Gauge boson

In particle physics, gauge bosons are bosonic particles which act as carriers of the fundamental forces of Nature. More specifically, elementary particles whose interactions are described by gauge theory exert forces on each other by the exchange of gauge bosons, usually as virtual particles.

Standard Model gauge bosons

In the Standard Model, there are three kinds of gauge bosons: photons, W and Z bosons, and gluons. Each corresponds to one of the three Standard Model interactions: photons are gauge bosons of the electromagnetic interaction, W and Z bosons carry the weak interaction, and the gluons carry the strong interaction. Due to colour confinement, isolated gluons do not occur at low energies. What could result instead are massive glueballs (as of 2006, these are not widely experimentally confirmed).

Number of gauge bosons

In a quantized gauge theory, gauge bosons are quanta of the gauge fields. Consequently, there are as many gauge bosons as there are generators of the gauge field. In quantum electrodynamics, the gauge group is U(1); in this simple case, there is only one gauge boson. In quantum chromodynamics, the more complicated group SU(3) has 8 generators, corresponding to the eight gluons. The three W and Z bosons correspond (roughly) to the 3 generators of SU(2) in GWS theory.

Massive gauge bosons

For technical reasons involving gauge invariance, gauge bosons are described mathematically by field equations for massless particles. Therefore, at a naive theoretical level all gauge bosons are required to be massless, and the forces that they describe are required to be long-ranged. The conflict between this idea and experimental evidence that the weak interaction ha very short range requires further theoretical insight.

According to the Standard Model, the W and Z bosons gain mass via the Higgs mechanism. In the Higgs mechanism, the four gauge bosons (of SU(2)×U(1) symmetry) of the unified electroweak interaction couple to a Higgs field. This field undergoes spontaneous symmetry breaking due to the shape of its interaction potential. As a result, the universe is permeated by a nonzero Higgs vacuum expectation value. This VEV couples to three of the electroweak gauge bosons (the W's and Z), giving them mass; the remaining gauge boson remains massless (the photon). This theory also predicts the existence of a scalar Higgs boson, which has not yet been observed.

Beyond the Standard Model

Grand unification theories

In grand unified theories (GUTs), additional gauge bosons called X bosons and Y bosons would exist. These would direct interactions between quarks and leptons, violating conservation of baryon number and causing proton decay. These bosons would be extremely heavy (even more so than the W and Z bosons) due to symmetry breaking. No evidence of such bosons (for example, due to proton decays seen in Super-Kamiokande) has ever been seen.

Gravitons

The fourth fundamental interaction, gravity, may also be carried by a boson, called the graviton. In the absence of experimental evidence and a mathematically coherent theory of quantum gravity, it is unknown whether this would be a gauge boson or not. The role of gauge invariance in general relativity is played by a similar symmetry: diffeomorphism invariance.