Polarization of Cosmic-Ray Muons at Sea Level

Abstract

Cosmic-ray muons at sea level are known to be partially longitudinally polarized. The object of this experiment was to measure the positive muon polarization by stopping the muons in a copper absorber and observing the up: down asymmetry in the decay electrons. Measurements were also made with iron and sulfur as control absorbers to test the symmetry of the apparatus. Since this experiment is performed in zero magnetic field, it has been possible to verify that unmagnetized iron depolarizes positive muons by precessing their magnetic moments in the strong, randomly oriented magnetic fields of the domains. The up: down ratio of decay electrons was 1.14\ifmmode\pm\else\textpm\fi{}0.02 for copper, 0.98\ifmmode\pm\else\textpm\fi{}0.02 for iron, and 1.02\ifmmode\pm\else\textpm\fi{}0.03 for sulfur. On the basis of the two-component neutrino theory, the muon polarization is found to be \ensuremath{\ge}(21\ifmmode\pm\else\textpm\fi{}3)%, where the inequality is due to depolarization effects. On the basis of cosmic-ray data on the muon energy spectrum as a function of altitude, the polarization is expected to be about 30%. The data of the present experiment suggest that in the absence of external magnetic fields copper depolarizes positive muons, as there appears to be a decrease in the up: down ratio with time lived before decay.