In particle physics, the W and Z bosons are vector bosons that are together known as the weak bosons or more generally as the intermediate vector bosons. These elementary particles mediate the weak interaction; the respective symbols are W+, W−, and Z0. The W± bosons have either a positive or negative electric charge of 1 elementary charge and are each other's antiparticles. The Z0 boson is electrically neutral and is its own antiparticle. The three particles each have a spin of 1. The W± bosons have a magnetic moment, but the Z0 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 pivotal in establishing 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", 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 were named for having 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 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. The Z boson is not involved in the absorption or emission of electrons or positrons. 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 with the electron (with the momentum transfer via the Z boson) since this behavior happens more often when the neutrino beam is present. In this process, the neutrino scatters off the electron (via exchange of a boson), transferring some of the neutrino's momentum to the electron.
Basic properties Physicists characterize particles by their energy equivalence at rest, using the mass energy equation to relate energy with mass. The W boson mass is 80.3692±0.0133 GeV and the Z boson is 91.1880±0.0020 GeV. 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 strong nuclear force is limited for different reasons; see Color confinement. All three bosons have particle spin s = 1 ħ. The emission of a W+ or W− boson either lowers or raises 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 Z0 boson can only change the spin, momentum, and energy of the other particle. (See also Weak neutral current.)
Relations to the 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 beta decay. Consider, for example, the beta decay of cobalt-60.
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 (often called a beta particle in this context) and an electron antineutrino:
Again, the neutron is not an elementary particle but a composite of an up quark and two down quarks (udd). It is 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:
which is immediately followed by decay of the W− itself:
Z bosons The Z0 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 spin and/or momentum. Z boson interactions involving neutrinos have distinct 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 confirmed shortly thereafter (also in 1973), in a neutrino experiment in the Gargamelle bubble chamber at CERN.
Predictions of the W+, W− and Z0 bosons
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