Nuclear Emulsion Evidence for Parity Nonconservation in the Decay Chainπ+→μ+→e+

Following a suggestion of Lee and Yang, the correlation between the initial direction of motion of the muon and the direction of emission of the positron in the decay chain ${\pi }^{+}\to {\mu }^{+}\to e^{+}$ was investigated in nuclear emulsion to detect a possible parity nonconservation in both decay interactions. Positive pions from the University of Chicago synchrocyclotron were brought to rest in emulsion carefully shielded from magnetic fields, and 2000 complete decay events were measured. A correlation $W\left(\theta \right)=1+acos\theta$ was found, with $a=-0.174\mathrm{}\pm 0.038$, clearly indicating a backward-forward asymmetry, i.e., a violation of parity conservation in both decay processes. Actually, following an argument of Lee, Oehme, and Yang, this asymmetry implies a noninvariance of either interaction with respect to both space inversion and charge conjugation, taken separately.