The W and Z bosons are together known as the
weak or more generally as the intermediate
vector bosons. These elementary particles
mediate the weak interaction; the respective
symbols are W+, W−, and Z. The W bosons
have either a positive or negative electric
charge of 1 elementary charge and are each
other's antiparticles. The Z boson is electrically
neutral and is its own antiparticle. The three
particles have a spin of 1. The W bosons have
a magnetic moment, but the Z has none. All
three of these particles are very short-lived,
with a half-life of about 3×10−25 s. Their
experimental discovery was a triumph for what
is now known as the Standard Model of particle
physics.
The W bosons are named after the weak force.
The physicist Steven Weinberg named the additional
particle the "Z particle", and later gave
the explanation that it was the last additional
particle needed by the model. The W bosons
had already been named, and the Z bosons have
zero electric charge.The two W bosons are
verified mediators of neutrino absorption
and emission. During these processes, the
W boson charge induces electron or positron
emission or absorption, thus causing nuclear
transmutation. The Z boson is not involved
in the absorption or emission of electrons
and positrons.
The Z boson mediates the transfer of momentum,
spin and energy when neutrinos scatter elastically
from matter (a process which conserves charge).
Such behavior is almost as common as inelastic
neutrino interactions and may be observed
in bubble chambers upon irradiation with neutrino
beams. Whenever an electron is observed as
a new free particle suddenly moving with kinetic
energy, it is inferred to be a result of a
neutrino interacting directly with the electron,
since this behavior happens more often when
the neutrino beam is present. In this process,
the neutrino simply strikes the electron and
then scatters away from it, transferring some
of the neutrino's momentum to the electron.
Because neutrinos are neither affected by
the strong force nor the electromagnetic force,
and because the gravitational force between
subatomic particles is negligible, such an
interaction can only happen via the weak force.
Since such an electron is not created from
a nucleon, and is unchanged except for the
new force impulse imparted by the neutrino,
this weak force interaction between the neutrino
and the electron must be mediated by an electromagnetically
neutral, weak-force boson particle. Thus,
this interaction requires a Z boson.
== Basic properties ==
These bosons are among the heavyweights of
the elementary particles. With masses of 80.4
GeV/c2 and 91.2 GeV/c2, respectively, the
W and Z bosons are almost 80 times as massive
as the proton – heavier, even, than entire
iron atoms. Their high masses limit the range
of the weak interaction. By way of contrast,
the photon is the force carrier of the electromagnetic
force and has zero mass, consistent with the
infinite range of electromagnetism; the hypothetical
graviton is also expected to have zero mass.
(Although gluons are also presumed to have
zero mass, the range of the color force is
limited for different reasons; see color confinement.)
All three bosons have particle spin s = 1.
The emission of a W+ or W− boson either
raises or lowers the electric charge of the
emitting particle by one unit, and also alters
the spin by one unit. At the same time, the
emission or absorption of a W boson can change
the type of the particle – for example changing
a strange quark into an up quark. The neutral
Z boson cannot change the electric charge
of any particle, nor can it change any other
of the so-called "charges" (such as strangeness,
baryon number, charm, etc.). The emission
or absorption of a Z boson can only change
the spin, momentum, and energy of the other
particle. (See also weak neutral current.)
== Weak nuclear force ==
The W and Z bosons are carrier particles that
mediate the weak nuclear force, much as the
photon is the carrier particle for the electromagnetic
force.
=== W bosons ===
The W bosons are best known for their role
in nuclear decay. Consider, for example, the
beta decay of cobalt-60.
6027Co → 6028Ni+ + e− + νeThis reaction
does not involve the whole cobalt-60 nucleus,
but affects only one of its 33 neutrons. The
neutron is converted into a proton while also
emitting an electron (called a beta particle
in this context) and an electron antineutrino:
n0 → p+ + e− + νeAgain, the neutron is
not an elementary particle but a composite
of an up quark and two down quarks (udd).
