Angular Distribution of Muons inπ−μDecay at Rest

Evidence for a significant departure from isotropy of the muon angular distribution from $\pi -\mu$ decay at rest is presented, based on measurements of projected angles in two emulsion stacks, exposed to the Dubna and CERN synchrocyclotrons. Extensive control experiments concerning observational bias, distortion, and inhomogeneous detection efficiencies prove that the observed lack of isotropy cannot be reduced to such spurious effects.