In
physics, the
W and Z bosons are the elementary particles that mediate the
weak nuclear force. Their discovery at
CERN in 1983 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-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
zero electric charge.
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 Z
0) 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/c
2, 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.
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 the explosion of both
supernovae and
neutron bombs:
:
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 2003, the only prediction of the Standard Model which has not been experimentally confirmed is the existence of the Higgs boson.
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
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.
See also
External links
Category:Boson
Category:Particle physics
Category:Standard Model
Category:Electroweak theory
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