CP VIOLATION

in particle physics, violation of the combined conservation laws associated with charge conjugation (C) and parity (P) by the weak nuclear force, which is responsible for reactions such as the decay of atomic nuclei. Charge conjugation is a mathematical operation that transforms a particle into an antiparticle, for example, changing the sign of the charge. Charge conjugation implies that every charged particle has an oppositely charged antimatter counterpart, or antiparticle. The antiparticle of an electrically neutral particle may be identical to the particle, as in the case of the neutral pi meson, or it may be distinct, as with the antineutron. Parity, or space inversion, is the reflection in the origin of the space coordinates of a particle or particle system; i.e., the three space dimensions x, y, and z become, respectively, -x, -y, and -z. Stated more concretely, parity conservation means that left and right and up and down are indistinguishable in the sense that an atomic nucleus throws off decay products up as often as down and left as often as right. For years it was assumed that charge conjugation and parity were exact symmetries of elementary processes, namely those involving electromagnetic, strong, and weak interactions. The same was held true for a third operation, time reversal (T), which corresponds to reversal of motion. Invariance under time implies that whenever a motion is allowed by the laws of physics, the reversed motion is also an allowed one. A series of discoveries from the mid-1950s caused physicists to alter significantly their assumptions about the invariance of C, P, and T. An apparent lack of the conservation of parity in the decay of charged K mesons into two or three pi mesons prompted the Chinese-born American theoretical physicists Chen Ning Yang and Tsung-Dao Lee to examine the experimental foundation of parity itself. In 1956 they showed that there was no evidence supporting parity invariance in weak interactions. Experiments conducted the next year verified decisively that parity was violated in the weak interaction beta decay. Moreover, they revealed that charge conjugation symmetry also was broken during this decay process. The discovery that the weak interaction 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 the American physicists James W. Cronin and Val Logsdon Fitch, demonstrated that the electrically neutral K meson, which was thought to break down into three pi mesons, decayed a fraction of the time into only two such particles, thereby violating CP symmetry. CP violation implied nonconservation of T, provided that the long-held CPT theorem was valid. In this theorem, regarded as one of the basic principles of quantum field theory, charge conjugation, parity, and time reversal are applied together. As a combination, these symmetries constitute an exact symmetry of all types of fundamental interactions. No completely satisfactory explanation of CP violation has yet been devised. The size of the effect, only about two parts per thousand, has prompted a theory that invokes a new force, called the superweak force, to explain the phenomenon. This force, much weaker than the nuclear weak force, is thought to be observable only in the K-meson system or in the neutron's electric dipole moment, which measures the average size and direction of the separation between charged constituents. Another theory, named the Kobayashi-Maskawa model after its inventors, posits certain quantum mechanical effects in the weak force between quarks as the cause of CP violation. The attractive aspect of the superweak model is that it uses only one variable, the size of the force, to explain everything. Furthermore, the model is consistent with all measurements of CP violation and its properties. The Kobayashi-Maskawa model is more complicated, but it does explain CP violation in terms of known forces. CP violation has important theoretical consequences. The violation of CP symmetry, taken as a kind of proof of the CPT theorem, 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 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 (see big-bang model).