Muon spin relaxation

Since its discovery in 1957, parity violation in the weak interaction has occupied both experimental and theoretical physicists in a broad effort directed toward its understanding. One manifestation of parity violation is found in the decay of spin‐polarized muons. In the earliest searches for suitable stopping materials for studying the weak interaction through the decay of positive muons, physicists noticed that the muon polarization remaining after thermalization depends markedly on the nature of the stopping environment, varying from about 10% in some liquids such as benzene to 100% in most metals. Data of this nature contained the beginnings of the technique of muon spin relaxation, also known as muon spin resonance or muon spin rotation. In this technique, which is akin to magnetic resonance, one monitors the spin polarization of muons to learn about the materials into which they have been injected. Muon spin relaxation now constitutes a significant research effort at the world's meson‐producing accelerators: at Brookhaven and LAMPF in the United States, Dubna and Leningrad in the Soviet Union, CERN and SIN in Switzerland, TRIUMF in Canada, KEK in Japan and NIKHEF in the Netherlands.