Isotropic and Nonisotropicπ−μDecays

Nuclear emulsion experiments on $\pi -\mu$ decay at rest are described and the muon angular distribution is compared with that obtained in previous experiments under radically different conditions. In the present experiment the pions were produced in a magnetic field of ${10}^4$ Oe, the angle of emission was 180°, the kinetic energy was ∼50 MeV and no filter was interposed between target and stack (flight path 765 cm), whereas in the previous experiment there was no field, the emission angle was 0°, the kinetic energy was ∼300 MeV, and the beam traversed 685 cm, of which 17 cm were of Cu. The muon angular distribution in this experiment is isotropic and significantly different from that of the previous one. Thus, "pion" history seems to have an essential influence on the properties of part of the decaying particles.