ALPACA air shower array to explore 100TeV gamma-ray sky in Bolivia

Andes Large area PArticle detectorfor Cosmic ray physics and Astronomy (ALPACA) is a new air showerarray project as a collaboration between Bolivia and Japan to explore the 100 TeV gamma-ray sky in the southern hemisphere. In a plateau near the Chacaltaya mountain at 4,740 m altitude, a surface detector array covering 82,800 m$^{2}$ with underground water Cherenkov muondetectors of total 5,400 m$^{2}$ area will be constructed. Because of 2 m soil overburden, the muondetectors can detect muonsof >1.2 GeV in air showerswith a high purity. Using the conventional surface array to determine the primary energyand the arrival direction, the underground muondetectors improve the gamma/hadron separation and also mass identification of primary cosmic rays. For gamma-ray showerswithin zenith angle of 45 degrees, ALPACA has a full effective area above 20TeV. At 20 TeV and 100 TeV, 99% and 99.9% hadron showersare rejected, respectively, while keeping the gamma-ray detection efficiency above 90%. Many interesting galactic objects can be observed with 0.2 degree angular resolutionat 100 TeV with >2,000 hours/year exposure. ALPACA enables us the first sensitive survey of the southern gamma-ray sky at 100 TeV energy range that is crucial to identify PeV accelerating objects. Preparation for infrastructure and con- struction of a pathfinderarray ALPAQUITA are ongoing. Scientific targets, expected performance of ALPACA including the prospects for some CR observations and current status are described.