Charge, parity and weak interactions
Weak interactions violate both charge (C) and parity (P) symmetry. To see this, consider the decay of a muon, µ-, into an electron, e-, plus an electron-antineutrino, v-barµ, and a muon-neutrino, vµ. (a) Experiments show that the electron is preferentially emitted in the direction opposite to the muon polarization. However, when we apply the P operation to this reaction (i.e. reverse all three directions in space), the result is a decay (b) that is suppressed in nature. Similarly, when we apply the C operation (i.e. replace all particles with their antiparticles), the result is a different decay (c) that is also suppressed in nature. But when we apply both the C and P operations to (a), the resulting reaction - in which an antimuon, µ+, preferentially emits a positron, e+, in the same direction as the polarization - is observed in experiments (d). This means that muon decays conserve charge-parity (CP) symmetry, but violate the two symmetries on their own. The polarization of the muon and its spin (red arrows) are unaffected by the C and P operations.