W and Z bosons

The W boson is an elementary particle, having an electric charge of just ±1, a mass of 80.4110 GeV (about 80 times the proton's mass), and weak isospin of the same. There exist three varieties of W bosons: positively-charged types, negatively-charged (antiparticles of each other) types, and the Z boson, which possesses no charge whatsoever. The discovery of the W Boson occurred in 1983, during a series of SPS accelerator-based experiments being conducted by Carlo Rubbia and Simon Van der Meer, working at the CERN laboratory. For their efforts, they were awarded the Nobel Prize, one year later.

W and Z bosons mediate the weak nuclear force. The W Boson is best known for mediating reactions for nuclear decay (fission). For example

n → p + e⁻ + ν_e

(neutron decays into proton + electron + anti-neutrino). This reaction is known as beta decay. The opposite process also occurs:

p + e⁻ → n + ν_e

(proton + electron goes to neutron + neutrino) and is called electron capture. Since protons are not fundamental particles (they are made up of quarks), it is the quarks that interact. The first example is then

d → W⁻ + u,

and then the W⁻ decays into an electron and electron-type neutrino.

That the W and Z bosons have mass is something of a conundrum. The W and Z are accurately described by a SU(2)xU(1) Gauge theory, but the bosons in a gauge theory must be massless. The photon is also massless because the photon and electromagnetism are 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. The most popular is called the Higgs mechanism, and requires an extra particle, the Higgs Boson.

The combination of the SU(2) gauge theory describing the W and Z, the electromagnetic interaction, and the Higgs mechanism is known as the Glashow-Weinberg-Salam model. Glashow, Weinberg, and Salam won the 1979 Nobel Prize in Physics for this work. These days it is very widely accepted, and has been adopted as part of the standard model of particle physics. At the present time (Sep 25, 2001), the only missing piece of this model is the Higgs Boson.