Reversal of the Parity Conservation Law in Nuclear Physics

In late 1956, experiments at the National Bureau of Standards demonstrated that the quantum mechanical law of conservation of parity does not hold in the beta decay of 60Co nuclei. This result, reported in the paper An experimental test of parity conservation in beta decay [1], together with ensuing experiments on parity conservation in
-meson decay at Columbia University, shattered a fundamental concept of nuclear physics that had been universally accepted for the previous 30 years. It thus cleared the way for a reconsideration of physical theories, especially those relating to symmetry, and led to new, far-reaching discoveries regarding the nature of matter and the universe. In particular, removal of the restrictions imposed by parity conservation first resolved a serious conflict in the theory of subatomic particles, known at the time as the tau-theta puzzle, and later led to a fuller understanding of the strong, electromagnetic, and weak interactions. The better understanding of their characteristics has led to a more unified theory of the fundamental forces.

The beta-decay experiments were carried out by C. S. Wu of Columbia University in collaboration with NBS staff members Ernest Ambler, Raymond W. Hayward, Dale D. Hoppes, and Ralph P. Hudson. The Bureau’s low temperature laboratory was chosen for the experiments because of its millikelvin-region research capability [2] and the staff"s experience in the spatial orientation of atomic nuclei [3], an essential feature of the beta-decay study.