Charge Conjugation Invariance

Invariances, in particle physics, are relationships between different processes.

Charge conjugation is the replacement of each particle in a process by the corresponding antiparticle. Charge conjugation invariance states that the rate of a process is identical to the rate of a similar process with all particles replaced by their antiparticles. Charge conjugation invariance is true for strong and electromagnetic interactions, but not for weak interactions.

In weak processes, it is found that there only left-handed neutrinos and only right-handed antineutrinos are involved.

Charge conjugation, C, turns a left-handed neutrino into a left-handed antineutrino (and right-handed neutrino to a right-handed antineutrino). But processes involving right handed-neutrinos or left-handed antineutrinos are not seen. So it is clear that charge conjugation does not apply to weak processes.

CP = Charge Conjugation with Parity Invariance

Parity, P, turns a left-handed neutrino or antineutrino into a right-handed one.  Thus CP, the combination of the two operations, turns a left-handed neutrino into a right handed antineutrino, and vice versa. This seems to match the pattern of weak interactions.

For some time after the discovery of the left-handedness of neutrinos, it was thought that CP is an exact invariance of weak processes.  However, in 1963 Fitch and Cronin discovered that a small fraction of processes in K-meson decays do not respect the relationships of CP  invariance. Certain decays that would be forbidden if this invariance were exact, were seen to occur at about one part in 1000 of the rate of typical weak decays.

CP and the B Factory

This CP invariance violation was observed in K decays.

In the Standard Model, it turns out that there is only one way to introduce a parameter that gives CP violation. The observed CP violation in K decay then leads to predicted relationships between a number of CP violating effects in B meson decays. A primary purpose of the B Factory at SLAC is to look for these effects and study whether or not they fit the pattern predicted by the Standard Model . Most physicists hope that they do not -- because this will then give some clues to the nature of physics beyond the Standard Model.