The ALPACA experiment: The project of the first sub-PeV gamma-ray observation in the southern sky

TL;DR

The ALPACA experiment aims to detect sub-PeV gamma rays in the southern sky to identify Galactic PeV cosmic ray sources, using a large air shower array with muon detectors at high altitude.

Contribution

This paper introduces the ALPACA project, detailing its design, progress, and sensitivity, marking the first effort to observe sub-PeV gamma rays in the southern hemisphere.

Findings

ALPAQUITA prototype will start data collection in 2022.

ALPACA will extend observations to 2024.

The experiment aims to identify Galactic PeVatrons.

Abstract

The ALPACA experiment is a project aiming to observe sub-PeV gamma rays for the first time in the southern hemisphere. The main goal of ALPACA is to identify PeVatrons, the accelerators of Galactic PeV cosmic rays, by observing sub-PeV pion-decay gamma rays generated in interactions between PeV cosmic rays and the interstellar medium. This new air shower experiment is located at an altitude of 4,740 m above sea level in the middle of Mt. Chakartaya in Bolivia. The air shower array consists of 401 scintillation counters covering an 83,000 m$^2$ surface area. In addition, a water-Cherenkov-type muon detector array with an area of 3,700 m$^2$ is installed to discriminate gamma rays from background cosmic rays. The prototype array ALPAQUITA will start data taking in 2022 and will extend to ALPACA in 2024. We report on a general introduction to ALPACA, the progress of the project, and the sensitivity to sub-PeV gamma rays.