Search for diffuse cosmic gamma rays above 200 TeV with the Utah‐Michigan array

Abstract

We have searched for γ‐rays in the cosmic radiation above 200 TeV using a two‐level array of scintillators designed to measure both the electromagnetic and muon content of extensive air showers. We find no evidence for an excess number of muon poor showers and conclude that γ‐rays comprise less than 0.4% of all cosmic rays above 200 TeV, and less than 0.05% above 1000 TeV (90% CL). The muon content of showers from the direction of the Galactic disk is the same as that of showers from other regions of the sky. The ratio of the flux of Galactic γ‐rays to that of cosmic rays is <8.0×10−5 (90% CL). This limit, based on muon measurements, represents a significant improvement over previous experiments and approaches predicted levels for diffuse γ‐rays from the Galactic plane.