Constraints on Gamma‐Ray Emission from the Galactic Plane at 300 TeV

We describe a new search for diffuse ultrahigh energy gamma-ray emissionassociated with molecular clouds in the galactic disk. The Chicago Air ShowerArray (CASA), operating in coincidence with the Michigan muon array (MIA), hasrecorded over 2.2 x 10^{9} air showers from April 4, 1990 to October 7, 1995.We search for gamma rays based upon the muon content of air showers arrivingfrom the direction of the galactic plane. We find no significant evidence fordiffuse gamma-ray emission, and we set an upper limit on the ratio of gammarays 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 degrees < l < 200degrees); -5 degrees < b < 5 degrees). This limit places a strong constraint onmodels for emission from molecular clouds in the galaxy. We rule outsignificant spectral hardening in the outer galaxy, and conclude that emissionfrom the plane at these energies is likely to be dominated by the decay ofneutral pions resulting from cosmic rays interactions with passive target gasmolecules.Comment: Astrophysical Journal, submitted, 11 pages, AASTeX Latex, 3 Postscript figure