W and Z bosons mediate the weak nuclear force. The W Boson is best known for mediating reactions for nuclear decay (fission). For example
(neutron decays into proton + electron + anti-neutrino). This reaction is known as beta decay. The opposite process also occurs:That the W and Z bosons have mass is something 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.
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