Constraints on gamma-ray emission from the Galactic plane at 300 TeV

We describe a new search for diffuse ultra-high-energy gamma-ray emission associated with molecular clouds in the Galactic disk. The Chicago Air Shower Array (CASA), operating in coincidence with the Michigan muon array (MIA), has recorded over 2.2 x 10⁹ air showers from 1990 April 4 to 1995 October 7. We search for gamma rays based upon the muon content of air showers arriving from the direction of the Galactic plane. We find no significant evidence for diffuse gamma-ray emission, and we set an upper limit on the ratio of gamma rays to normal hadronic cosmic rays at less than 2.4 x 10^-5 at 310 TeV (90% confidence limit) from the Galactic plane region: (50°<l < 200° ; -5° < b < 5°). This limit places a strong constraint on models for emission from molecular clouds in the Galaxy. We rule out significant spectral hardening in the outer Galaxy, and conclude that emission from the plane at these energies is likely to be dominated by the decay of neutral pions resulting from cosmic-ray interactions with passive target gas molecules.