Everything about Gauge Boson totally explained
In
particle physics,
gauge bosons are
bosonic particles that act as carriers of the
fundamental forces of nature. More specifically,
elementary particles whose
interactions are described by
gauge theory exert forces on each other by the exchange of gauge bosons, usually as
virtual particles.
Standard Model gauge bosons
In the
Standard Model, there are three kinds of gauge bosons:
photons,
W and Z bosons, and
gluons. Each corresponds to one of the three Standard Model interactions: photons are gauge bosons of the
electromagnetic interaction, W and Z bosons carry the
weak interaction, and the gluons carry the
strong interaction. Due to
color confinement, isolated gluons don't occur at low energies. What could result instead are massive
glueballs (
as of 2006, these are not widely confirmed experimentally).
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's 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 naive 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 interaction has 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. 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 hasn't yet been observed.
Beyond the Standard Model
Grand unification theories
In
grand unified theories (GUTs), additional gauge bosons called
X and Y bosons would exist. These would direct interactions between
quarks and
leptons, violating conservation of
baryon number and causing
proton decay. These bosons would be extremely heavy (even more so than the
W and Z bosons) due to symmetry breaking. No evidence of such bosons (for example, due to proton decays seen in
Super-Kamiokande) has ever been seen.
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's 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.
Z' boson
See section
Z' boson Further Information
Get more info on 'Gauge Boson'.
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