The 1959 Wu experiment showed that parity symmetry is violated in the weak interaction.
Chien-Shiung Wu and collaborators studied the beta decay of cobalt-60 nuclei cooled to very low temperatures and aligned in a magnetic field. If nature treated a process and its mirror image identically, electrons should have been emitted symmetrically relative to the nuclear spin. Instead, the electrons showed a preferred direction.
This asymmetry demonstrated that the weak nuclear force distinguishes left from right. The result confirmed a prediction made by Tsung-Dao Lee and Chen-Ning Yang, who had proposed testing parity conservation in weak interactions. Lee and Yang received the 1957 Nobel Prize for their theoretical work, while Wu’s experimental contribution was central to the discovery.
The experiment concerned parity, not ordinary geometric rotation. It also did not show that every physical interaction violates parity; the effect is specifically associated with the weak interaction.