Zenith-Angle Dependence of Cosmic-Ray Muons

The zenith-angle distribution of high-energy cosmic-ray muons has been further investigated using a sample of 2 × ${10}^5$ muons observed in the Utah underground detector. The data have been analyzed in such a way that the angular dependence can be extracted without substantial sensitivity to the parameters of cosmic-ray phenomenology. The zenithangle distribution is flatter than expected on the basis of muon production via pion and kaon decay. An isotropic component (of unspecified origin) is required which rises to a maximum contribution of about 0.5 of the vertical intensity in the muon energy range 1-3 TeV and threafter decreases.