In particle physics, a gauge boson is a elementary particle that acts as the force carrier for elementary fermions.
, W and Z bosons, and are gauge bosons. All known gauge bosons have a spin of 1 and therefore are . For comparison, the Higgs boson has spin zero and the hypothetical graviton has a spin of 2.
Gauge bosons are different from the other kinds of bosons: first, fundamental (the Higgs boson); second, , which are composite bosons, made of ; third, larger composite, non-force-carrying bosons, such as certain .
Gauge bosons in the Standard Model
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
, which carry the strong interaction.
Isolated gluons do not occur because they are Color charge and subject to colour confinement.
Multiplicity of gauge bosons
In a quantized
gauge theory, gauge bosons are
quantum of the
gauge theory. 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, the photon. In quantum chromodynamics, the more complicated group SU(3) has eight generators, corresponding to the eight gluons. The three W and Z bosons correspond (roughly) to the three generators of SU(2) in electroweak theory.
Massive gauge bosons
Gauge invariance requires that gauge bosons are described mathematically by field 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 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 non-zero Higgs vacuum 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 scalar Higgs boson, which has been observed in experiments at the LHC.
Beyond the Standard Model
Grand unification theories
The Georgi–Glashow model predicts additional gauge bosons named X and Y bosons. The hypothetical X and Y bosons mediate interactions between quarks and
, hence violating conservation of
baryon number and causing
proton decay. Such bosons would be even more massive than W and Z bosons due to symmetry breaking. Analysis of data collected from such sources as the
Super-Kamiokande neutrino detector has yielded no evidence of X and Y bosons.
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.
W′ and Z′ bosons
W′ and Z′ bosons refer to hypothetical new gauge bosons (named in analogy with the Standard Model W and Z bosons).
See also
-
1964 PRL symmetry breaking papers
-
Boson
-
Glueball
-
Quantum chromodynamics
-
Quantum electrodynamics
External links
