Dawn of sub-PeV gamma-ray astronomy in the southern hemisphere

Origin of cosmic rays (CRs) has been a century-long problem of astrophysics. According to the standard scenario, CR protons are accelerated up to a few PeV in the Milky Way galaxy. So the identification of PeV accelerator is crucial in the cosmic-ray physics. Due to the deflection of PeV proton’s trajectory by the galactic magnetic field, their arrival direction at the earth does not point to their origin. On the other hand, neutral pions are produced through the interaction of PeV protons with interstellar matter and the decayed sub-PeV (>100TeV) photons point to their origin. The first successful observation of sub-PeV photons was reported by the Tibet ASgamma experiment in 2019 and it is followed by the HAWC and LHAASO experiments. They found that the northern sky is rich with sub-PeV objects. More high-energy objects are known in the southern sky, where the galactic plane near the center of the galaxy is visible. A new experiment ALPACA is under construction and partly in operation in the Bolivian Andes at an altitude of 4,700m. ALPACA uses the technique of the Tibet ASgamma experiment to reveal the sub-PeV sky in the southern hemisphere for the first time. In this seminar, I will review the gamma-ray astronomy to search for the origin of cosmic rays, especially the recent progress of sub-PeV gamma-ray astronomy in the northern hemisphere, then I will present the status and prospects of the ALPACA experiment.