Fred Gilman earned
his Ph.D. in theoretical physics at Princeton. After 23 years at Stanford Linear
Accelerator Lab he spent five years as associate director of the Superconducting
Supercollider in Texas. He joined the faculty of Carnegie Mellon in 1995, accepting the
endowed Buhl Chair in Theoretical Physics.In the study of the fundamental forces and constituents of matter, symmetries of the force laws, or interactions, play a very important role. One of the great discoveries of the 1950s was that space reflections - what you see when you look at things in a mirror - were not a symmetry of the weak interactions. Indeed, this symmetry (symbolized by the letter P, for parity) was found to be so strongly violated that a weak interaction process, such as a radioactive decay involving electrons and neutrinos, and its mirror image generally do not both occur in nature. At the same time it was found that the expected symmetry under changing all particles to antiparticles and vice versa (symbolized by the letter C for charge conjugation) proved also not to be a symmetry of the weak interactions. However, in the theory that emerged at the end of that decade to successfully describe the weak interactions, the combined transformation of doing both a mirror reflection and changing particles to antiparticles, symbolized CP, was still a "good" symmetry.
This situation lasted until 1964 when an experiment found a very small, but definite violation of CP symmetry in particles known as K mesons. From then until the present day, particle physicists have been trying to understand this effect, which won the leaders of the experiment that discovered it, Jim Cronin and Val Fitch (Fitch is on the Carnegie Mellon Physics Department's Advisory Board), the Nobel Prize in Physics. A long series of experiments confirmed and measured with increasing accuracy the basic effect in various ways. All these measurements may be summarized theoretically in terms of one parameter that gives a small (about 2 in a thousand) admixture with the "wrong" CP property to the neutral K mesons. A basic model, set forth by Lincoln Wolfenstein of our department shortly after the initial discovery, proposes violation of the CP symmetry arises from some, as yet unknown, "superweak" interaction.
More than a decade later, after the basic features
of what is now called the Standard Model of the constitutents of matter and their
interactions had been developed, the theoretical situation changed dramatically. With six
quarks (the last of which, the long-sought top quark, was finally found this past year)
and six leptons as the building blocks of all matter, it is possible to naturally explain
CP violation as a result of a phase difference between the weak interaction amplitudes of
the quarks. It was possible, at least in principle, to get the right magnitude for the
effect seen by Fitch and Cronin that corresponds to the superweak theory, but now this
would have a calculable relationship to a fundamential parameter of the Standard Model. In
1978, my student, Mark Wise, and I pointed out that there should be an additional
(non-superweak), but much smaller, effect in the decays of K mesons. This has been pursued
over more than a decade in a series of experiments at Fermilab and at CERN, with still
conflicting results - the Fermilab result is statistically consistent with no effect,
while the CERN experiment gives a positive result. Both experimental teams are in the
course of gearing up for the next round experiments that will be carried out in the next
couple of years. With 10 times more precision, they should settle the experimental
conflict unambiguously, and be able to see if there is a non-superweak effect.
In the 1980s it was understood that much bigger effects of CP symmetry violation could be seen in the decays of B mesons - particles containing the b-quark and 10 times heavier than K mesons. Many theorists noted that the phase differences between quark decay amplitudes could be very large for B's, while small for K's, and moreover could be very cleanly measured in certain B decays. Further, the Standard Model provides a characteristic pattern that can be tested for consistency between different decays. Both the Stanford Linear Accelerator Center and the KEK National High Energy Physics Laboratory in Japan are now building "B-Factories" - high intensity colliding beam accelerators - that should produce large enough samples of B decays to test the theoretical predictions coming from the Standard Model when they start operation at the turn of the century. Theorists meanwhile continue to dream up alternatives to the Standard Model and propose ways of finding out which of the alternatives, if any, might lead toward understanding the origin of CP violation.
