The W Boson is an elementary particle; it is one of the carriers of the weak interaction. It has a charge of ±1 (in units of the electron charge), a mass of 80.4110 GeV (about 80 times the mass of the proton), and weak isospin of the same. There exist two varieties of W boson, positively and negatively charged, which are antiparticles of each other. The third carrier of the weak force is the Z boson, which is uncharged. The W Boson was discovered in 1983 by the UA1 and UA2 experiments at the CERN laboratory, using the SPS accelerator. Carlo Rubbia and Simon Van der Meer won the Nobel Prize in 1984 for this discovery.
The W Boson is best known for mediating reactions for nuclear decay (fission). For example n -> p + e- + nu_e_bar (neutron decays into proton + electron + anti-neutrino). This reaction is known as beta decay. The opposite process also occurs: p + e- -> n + nu_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 somewhat of a conundrum. The W and Z are accurately described by a SU(2) 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.