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The origin of cosmic rays with energies above 100 TeV remains a profound astrophysical mystery. If such cosmic rays are accelerated by a handful number of point sources, such as Cygnus X-3, then the gamma ray emission from such sources should have already been detected by the current generation of large area air shower experiments which have been operating during the past few years. However, no continuous point source emission has been detected. Therefore we consider the implications of a new search for gamma ray emission from diffuse sources.
The Chicago Air Shower Array (CASA) which operates in coincidence with the Michigan muon array (MIA) is the world's most sensitive experiment to gamma rays with energies $\simgreat 100$ TeV, and is well-suited for studies of diffuse sources based upon the muon content of air showers. We describe a search for diffuse gamma ray emission from molecular cloud regions observed by CASA-MIA. Such emission would be indicated by an excess of muon-poor showers from the direction of the source. If we conservatively assume that the flux of cosmic rays is uniform in the galaxy, the we predict that the diffuse emission should be detectable by CASA-MIA within the lifetime of the experiment. However, if there are sources of cosmic rays in close proximity to certain molecular clouds, then spectrum of gamma rays from these clouds will be stronger and harder. By searching for such enhancements in the diffuse emission, and by correlating the CASA-MIA results with emission detected at lower energies by EGRET, we may identify or constrain the nature of cosmic rays sources in both energy regimes.
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