parityviolation

Parity violation is the phenomenon in which the laws of physics are not invariant under a parity transformation, a mirror reflection that inverts spatial coordinates. In such a transformation, left-handed and right-handed coordinate systems are swapped. If a process looks the same in a mirror, it conserves parity; if not, parity is violated. Parity violation is most prominently associated with the weak nuclear interaction.

The first clear evidence came in 1957 from beta-decay experiments led by Chien-Shiung Wu on cobalt-60, which showed that emitted electrons were arranged asymmetrically with respect to the nuclear spin. This violated mirror symmetry and demonstrated that the weak force does not treat left and right equally. Subsequent experiments, including studies of polarized particles and neutrino helicity, established that weak interactions couple preferentially to left-handed states, indicating maximal parity violation in charged weak currents.

The Standard Model explains parity violation through the V-A (vector minus axial vector) structure of the weak interaction. Charged-current processes couple only to left-handed fermions (and right-handed antifermions), yielding strong (maximal) parity violation. Electromagnetic and strong interactions conserve parity, while neutral-current interactions mediated by the Z boson also exhibit parity-violating couplings due to their axial-vector components. Precision measurements of parity-violating effects in electron scattering and beta decay have become important tests of the electroweak theory and have helped determine parameters such as the weak mixing angle.

Pariy violation is distinct from CP violation and is compatible with CPT invariance. It remains a foundational aspect of the Standard Model, reflecting the chiral nature of the weak force and guiding ongoing experimental tests of electroweak theory.