History

The experimental technique of µSR applied to condensed matter originates from the discovery of the violation of parity in decays regulated by weak interactions Phys. Rev. 104, 254 (1956). The Nobel Prize was awarded to T.D. Lee and C.N. Yang in 1957 "for their penetrating investigation of the so-called parity laws'' (from the motivation of the prize). These investigation had been confirmed by C.S. Wu at al., Phys. Rev. 105 (1957) 1413 in their experiment on the β-decay of {$^{60}Co$}. The discovery is discussed in this Phys. Rev. Focus 22, 19

The essence of parity violation is more directly grasped from the parallel muon decay experiment Richard L. Garwin, Leon M. Lederman, and Marcel Weinrich, Phys. Rev. 105, 1415 (1957).

The first column and the main picture from this work are shown on the left. Due to parity violation the positron counts from the decays of a muon population are made to oscillate in time, as the spins of the muons are coherently precessing around an external magnetic field. This implies that the muons preferentially emit their decay positron along the direction of their spin, instead of uniformly distributed in all directions, as it would happen for parity conserving events.