It is in fact one of the down quarks that
interacts in beta decay, turning into an up
quark to form a proton (uud). At the most
fundamental level, then, the weak force changes
the flavour of a single quark:
d → u + W−which is immediately followed
by decay of the W− itself:
W− → e− + νe
=== Z boson ===
The Z boson is its own antiparticle. Thus,
all of its flavour quantum numbers and charges
are zero. The exchange of a Z boson between
particles, called a neutral current interaction,
therefore leaves the interacting particles
unaffected, except for a transfer of momentum.
Z boson interactions involving neutrinos have
distinctive signatures: They provide the only
known mechanism for elastic scattering of
neutrinos in matter; neutrinos are almost
as likely to scatter elastically (via Z boson
exchange) as inelastically (via W boson exchange).
The first prediction of Z bosons was made
by Brazilian physicist José Leite Lopes in
1958, by devising an equation which showed
the analogy of the weak nuclear interactions
with electromagnetism. Steve Weinberg, Sheldon
Glashow and Abdus Salam later used these results
to develop the electroweak unification, in
1973. Weak neutral currents via Z boson exchange
were confirmed shortly thereafter (also in
1973), in a neutrino experiment in the Gargamelle
bubble chamber at CERN.
== Predicting the W and Z ==
Following the spectacular success of quantum
electrodynamics in the 1950s, attempts were
undertaken to formulate a similar theory of
the weak nuclear force. This culminated around
1968 in a unified theory of electromagnetism
and weak interactions by Sheldon Glashow,
Steven Weinberg, and Abdus Salam, for which
they shared the 1979 Nobel Prize in Physics.
Their electroweak theory postulated not only
the W bosons necessary to explain beta decay,
but also a new Z boson that had never been
observed.
The fact that the W and Z bosons have mass
while photons are massless was a major obstacle
in developing electroweak theory. These particles
are accurately described by an SU(2) gauge
theory, but the bosons in a gauge theory must
be massless. As a case in point, the photon
is massless because electromagnetism is described
by a U(1) gauge theory. Some mechanism is
required to break the SU(2) symmetry, giving
mass to the W and Z in the process. One explanation,
the Higgs mechanism, was forwarded by the
1964 PRL symmetry breaking papers. It predicts
the existence of yet another new particle;
the Higgs boson. Of the four components of
a Goldstone boson created by the Higgs field,
three are "eaten" by the W+, Z0, and W−
bosons to form their longitudinal components,
and the remainder appears as the spin 0 Higgs
boson.
The combination of the SU(2) gauge theory
of the weak interaction, the electromagnetic
interaction, and the Higgs mechanism is known
as the Glashow-Weinberg-Salam model. Today
it is widely accepted as one of the pillars
of the Standard Model of particle physics,
particularly given the 2012 discovery of the
Higgs boson by the CMS and ATLAS experiments.
The model predicts that W and Z bosons have
the following masses:
M
W
=
1
2
v
g
M
Z
=
1
2
v
g
2
+
g
′
2
{\displaystyle {\begin{aligned}M_{W}&={\tfrac
{1}{2}}vg\\M_{Z}&={\tfrac {1}{2}}v{\sqrt {g^{2}+{g'}^{2}}}\end{aligned}}}
where g is the SU(2) gauge coupling, g' is
U(1) gauge coupling, and v is the Higgs vacuum
expectation value.
== Discovery ==
Unlike beta decay, the observation of neutral
current interactions that involve particles
other than neutrinos requires huge investments
in particle accelerators and detectors, such
as are available in only a few high-energy
physics laboratories in the world (and then
only after 1983). This is because Z-bosons
behave in somewhat the same manner as photons,
but do not become important until the energy
of the interaction is comparable with the
relatively huge mass of the Z boson.
The discovery of the W and Z bosons was considered
a major success for CERN. First, in 1973,
came the observation of neutral current interactions
as predicted by electroweak theory. The huge
Gargamelle bubble chamber photographed the
tracks of a few electrons suddenly starting
to move, seemingly of their own accord. This
is interpreted as a neutrino interacting with
the electron by the exchange of an unseen
Z boson. The neutrino is otherwise undetectable,
so the only observable effect is the momentum
imparted to the electron by the interaction.
