Weinberg-Salam Theory
Antoine-Henri Becquerel's discovery of radioactivity in 1896 was also the indirect discovery of the weak nuclear force. Becquerel's photographic plate was fogged by beta particles (electrons) emitted by uranium nuclei in a process known as weak beta decay. A neutron sitting in a uranium nucleus becomes a proton while emitting an electron and an antineutrino. Physicists now understand that protons and neutrons are not fundamental particles, but rather are each composed of three quarks; at the most fundamental level, then, this process transforms a down quark into an up quark: Every type of particle, massive or massless, charged or neutral, quark or lepton, interacts via the weak force. In comparison, leptons do not feel the strong force, and neutral particles do not feel the electromagnetic force.
The first theory of weak interactions was devised by Enrico Fermi in 1933, and was quite successful in modeling beta decay, a low energy process. It was recognized by the 1950s, however, that the theory could not be valid at high energies. The problem with Fermi's theory was that the weak force was not mediated by an intermediate particle.
Quantum electrodynamics (QED), the quantum field theory of the electromagnetic interaction, had shown that charged particles interact via the exchange of a photon, the quantum of the electromagnetic field. This is true for all interactions; matter particles interact by exchanging field quanta. It is proposed that gravity is mediated by the exchange of, as of yet unobserved, gravitons, the strong force by the exchange of gluons, and the weak force by the exchange of intermediate vector bosons. A boson is a particle of integer spin; vector refers to spin-1 particles. The properties of the exchange particle can be inferred from the characteristics of the force. The intermediate vector bosons had to be quite massive, as the weak force has a very short range. And three were needed, the positively charged W- , the negatively charged W+ , and the neutral Z0 .
It was not difficult to write down the field theory of particles interacting through massive vector bosons, but the new theory had a much more serious and seemingly fatal flaw, it was non-renormalizable. There was no consistent field theory of massive vector bosons.
By the mid 1960s formal developments in quantum field theory placed in the hands of theorists the tools required to construct a viable theory of weak interactions. Gauge theories came first. A field theory of matter particles only, modified to be invariant under a particular set of symmetry transformations (those that form a mathematical group, called the gauge group), yields a field theory with exchange particles as well. The particular gauge group chosen determines both the number of exchange particles and their spin. The theory is renormalizable, but the exchange particles are massless.
In any quantum field theory the state of lowest energy is called the vacuum state. It is possible to construct theories in which the vacuum does not share all the symmetries of the theory; this is called spontaneous symmetry breaking. By adding to the model a massless spin-0 particle, which breaks the symmetry of the vacuum in a particular way, some of the exchange particles can become massive, without mass being put into the theory. These spontaneously broken gauge theories contain massive vector bosons, and, as was later demonstrated by the Dutch physicist Gerardus 't Hooft, are still renormalizable.
By 1967 all the pieces were in place, and were independently exploited by the American Steven Weinberg and the Pakistani Abdus Salam. They constructed a theory invariant which contains four massless vector bosons. They employed the above-mentioned Higgs mechanism to generate mass (named after its discoverer, the Scottish physicist Peter Higgs). After some deft mathematical manipulation, they had a renormalizable theory with three massive exchange particles, the W+ , the W- , and the Z0 , and one massless boson, the photon. Not only had they produced a theory of the weak interaction, but it incorporated the electromagnetic interaction as well. It was the first unified quantum field theory, and is today called the theory of the electroweak interaction.
Every facet of the Weinberg-Salam model, save one, has been experimentally verified. The three massive bosons were discovered in 1982 at CERN, the European high-energy physics laboratory. They had just the masses predicted by the electroweak model. What has not yet been seen, though, is the Higgs boson, the particle required to make the W± and the Z0 massive. It is possible that the Higgs is just too massive to be produced by present-day particle accelerators. Although the Higgs boson enters the theory as a massless particle, it, too, develops a mass. It is hoped that it will be observed when the next generation of accelerators come on-line early in the twenty-first century.
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