Electroweak Theory
The physical world appears to be a terribly complex place, with many different kinds of particles reacting with each other in a great variety of ways. Yet, scientists are convinced that this apparent " messiness" is an illusion and that fundamentally, the physical world operates on only a few basic principles. The search for underlying simplicity has gone on since the days of ancient Greece.
The first breakthrough in that search occurred in the 1860s when James Clerk Maxwell showed the relationship between two apparently independent forces, electricity and magnetism. Maxwell developed a series of mathematical equations that could be used to describe light and, later, radio waves, x-rays, and other forms of electromagnetic radiation.
During this century, scientists have become convinced that only four fundamental forces exist in nature. In addition to the familiar forces of electromagnetism and gravitation, these include the strong and weak forces which seem to operate only at the atomic and subatomic level. They began looking for a way to unify these forces, that is, to look for a small set of equations that can be used to describe all forces as the manifestation of a single fundamental force.
A fundamental problem in this search is that the four primary forces operate over very different ranges. The gravitational and electromagnetic forces affect objects over infinite distances, for example, while the strong force can not be felt beyond a distance of about 10-15 meter. In order to deal with such widely different forces, scientists work with a special kind of mathematics called "gauge theory." A gauge theory changes the scale of one frame of reference so that it can be compared with a different frame of reference.
The first successful use of gauge theory in the attempt to unify forces was achieved in the late 1960s. Steven Weinberg, Abdus Salam, and Sheldon Glashow developed a series of equations that show how electromagnetism and the weak force can be viewed as two different expressions of a single force, the electroweak force.
The weak force is still not very well understood. Scientists do know that it is responsible for beta decay in the atomic nucleus. As a result of this force, a neutron decays into a proton, an electron, and an electron neutrino.
The Weinberg-Salam-Glashow theory predicts a number of phenomena that should be observed as a consequence of the electroweak force. One of these predictions is the existence of weak neutral current interactions, where no electrical charge is exchanged. Prior to the theory, only charged current interactions, where charge is exchanged, had been observed.
An example of a charge interaction is one in which a muon neutrino and an electron interact to produce an electron neutrino and a muon. An example of a neutral interaction is the interaction between a muon neutrino and an electron that results in the formation of the same two particles.
In 1973, the first evidence for a weak neutral current was observed in the giant accelerator at the European Center for Nuclear Research (CERN).
As a way of extending the electroweak theory to all elementary particles, Glashow had also predicted that certain particles would possess a property he called "charm." In November 1974, "charm " was discovered as a component of a new particle, called J/, discovered at almost exactly the same time in two laboratories in the United States. The particle's unusual name is a combination of the name given by one group of discoverers at the Stanford Linear Accelerator (J) and a second name given by the second group at the Brookhaven National Laboratory ().
A third prediction of the Weinberg-Salam-Glashow theory is the existence of three force particles needed to carry the electroweak interaction. A force particle is an elementary particle that carries one of the fundamental forces. The particle that carries the electromagnetic force, for example, is the photon. According to the Weinberg-Salam-Glashow theory, the electroweak force should be carried by three different particles, in addition to the photon. Those particles were named the W+,
W-, and Z0 bosons.
Unlike the photon, which has no rest mass, these three particles were predicted to have very large masses, about a hundred times that of the proton. In a series of elegant experiments, all three bosons were discovered in experiments conducted at CERN in 1983 by a large research team under the direction of Carlo Rubbia. Today, the evidence for the existence of an electroweak force appears to be very convincing.
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