Gauge boson certain atoms. The Standard Model of particle physics recognizes four kinds of gauge bosons: photons, which carry the electromagnetic interaction; W and Z bosons, which carry the weak interaction; and gluons, which carry the strong interaction. Isolated gluons do not occur because they are color-charged and subject to color confinement. 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
W and Z bosons Their electroweak theory postulated not only the W bosons necessary to explain beta decay, but also a new Z boson that had never been observed. The fact that the W and Z bosons have mass while photons are massless was a major obstacle in developing electroweak theory. These particles are accurately described by an SU(2) gauge theory, but the bosons in a gauge theory must be massless. As a case in point, the photon is massless because electromagnetism is described by a U(1) gauge theory. Some mechanism is required to break the SU(2) symmetry, giving mass to the W and