First, in 1973, came the observation of neutral current interactions as predicted by electroweak theory. The discovery of the W and Z particles is a major CERN success story. As of 2007, despite intensive search for the Higgs boson carried out at CERN and Fermilab, its existence remains the main prediction of the Standard Model not to be confirmed experimentally. These days it is widely accepted as one of the pillars of the Standard Model of particle physics. 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. It predicts the existence of yet another new particle, the Higgs boson. One explanation, the Higgs mechanism, was forwarded by Peter Higgs in the late 1960s. Some mechanism is required to break the SU(2) symmetry, giving mass to the W and Z in the process. ![]() As a case in point, the photon is massless because electromagnetism is described by a U(1) gauge theory. These particles are accurately described by an SU(2) gauge theory, but the bosons in a gauge theory must be massless. The fact that the W and Z bosons have mass while photons are massless was a major obstacle in developing electroweak theory. Their electroweak theory postulated not only the W bosons necessary to explain beta decay, but also a new Z boson that had never been observed. This culminated around 1968 in a unified theory of electromagnetism and weak interactions by Sheldon Glashow, Steven Weinberg, and Abdus Salam, for which they shared the 1979 Nobel Prize in physics. 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.įollowing the spectacular success of quantum electrodynamics in the 1950s, attempts were undertaken to formulate a similar theory of the weak nuclear force. 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. Which is immediately followed by decay of the W − itself:īeing its own antiparticle, the Z boson has all zero quantum numbers. At the most fundamental level, then, the weak force changes the flavor of a single quark: It is in fact one of the down quarks that interacts in beta decay, turning into an up quark to form a proton (uud). The neutron is converted into a proton while also emitting an electron (called a beta particle in this context) and an antineutrino:Īgain, the neutron is not an elementary particle but a composite of an up quark and two down quarks (udd). This reaction does not involve the whole cobalt-60 nucleus, but affects only one of its 33 neutrons. Consider, for example, the beta decay of cobalt-60, an important process in supernovae explosions. The W boson is best known for its role in nuclear decay. The W and Z bosons are carrier particles that mediate the weak nuclear force, much like the photon is the carrier particle for the electromagnetic force. The Z 0 boson cannot change either electric charge nor any other charges (like strangeness, charm, etc.), only spin and momentum, so it never changes the generation or flavor of the particle emitting it (see weak neutral current). At the same time a W boson can change the generation of the particle, for example changing a strange quark to an up quark. The emission of a W + or W – boson can either raise or lower electric charge of the emitting particle by 1 unit, and alter the spin by 1 unit. The electromagnetic force, by contrast, has an infinite range because its boson (the photon) is massless. The mass of these bosons are significant because they limit the range of the weak nuclear force. With a mass of 80.4 and 91.2 GeV/c 2, respectively, the W and Z 0 particles are almost 100 times as massive as the proton-heavier than entire atoms of iron. ![]() ![]() ![]() These bosons are heavyweights among the elementary particles. All three particles are very short-lived with a mean life of about 3 × 10 −25 seconds. The Z boson (or Z 0) is electrically neutral and is its own antiparticle. Two kinds of W bosons exist with +1 and −1 elementary units of electric charge the W + is the antiparticle of the W −.
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