Chacaltaya Observatory of Cosmic Physics has been jointly operated with Bolivia since 1962 at Mt.Chacaltaya, Bolivia, as the world-highest cosmic-ray laboratory (16˚21’S, 68˚08’W, 5300m a.s.l.).
The air-shower experiment, BASJE (Bolivia Air Shower Joint Experiment), aims to investigate the origin of primary cosmic rays around and above the knee region (~1015 eV) by measuring the mass composition, the energy spectrum and the arrival direction distribution. As a result, the mass composition of primary cosmic rays becomes heavier with the increasing energy up to the knee region. This project is finished at the end of March, 2016.
After the BASJE experiment, preparation of the ALPACA (Andes Large area PArticle detector for Cosmic ray physics and Astronomy) experiment, which aims at wide field-of-view observation of cosmic gamma rays in the 100 TeV region, started at the foot of Mt. Chacaltaya (4740 m above sea level). The 100 TeV gamma rays are decay product of neutral pions produced by primary cosmic rays accelerated up to the knee energy region interacting with matter surrounding a yet-unidentified cosmic ray accelerator (PeVatron). Therefore, observation of 100 TeV gamma rays will be a key experiment to locate an unknown PeVatron.