Cp Symmetry Violation

The symmetry known as CP is a fundamental relation between matter and antimatter. The discovery of its violation by Christenson, Cronin, Fitch, and Turlay (1964) has given us important insights into the structure of particle interactions and into why the Universe appears to contain more matter than antimatter. In 1928, Paul Dirac predicted that every particle has a corresponding antiparticle. If the particle has quantum numbers (intrinsic properties), such as electric charge, the antiparticle will have opposite quantum numbers. Thus, an electron, with charge −|e|, has as its antiparticle a positron, with charge +|e| and the same mass and spin. Some neutral particles, such as the photon, the quantum of radiation, are their own antiparticles. Others, like the neutron, have distinct antiparticles; the neutron carries a quantum number known as baryon number B = 1, and the antineutron has B = –1. (The prefix baryis Greek for heavy.) The operation of charge reversal, or C, carries a particle into its antiparticle. Many laws of physics are invariant under the C operation; that is, they do not change their form, and, consequently, one cannot tell whether one lives in a world made of matter or one made of antimatter. Many equations are also invariant under two other important symmetries: space reflection, or parity, denoted by P, which reverses the direction of all spatial coordinates, and time reversal, denoted by T, which reverses the arrow of time. By observing systems governed by these equations, we cannot tell whether our world is reflected in a mirror or in which direction its clock is running. Maxwell’s equations of electromagnetism and the equations of classical mechanics, for example, are Enrico Fermi Institute Report No. 01-40, hep-ph/0109240, to be published in Macmillan Encylopedia of Physics, Supplement: Elementary Particle Physics, edited by John S. Rigden, Jonathan Bagger, and Roger H. Stuewer (Macmillan Reference USA, New York, 2002).