Parity Experiments in Beta Decays

Authors(s):C. S. Wu Publication:Reviews of Modern Physics Publication Date:July 1, 1959 Publisher: American Physical Society Citation:RevModPhys,31.783 Link:

The frontier of parity study has now advanced to the field of strange particles. The atmosphere in the field of beta decay appears unusually calm and quiet after the storm. I will try to piece together the jigsaw picture and to see what sorts of puzzles in beta decay have fallen into shape. Most urgent of all is the question, whether there are still any missing pieces, and if there are, what are they?

Prior to the discovery of parity nonconservation in beta decay, the (S, T) combination had been the favorite choice based mainly on He6 (β-v) angular correlation results. In fact, the (β-ν) angular correlation was the only means used to investigate the type of beta interaction in those days. But this type of experiment was known to be difficult. Wu and Schwarzschild made a detailed examination of the old Ηe6 experiment and pointed out that the effective volume of the He6 source in the hole of the pumping diaphragm was not correctly taken into account. Had this been done properly, the results of Ηe6 would not then have implied the tensor interaction. However, in spite of its many limitations, β-ν correlation is still an effective and powerful method in yielding information on the beta interactions. The first sign of warning against the (S, T) combination in beta interaction came in May of 1957 when Herrmannsfeldt et al. published their (β-v) correlation results on A35, which decays mainly through Fermi interaction, and the results strongly supported the vector interaction instead of scalar as was once believed.

Recently, this group has remeasured the (β-v) correlation in He6 with the same apparatus and obtained the correlation coefficient X= — 0.39+0. 02, which certainly favors axial vector in He6.