The discovery of the W and Z bosons themselves
had to wait for the construction of a particle
accelerator powerful enough to produce them.
The first such machine that became available
was the Super Proton Synchrotron, where unambiguous
signals of W bosons were seen in January 1983
during a series of experiments made possible
by Carlo Rubbia and Simon van der Meer. The
actual experiments were called UA1 (led by
Rubbia) and UA2 (led by Pierre Darriulat),
and were the collaborative effort of many
people. Van der Meer was the driving force
on the accelerator end (stochastic cooling).
UA1 and UA2 found the Z boson a few months
later, in May 1983. Rubbia and van der Meer
were promptly awarded the 1984 Nobel Prize
in Physics, a most unusual step for the conservative
Nobel Foundation.The W+, W−, and Z0 bosons,
together with the photon (γ), comprise the
four gauge bosons of the electroweak interaction.
== Decay ==
The W and Z bosons decay to fermion–antifermion
pairs but neither the W nor the Z bosons can
decay into the higher-mass top quark. Neglecting
phase space effects and higher order corrections,
simple estimates of their branching fractions
can be calculated from the coupling constants.
=== W bosons ===
W bosons can decay to a lepton and neutrino
or to an up-type quark and a down-type quark.
The decay width of the W boson to a quark–antiquark
pair is proportional to the corresponding
squared CKM matrix element and the number
of quark colours, NC = 3. The decay widths
for the W bosons are then proportional to:
Here, e+, μ+, τ+ denote the three flavours
of leptons (more exactly, the positive charged
antileptons). νe, νμ, ντ denote the three
flavours of neutrinos. The other particles,
starting with u and d, all denote quarks and
antiquarks (factor NC is applied). The various
Vij denote the corresponding CKM matrix coefficients.
Unitarity of the CKM matrix implies that
|Vud|2 + |Vus|2 + |Vub|2 =
|Vcd|2 + |Vcs|2 + |Vcb|2 = 1. Therefore, the
leptonic branching ratios of the W boson are
approximately B(e+νe) = B(μ+νμ) = B(τ+ντ)
= ​1⁄9. The hadronic branching ratio is
dominated by the CKM-favored ud and cs final
states. The sum of the hadronic branching
ratios has been measured experimentally to
be 67.60±0.27%, with B(l+νl) = 10.80±0.09%.
=== Z bosons ===
Z bosons decay into a fermion and its antiparticle.
As the Z boson is a mixture of the pre-symmetry-breaking
W0 and B0 bosons (see weak mixing angle),
each vertex factor includes a factor T3 − Q
sin2 θW; where T3 is the third component
of the weak isospin of the fermion, Q is the
electric charge of the fermion (in units of
the elementary charge), and θW is the weak
mixing angle. Because the weak isospin is
different for fermions of different chirality,
either left-handed or right-handed, the coupling
is different as well.
The relative strengths of each coupling can
be estimated by considering that the decay
rates include the square of these factors,
and all possible diagrams (e.g. sum over quark
families, and left and right contributions).
This is just an estimate, as we are considering
only tree-level diagrams in the Fermi theory.
Here, L and R denote either the left- or right-handed
chirality of the fermions, respectively. (The
right-handed neutrinos do not exist in the
standard model. However, in some extensions
beyond the standard model they do.) The notation
x = sin2 θW is used.
== See also ==
Bose–Einstein statistics
Boson – A class of particle that follows
Bose–Einstein statistics
Higgs boson – Elementary particle related
to the Higgs field giving particles mass
List of particles
Mathematical formulation of the Standard Model
W′ and Z′ bosons – Hypothetical gauge
bosons that arise from extensions of the electroweak
symmetry of the Standard Model
X and Y bosons: analogous pair of bosons predicted
by the Grand Unified Theory
ZZ diboson
