


The discovery that the weak force conserves neither charge conjugation nor parity separately, however, led to a quantitative theory establishing combined CP as a symmetry of nature. Physicists reasoned that if CP were invariant, time reversal T would have to remain so as well. But further experiments, carried out in 1964 by a team led by the American physicists James W. Cronin and Val Logsdon Fitch, demonstrated that the electrically neutral K-meson—which normally decays via the weak force to give three pi-mesons—decayed a fraction of the time into only two such particles and thereby violated CP symmetry. CP violation implied nonconservation of T, provided that the long-held CPT theorem was valid. The CPT theorem, regarded as one of the basic principles of quantum field theory, states that all interactions should be invariant under the combined application of charge conjugation, parity, and time reversal in any order. CPT symmetry is an exact symmetry of all fundamental interactions.

The theoretical description of subatomic particles and forces known as the Standard Model contains an explanation of CP violation, but, as the effects of the phenomenon are small, it has proved difficult to show conclusively that this explanation is correct. The root of the effect lies in the weak force between quarks, the particles that make up K-mesons. The weak force appears to act not upon a pure quark state, as identified by the “flavour” or type of quark, but on a quantum mixture of two types of quarkquarks. In 1972 the Japanese theoretical physicists Kobayashi Makoto Kobayashi and Maskawa Toshihide Maskawa proposed that CP violation would be an inherent prediction of the Standard Model of particle physics if there were six types of quark. quarks. (In 2008 Kobayashi and Maskawa were awarded the Nobel Prize for Physics for their “discovery of the origin of the broken symmetry which predicts the existence of at least three families of quarks in nature.”) They realized that with six types of quarkquarks, quantum mixing would allow very rare decays that would violate CP symmetry. Their predictions were borne out by the discovery of the third generation of quarks, the bottom and top quarks, in 1977 and 1995, respectively.


CP violation has important theoretical consequences. The violation of CP symmetry enables physicists to make an absolute distinction between matter and antimatter. The distinction between matter and antimatter may have profound implications for cosmology. One of the unsolved theoretical questions in physics is why the universe is made chiefly of matter. With a series of debatable but plausible assumptions, it can be demonstrated that the observed imbalance or asymmetry in the matter-antimatter ratio may have been produced by the occurrence of CP violation in the first seconds after the big bang—the violent explosion that is thought to have resulted in the formation of the universe.