Elementary particles that are force carriers
The
Standard Model
of elementary particles, with the gauge bosons in the fourth column in red
In
particle physics
, a
gauge boson
is a
bosonic
elementary particle
that acts as the
force carrier
for elementary
fermions
.
[1]
[2]
Elementary particles whose interactions are described by a
gauge theory
interact with each other by the exchange of gauge bosons, usually as
virtual particles
.
Photons
,
W and Z bosons
, and
gluons
are gauge bosons. All known gauge bosons have a
spin
of 1; for comparison, the
Higgs boson
has spin zero and the hypothetical
graviton
has a spin of 2. 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
[
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]
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
.
[3]
Isolated gluons do not occur because they are
colour-charged
and subject to
colour confinement
.
Multiplicity of gauge bosons
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]
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, 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
[
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]
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 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 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
.
[4]
Beyond the Standard Model
[
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]
Grand unification theories
[
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]
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.
[
citation needed
]
Gravitons
[
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]
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
[
clarification needed
]
symmetry:
diffeomorphism invariance
.
W′ and Z′ bosons
[
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]
W′ and Z′ bosons refer to hypothetical new gauge bosons (named in analogy with the
Standard Model
W and Z bosons
).
See also
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]
References
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]
External links
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]