Anderson, an American, found them in cosmic rays; these particles became known as positrons. Here was a genuine advance, for this represented the first prediction of a new particle's existence, entirely from theory. The general view at the time was that a Dirac-like theory would soon come forth to describe the proton and neutron. At that point, physicists would have dug down to bedrock in their search for nature's ultimate secrets.

Today, after six decades and many more disappointed hopes, the field of physics is in a rather similar situation. Once again we have a set of powerful theories—the Standard Model, which offers great predictive power. Indeed, it not only accounts for all physical experiments performed to date, it has even shown its power by once again successfully predicting the existence of new particles. Yet today's researchers are not satisfied. Important features of the Standard Model remain unconfirmed; not all of its predictions have yet been borne out. Furthermore, even if one accepts it without reservation, it raises a number of new questions, which lie within the reach of experiments. In pursuing these matters, today's physicists are setting an agenda for the coming century.

The road to the Standard Model has not been smooth. It began just after the war, as the federal government allocated funds for construction of an increasingly powerful series of particle accelerators. In studies of the atom's nucleus, they quickly replaced the older technique of relying on observations of cosmic rays. The new accelerators produced beams of particles, such as electrons or protons, that were particularly intense. They also offered high energy, and experimenters could control this energy, turning it up and down.

The standard technique was to direct such a beam onto a target made of some material and observe the debris that came out as particles with that beam "split the atom"—or, more properly, shattered some of the target's nuclei. New types of particles might lie within those sprays of nuclear debris, and experimenters were not disappointed. Indeed, during the years after 1950, they found themselves with an embarrassment of riches. There appeared to be not one or two but rather dozens of new particles. So far as anyone could tell, they might all be as fundamental as the proton or the electron.

These new particles generally were very short lived. Unlike electrons and even positrons, they did not form well-defined tracks within a detector. Instead, they decayed into other particles, in as little as 10-16

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