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
Elementary particle general relativity. There may be hypothetical elementary particles not described by the Standard Model, such as the graviton, the particle that would carry the gravitational force, and sparticles, supersymmetric partners of the ordinary particles. The 12 fundamental fermions are divided into 3 generations of 4 particles each. Half of the fermions are leptons, three of which have an electric charge of −1, called the electron (), the muon (), and the tau (), the other three leptons are neutrinos (, , ), which are the only elementary fermions with no electric or color charge. The remaining six particles are quarks