Gauge bosons are different from the other kinds of bosons: first, fundamentalscalar bosons (the Higgs boson); second,mesons, which arecomposite bosons, made ofquarks; third, larger composite, non-force-carrying bosons, such as certainatoms.
In aquantizedgauge theory, gauge bosons arequanta of thegauge fields. Consequently, there are as many gauge bosons as there are generators of the gauge field. Inquantum electrodynamics, the gauge group isU(1); in this simple case, there is only one gauge boson, the photon. Inquantum chromodynamics, the more complicated groupSU(3) has eight generators, corresponding to the eight gluons. The three W and Z bosons correspond (roughly) to the three generators ofSU(2) inelectroweak theory.
Gauge invariance requires that gauge bosons are described mathematically byfield equations for massless particles. Otherwise, the mass terms add non-zero additional terms to the Lagrangian under gauge transformations, violating gauge symmetry. Therefore, at a naïve 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 and strong interactions have a very short range requires further theoretical insight.
According to the Standard Model, the W and Z bosons gain mass via theHiggs mechanism. In the Higgs mechanism, the four gauge bosons (of SU(2)×U(1) symmetry) of the unifiedelectroweak interaction couple to aHiggs field. This field undergoesspontaneous symmetry breaking due to the shape of its interaction potential. As a result, the universe is permeated by a non-zero Higgsvacuum expectation value (VEV). This VEV couples to three of the electroweak gauge bosons (W+, W− and Z), giving them mass; the remaining gauge boson remains massless (the photon). This theory also predicts the existence of a scalarHiggs boson, which has been observed in experiments at theLHC.[4]
TheGeorgi–Glashow model predicts additional gauge bosons named X and Y bosons. The hypothetical X and Y bosons mediate interactions between quarks andleptons, hence violating conservation ofbaryon number and causingproton decay. Such bosons would be even more massive than W and Z bosons due tosymmetry breaking. Analysis of data collected from such sources as theSuper-Kamiokandeneutrino detector has yielded no evidence of X and Y bosons.[5]
