Gluon is an elementary gauge boson associated with the strong interaction described by quantum chromodynamics. Gluons carry colour charge and mediate interactions between quarks. Unlike photons, which are electrically neutral, gluons themselves carry combinations of colour and anticolour quantum numbers. This feature allows gluons to interact directly with one another as well as with quarks. Gluons are massless in the Standard Model and have spin one. Their self interaction is a defining feature of nonabelian gauge theory and is central to the distinctive behaviour of the strong interaction.
Quantum chromodynamics uses a gauge symmetry based on the colour group with eight independent gluon gauge fields. Although one might naively expect a colour and anticolour combination to produce nine possibilities, the underlying gauge structure produces eight physical gluon colour states. Gluons can change the colour state of a quark during interaction while preserving the appropriate overall gauge structure. The mathematics of the theory is substantially more complicated than electromagnetism because the gauge bosons themselves carry the relevant charge.
Gluon self interaction leads to asymptotic freedom and confinement. At very short distances or high momentum transfers, the effective strong coupling becomes weaker, allowing quarks and gluons to behave approximately as weakly interacting constituents in high energy processes. At larger distances, the interaction becomes strong and isolated quarks and gluons are not observed as free particles under ordinary conditions. Instead they become confined within colour neutral hadrons such as protons, neutrons and mesons.
Gluons contribute substantially to the mass and internal energy of ordinary hadrons. The proton mass, for example, cannot be explained simply by adding the small rest masses of its three valence quarks. A large part arises from the energy associated with quark motion, gluon fields and the strongly interacting quantum vacuum structure. Through mass energy equivalence, this internal energy contributes to the observed mass of the composite particle. This provides an important example of how most of the mass of ordinary visible matter is associated with strong interaction dynamics rather than simply elementary quark rest masses.
Gluons cannot normally be detected as isolated long lived particles because of colour confinement. In high energy collisions, energetic quarks and gluons produced in a hard interaction undergo parton showering and hadronisation, producing sprays of hadrons known as jets. Experimental physicists identify jets and study their properties to infer the presence and behaviour of the underlying quarks and gluons. Gluon initiated jets have statistical and structural differences from quark initiated jets, allowing sophisticated collider analyses to distinguish them probabilistically.
Gluon interactions are essential to the running of the strong coupling constant. Quantum corrections alter the effective strength of the strong interaction with energy scale. Gluon self interactions contribute importantly to the sign of the beta function that produces asymptotic freedom in quantum chromodynamics. This behaviour is fundamentally different from the electromagnetic coupling evolution and explains why high energy scattering can be treated with perturbative techniques while low energy hadronic physics becomes strongly coupled and requires different theoretical approaches.
Gluons play a major role in particle accelerators and collider physics. High energy collisions frequently involve quarks and gluons as constituents of hadrons, and gluon initiated processes contribute substantially to particle production at modern colliders. Understanding parton distribution functions is therefore essential because gluons carry a significant and scale dependent fraction of the momentum of protons and other hadrons. Precision measurements of jets, heavy particles and other final states provide information about gluon distributions and quantum chromodynamic predictions.
Gluon should not be confused with a generic carrier of force that is directly observable as an isolated particle. It is a gauge boson of the strong interaction but its colour charge causes self interaction and confinement. It also differs fundamentally from the photon because the photon carries no electric charge and does not self interact through the same gauge mechanism. Gluons are massless at the fundamental level yet participate in the dynamics responsible for massive composite hadrons. Their physical effects are therefore often observed indirectly through hadronic structure and high energy collision products rather than through free gluon detection.
