Wu Experiment

The Wu experiment was a 1956 and 1957 test of parity conservation in the weak nuclear interaction, carried out by Chien-Shiung Wu with a team from Columbia University and the low-temperature physics group at the United States National Bureau of Standards. Acting on a suggestion from Tsung-Dao Lee and Chen-Ning Yang, Wu cooled radioactive cobalt-60 nuclei to near absolute zero inside a magnetic field to align their spins, then measured the direction in which beta particles were emitted as the nuclei decayed. If parity were conserved, equal numbers should have flown off in each direction; instead, far more emerged opposite the direction of nuclear spin, showing that the weak interaction distinguishes a process from its mirror image. The result, confirmed within weeks by a second experiment on muon decay, was announced in January 1957 and directly confirmed the parity violation Lee and Yang had proposed months earlier. Wu's own role went unrecognized by the Nobel committee, an omission Yang himself later called a mistake.