W and Z bosons

In physics, the W and Z bosons are the elementary particles that mediate the weak force. Their discovery has been heralded as a major success for the Standard Model of particle physics.

The W particle is named after the weak nuclear force. The Z particle was semi-humorously 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 zero electric charge.

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Basic properties

Two kinds of W bosons exist with +1 and −1 elementary units of electric charge; the W⁺ is the antiparticle of the W⁻. The Z boson (or Z⁰) is electrically neutral and is its own antiparticle. All three particles are very short-lived with a mean life of about 3 × 10⁻²⁵ seconds.

These bosons are heavyweights among the elementary particles. With a mass of 80.4 and 91.2 GeV/c², respectively, the W and Z⁰ particles are almost 100 times as massive as the proton—heavier than entire atoms of iron. The mass of these bosons are significant because they limit the range of the weak nuclear force. The electromagnetic force, by contrast, has an infinite range because its boson (the photon) is massless.

All three types have a spin of 1.

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. 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 Z⁰ 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).

The weak nuclear force

  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 W boson is best known for its role in nuclear decay. Consider, for example, the beta decay of cobalt-60, an important process in supernovae explosions.

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.

Predicting the W and Z

  Following the spectacular success of quantum electrodynamics in the 1950s, attempts were undertaken to formulate a similar theory of the weak nuclear force. 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. 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.

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 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.

Discovery of the W and Z

The discovery of the W and Z particles is a major CERN success story. First, in 1973, came the observation of neutral current interactions as predicted by electroweak theory. The huge Gargamelle bubble chamber photographed the tracks of a few electrons suddenly starting to move, seemingly of their own accord. This is interpreted as a neutrino interacting with the electron by the exchange of an unseen Z boson. The neutrino is otherwise undetectable, so the only observable effect is the momentum imparted to the electron by the interaction.

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 Super Proton Synchrotron, 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.

See also

• Standard model (mathematical formulation)
• List of particles
• X and Y bosons: analogous pair of bosons predicted by GUT
• W' boson
• Z' boson