12. CP violation in meson decays 1 12. CP VIOLATION IN MESON DECAYS

The CP transformation combines charge conjugation C with parity P. Under C, particles and antiparticles are interchanged, by conjugating all internal quantum numbers, e.g., Q! −Q for electromagnetic charge. Under P, the handedness of space is reversed, ~x! −~x. Thus, for example, a left-handed electron e−L is transformed under CP into a right-handed positron, e+R. If CP were an exact symmetry, the laws of Nature would be the same for matter and for antimatter. We observe that most phenomena are C- and P-symmetric, and therefore, also CP-symmetric. In particular, these symmetries are respected by the gravitational, electromagnetic, and strong interactions. The weak interactions, on the other hand, violate C and P in the strongest possible way. For example, the charged W bosons couple to left-handed electrons, e−L, and to their CP-conjugate right-handed positrons, e R, but to neither their C-conjugate left-handed positrons, e+L, nor their P-conjugate right-handed electrons, e−R. While weak interactions violate C and P separately, CP is still preserved in most weak interaction processes. The CP symmetry is, however, violated in certai