In the 1957 Wu experiment with cobalt-60, which symmetry principle was shown to fail in weak nuclear decay?

In the 1957 Wu experiment with cobalt-60, parity conservation was shown to fail in weak nuclear decay.

Chien-Shiung Wu and collaborators cooled radioactive cobalt-60 nuclei to very low temperatures and aligned their spins in a magnetic field. They then measured the directions of electrons emitted during beta decay. The electrons were emitted preferentially in one direction relative to the nuclear spin, revealing a left–right asymmetry.

If parity were conserved, a mirror-reflected version of the decay would have to be equally probable. The observed asymmetry showed that the weak interaction distinguishes left from right. This overturned the widely held assumption that all fundamental interactions respected parity.

Tsung-Dao Lee and Chen-Ning Yang proposed that parity might fail in weak interactions and received the 1957 Nobel Prize for that theoretical work. Wu’s experiment supplied decisive evidence, but she was not included in that prize. The result does not mean that all physical processes violate parity; the effect is characteristic of the weak interaction.