A gluon is an elementary particle of the Standard Model that mediates the strong interaction, binding quarks into composite particles and participating in the interactions that determine their structure. Gluons are the force-carrying particles of quantum chromodynamics (QCD). Unlike the electrically neutral carriers of electromagnetic interactions, they themselves possess the charge associated with the force they transmit. This allows gluons to interact directly with one another, a defining feature of the strong interaction. (pdg.lbl.gov)
Physical properties
A gluon is a boson with spin 1. In the Standard Model, it has zero rest mass and zero electric charge. Its electrically neutral character does not make it uncharged in every sense: it carries color charge, the quantum property responsible for strong interactions. “Color” is a technical label, unrelated to visible colors or the optical properties of matter. (pdg.lbl.gov)
The photon, which mediates electromagnetism, provides a useful comparison. Photons carry no electric charge, whereas gluons carry color charge. Consequently, QCD contains direct gluon self-interactions that have no equivalent among the basic photon interaction vertices of quantum electrodynamics. These interactions make the gluon field fundamentally nonlinear and contribute to the difficulty of calculating strong-interaction phenomena. (hep.phy.cam.ac.uk)
Eight gluons and gauge symmetry
QCD is a gauge theory based on the mathematical symmetry group SU(3). Quarks transform in its three-dimensional fundamental representation, while gluons transform in its eight-dimensional adjoint representation. There are therefore eight independent gluon fields, commonly described as eight types of gluon. These are color states of the same kind of elementary particle, not eight particles with different masses or interaction strengths. (pdg.lbl.gov)
A schematic description assigns a gluon a color and an anticolor. This is useful for following color flow, but nine simple color–anticolor combinations do not correspond to nine physical gluons. The combinations separate into an eight-dimensional octet and a one-dimensional color singlet; the singlet is not a gluon of QCD. The octet includes linear combinations of color–anticolor states, so treating all eight gluons as separately labeled pairs is only an approximation to their mathematical description. (hep.phy.cam.ac.uk)
In quantum field theory, gluons are excitations of these gauge fields. The QCD interaction terms allow quarks to emit or absorb gluons and include vertices involving three or four gluons. Gluon emission changes color but does not change quark flavor: an up quark remains an up quark, for example. The field description, rather than a picture of miniature objects connected by literal glue, specifies these interactions. (pdg.lbl.gov)
Confinement and energy dependence
Under ordinary conditions, gluons are not observed as isolated particles. Together with quarks, they are subject to color confinement: observable strongly interacting particles occur as color-neutral combinations called hadrons. When energetic quarks and gluons emerge from a collision, they develop into hadrons through hadronization, rather than arriving at a detector as freely propagating colored particles. (api.repository.cam.ac.uk)
The interaction strength depends on the momentum scale at which it is probed. At sufficiently high momentum transfers, corresponding to short distances, the effective QCD coupling becomes weaker. This property, asymptotic freedom, permits calculations using perturbation theory, an expansion in the interaction strength. At lower momentum scales the coupling grows, and such expansions generally cease to describe hadron formation reliably. (hep.phy.cam.ac.uk)
For this strongly coupled regime, lattice QCD provides a complementary approach. It represents quark and gluon fields on a discrete spacetime lattice and uses numerical calculations to investigate their collective behavior. This connects the underlying theory to properties of hadrons and to the behavior of strongly interacting matter at high temperatures. (energy.gov)
Experimental discovery
Gluons were discovered in 1979 through experiments at the PETRA electron–positron collider at DESY in Hamburg, Germany. The JADE, MARK J, PLUTO, and TASSO collaborations contributed evidence. The central signature was a collision producing three distinct particle jets: concentrated streams of hadrons whose arrangement indicated an underlying quark, antiquark, and radiated gluon. (desy.de)
In the simplest quark–antiquark production process, two approximately opposing jets are expected. A sufficiently energetic gluon radiated by either constituent produces a third jet. The observed three-jet configurations supplied direct experimental evidence for gluon production, although the detectors recorded hadrons rather than an isolated gluon. Subsequent measurements of jet distributions supported the gluon’s spin-1 assignment. The discovery helped establish QCD as the theory of strong interactions. (desy.de)
Gluons in matter
A proton or neutron is not adequately described as three quarks held together by a static connector. Its internal structure includes dynamical gluon fields and quark–antiquark contributions. Most of its mass arises from the energy associated with these interacting constituents, rather than simply from the rest masses of its valence quarks. Massless gluons can therefore contribute to the mass of a composite system through the energy they carry. (energy.gov)
At extreme temperatures and energy densities, matter can form a quark–gluon plasma, in which quarks and gluons are no longer confined within individual hadrons. Heavy-ion collisions recreate small, short-lived regions of this state. Experiments at CERN’s Large Hadron Collider investigate its properties through particle production, collective motion, and the energy loss of energetic jets passing through the medium. This plasma remains an interacting many-body system, not a collection of permanently isolated gluons. (home.cern)