A Parity (P) operation on a system of interacting particles means to replace that system with its mirror image. It is a spatial inversion operation that has the effect of changing left-handed particles to right-handed ones and vice versa. P violation occurs when the rate for a particle interaction is different for the mirror image of that interaction. Electromagnetic and strong nuclear forces have the same strength for left-handed and right-handed particles. So parity is a good symmetry for these interactions and is said to be conserved by them. But the weak nuclear force is asymmetric for right-handed and left-handed particles and thus violates parity. This was first observed in charged current (exchange of W+ or W- particles) interactions in 1956, by Madame Wu and collaborators studying the radioactive decay of 60Co (isotope 60 of Cobalt). Parity violation in neutral current (exchange of Z0 particles) interactions was first observed at SLAC in 1978, by Charles Prescott and collaborators studying the scattering of electrons from protons in a liquid hydrogen target.
Parity is one of three important discrete operations in particle physics. The other two are Charge (C) and Time (T). A C operation changes particles to anti-particles in a system of interacting particles, while a T operation reverses the direction of time in that system. Combinations of these operations, such as CP and CPT, are also very important. Tests of whether particle interactions are symmetric (ie. have the same strength) for a process and its image process under a C, P, T, CP, or CPT operation elucidate the underlying physics. Symmetry conservation and non-conservation (violation) can have dramatic effects for both particle physics and cosmology. The combined symmetry CPT is believed to be conserved for all particle interactions. Consequences of CPT symmetry are that a particle and its anti-particle should have the same mass and lifetime. No violations of CPT have yet been observed. CP violation, however, has been observed as discussed below. CP violation is especially intriguing, since it is believed to be a necessary ingredient to explain the preponderance of matter over antimatter in the universe -- ie. to explain why we exist at all!
Charge (C) Violation is also observed in weak interactions, but not in electromagnetic or strong interactions. C Violation occurs when the rate for a particle interaction is different if all the particles in the interaction are changed to anti-particles. C Violation was first demonstrated in 1957 by physicists at the University of Liverpool studying the decay of muons to electrons and anti-muons to positrons and then analyzing the polarization of the electrons and positrons. It was found that muons decayed to left-handed electrons but not to right-handed electrons, and that anti-muons decayed to right-handed positrons but not to left-handed positrons.
When C and P Violation were first observed in 1956 and 1957, it was expected that when C and P operations were combined together (the CP operation), symmetry would still be preserved. Otherwise, CP violation would necessarily imply T violation in order to conserve CPT. However, CP was observed to be violated at a small level in the weak interactions of kaons (particles that contain a down-type quark and a strange-type quark) in 1964 by James Cronin and Val Fitch and collaborators, for which they won the Nobel Prize in physics in 1980. Since then, CP violation has not been observed outside of the kaon system but it is expected to be observed soon in the weak interactions of particles involving bottom quarks. This is the main program for the BaBar experiment at SLAC.
Time (T) Violation has also recently been observed in the weak interactions of kaons by the CPLEAR experiment at CERN and the KTEV experiment at Fermilab. T Violation means that the rate for a particle interaction is different for the time-reversed process.
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Last updated: 04-09-2001 by M. Woods