosons of quantum chromodynamics (QCD), the theory of the strong interaction. They are massless vector particles that carry color charge and act as the mediators of forces between quarks and between gluons themselves.

In QCD the color symmetry is SU(3), which yields eight independent gluon fields. Each gluon corresponds to one generator of SU(3). Unlike photons, gluons carry color charge and can interact with each other, giving rise to three-gluon and four-gluon vertices. This non-Abelian structure underpins many features of the strong interaction.

Because color charges do not commute, gluons participate in self-interactions and the strong coupling runs with energy. At high energies the coupling becomes small (asymptotic freedom), while at large distances it becomes strong, leading to confinement: free quarks and gluons are not observed under normal conditions.

Gluons couple to quarks and bind them into hadrons such as protons, neutrons, and mesons. They also contribute to the internal momentum and spin structure of hadrons and drive jet production in high-energy collisions.

Experimental evidence for gluons includes three-jet events in electron-positron annihilation observed in the late 1970s, which were interpreted as radiated gluons. Additional support comes from scaling violations in deep inelastic scattering and lattice QCD studies, which also anticipate bound states of gluons called glueballs, though these remain unambiguous experimentally.