A large underground water Cherenkov muon detector array with the Tibet air shower array for the Gamma-ray astronomy in the 100 TeV region: Overview and physics goal

The Tibet air shower array with effective area 37,000 m2 located at 4300m in altitude, in Tibet, China, is sensitive to celestial gamma rays with energies above a few TeV. We have a plan to add a large underground muon detector array to it in order to discriminate cosmic gamma rays from cosmic-ray hadrons and to increase its sensitivity by approximately one order of magnitude to cosmic gamma rays in the 100 TeV energy region. The overview of the plan will be briefly described.