The W particle is named after the weak nuclear force. The Z particle was semi-humourously given its name because it was said to be the last particle to need discovery. Another explanation is that the Z particle derives its name from the fact that it has zeroelectric charge.
These bosons are heavyweights among the elementary particles. With a mass of 80.4 and 91.2 GeV/c2, respectively, the W and Z particles are almost 100 times as massive as the proton—heavier than atoms of iron. The mass of these bosons is significant because it limits the range of the weak nuclear force. The electromagnetic force, by contrast, has an infinite range because its boson (the photon) is massless.
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 antineutrino:
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 flavor of a single quark:
which is immediately followed by decay of the W− itself:
Being its own antiparticle, the Z boson has all zero quantum numbers. 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. Unlike beta decay, the observation of neutral current interactions requires huge investments in particle accelerators and detectors, such as are available in only a few high-energy physics laboratories in the world.
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 in the late 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 2003, the only prediction of the Standard Model which has not been experimentally confirmed is the existence of the Higgs boson.
The discovery of the W and Z particles 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 SPS, where unambiguous signals of W particles 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, 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 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.
