The standard model of matter accepted by most physicists today postulates two fundamental types of particles. One type includes those particles of which matter is made, namely quarks and leptons. The second type consists of mediating particles that transmit the four fundamental forces, namely the graviton (for the gravitational force), the gluon (for the strong force), the photon (for the electromagnetic force) and the W and Z bosons (for the weak force). Of these mediating particles, only the photon had actually been observed prior to the 1980s. Even today, the graviton and gluon are only hypothetical particles for whose existence there is very little or no empirical evidence.
The existence of W and Z bosons was predicted as a consequence of the electroweak theory developed by Steven Weinberg, Abdus Salam, and Sheldon Glashow in the early 1970s. Since the weak force acts only over very short distances (about 10-15 cm), the mass of its mediating particle(s) must be very large. Initial calculations predicted a mass of 80 GeV for the W bosons and 92 GeV for the Z boson.
Another property of these particles was predicted by the electroweak theory. The interactions of particles that occur during electroweak changes may or may not involve an exchange of electrical charges. This property requires that a positively charged boson, a negatively charged boson, and a neutral boson all be available for electroweak interactions. As a result, scientists expected to look for a positively charged W boson (W+), a negatively charged W boson (W-), and a neutral Z boson (Zo).
The search for these particles began in 1981 in the Super Proton Synchrotron collider located at the European Center for Nuclear Research (CERN) in Geneva. In this collider, a beam of protons rotating in one direction is caused to collide with a beam of antiprotons traveling in the opposite direction. A total energy of 540 GeV is released in each such collision. This level of energy is sufficient to result in the formation of at least some W and Z bosons.
Between February and May of 1983, CERN researchers announced that they had obtained evidence for the existence of all three bosons. In each case, they identified the particle on the basis of its decay products since no one of the particles survives more than 10-20 seconds. All told, no more than about a half dozen events out of more than a billion fit the predicted decay sequence. Sill, most scientists have become convinced that the W and Z bosons had been observed. They further accepted this result as strong confirmation of the Salam-Weinberg-Glashow electroweak theory.
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