In particle physics, a gauge boson is a force
carrier, a bosonic particle that carries any
of the fundamental interactions of nature,
commonly called forces.
Elementary particles, whose interactions are
described by a gauge theory, interact with
each other by the exchange of gauge bosons—usually
as virtual particles.
All known gauge bosons have a spin of 1.
Therefore, all known gauge bosons are vector
bosons.
Gauge bosons are different from the other
kinds of bosons: first, fundamental scalar
bosons (the Higgs boson); second, mesons,
which are composite bosons, made of quarks;
third, larger composite, non-force-carrying
bosons, such as certain atoms.
== 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 gluons, which carry the strong interaction.Isolated
gluons do not occur because they are color-charged
and subject to color confinement.
=== Multiplicity 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 eight generators, corresponding
to the eight gluons.
The three W and Z bosons correspond (roughly)
to the three 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 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 nonzero Higgs vacuum expectation value (VEV).
This VEV couples to three of the electroweak
gauge bosons (the Ws 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 leptons, 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
