W and Z bosons

The W and Z bosons (together known as the weak bosons) are the elementary particles that mediate the weak interaction; their symbols are W+
, W−
and Z. The W bosons have a positive and negative electric charge of 1 elementary charge respectively and are each other's antiparticle. The Z boson is electrically neutral and its own antiparticle. All three of these particles are very short-lived with a half-life of about 3×10⁻²⁵ s. Their discovery was a major success for what is now called 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"^[3], later giving the explanation that it was the last additional particle needed by the model – the W bosons had already been named – and that it has zero electric charge.^[4]

The two W bosons are best known as mediators of neutrino absorption and emission, where their charge is associated with electron or positron emission or absorption, always causing nuclear transmutation. The Z boson is most easily detected as a necessary theoretical force-mediator, whenever neutrinos scatter elastically from matter, something that must happen without production or absorption of new charged particles. Such behavior (which is almost as common as inelastic neutrino interactions) is seen in bubble chambers irradiated with neutrino beams. Whenever an electron simply "appears" in such a chamber as a free particle, and begins to move as a result of an impulse in the direction of the neutrinos, and this behavior happens more often when the neutrino beam is present, it is inferred to be a result of a neutrino interacting directly with the electron. Such an interaction can only happen via the weak force. Since such an electron is not created from a nucleon, and remains unchanged except for the impulse imparted by the neutrino, this weak force interaction between the neutrino and the electron must be mediated by a particle with no charge, and so must be mediated by 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 100 times as massive as the proton—heavier than entire atoms of iron. The masses of these bosons are significant because they act as the force carriers of a quite short-range fundamental force: their high masses thus limit the range of the weak nuclear force. By way of contrast, the electromagnetic force has an infinite range because its force carrier, the photon, has zero rest mass.

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 obviously 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 like the photon is the carrier particle for the electromagnetic force. The W bosons are best known for their role in nuclear decay. Consider, for example, the beta decay of cobalt-60, an important process in supernova explosions.

60
27Co → 60
28Ni + e−
+ ν
e

This 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−
+ ν
e

Again, 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−
+ ν
e

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). Weak neutral currents via Z boson exchange were predicted in 1973 by Abdus Salam, Sheldon Glashow and Steven Weinberg,^[5] and confirmed shortly thereafter in 1974, in a neutrino experiment in the Gargamelle bubble chamber at CERN.

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 rest mass of the Z boson.

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.^[6] 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 Peter Higgs and others in the mid 1960s. It predicts the existence of yet another new particle; the 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. These days it is widely accepted as one of the pillars of the Standard Model of particle physics. As of 13 December 2011, intensive search for the Higgs boson carried out at CERN has indicated that if the particle is to be found, it seems likely to be found around 125 GeV. Follow the Higgs boson article for updates.

Discovery

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 conducted by Carlo Rubbia and Simon van der Meer. The actual experiments were called UA1 (led by Rubbia) and UA2 (led by Peter Jenni),^[7] 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.^[8]

The W+
, W−
, and Z0
bosons, together with the photon (γ), build up 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, N_C = 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). ν
e, ν
μ, ν
τ denote the three flavours of neutrinos. The other particles, starting with u and d, all denote quarks and antiquarks (factor N_C is applied). The various V_ij denote the corresponding CKM matrix coefficients.

Unitarity of the CKM matrix implies that |V_ud|² + |V_us|² + |V_ub|² =  |V_cd|² + |V_cs|² + |V_cb|² = 1. Therefore the leptonic branching ratios of the W boson are approximately B(e+
ν
e) = B(μ+
ν
μ) = B(τ+
ν
τ) = ¹⁄₉ (~11.11%). The hadronic branching ratio is dominated by the CKM-favored ud and cs final states, and the sum of the hadronic branching ratios is roughly ²⁄₃ (~66.67%). The branching ratios have been measured experimentally: B(l⁺ν_l) = 10.80±0.09% and B(hadrons) =67.60±0.27%.^[9]

Z bosons

Z bosons decay into a fermion and its antiparticle. The decay width of a Z boson to a fermion–antifermion pair is proportional to the square of the weak charge T₃ − Qx, where T₃ is the third component of the weak isospin of the fermion, Q is the charge of the fermion (in units of the elementary charge), and x = sin²θ_W, where θ_W 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 decay width of the Z boson for quarks is also proportional to N_C.

Here, L and R denote the left- and right-handed chiralities 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.^[11][12]

See also

• Standard Model (mathematical formulation)
• List of particles
• X and Y bosons: analogous pair of bosons predicted by the Grand Unified Theory
• W' and Z' bosons
• Boson
• Bose–Einstein statistics

References

External links

• The Review of Particle Physics, the ultimate source of information on particle properties.
• The W and Z particles: a personal recollection by Pierre Darriulat
• When CERN saw the end of the alphabet by Daniel Denegri
• W and Z page from CERN
• W and Z particles at Hyperphysics
• Z particle at Everything2