A new air shower array in the Southern Hemisphere looking for the origins of Cosmic rays: the ALPACA experiment

TL;DR

ALPACA is a southern-hemisphere air-shower array planned in Bolivia to study the origins of Galactic cosmic rays by detecting high-energy gamma rays using a surface detector array and underground muon detectors. The paper describes the ALPAQUITA prototype (≈1/4 of full ALPACA), its detector design, MC-based gamma/hadron discrimination performance, and initial Moon shadow measurements used to calibrate angular resolution. It outlines the construction plan to extend the surface array and add underground MD pools, with projected capabilities to detect multiple TeV gamma-ray sources within a year. The results demonstrate feasibility and outline a path toward full southern-sky gamma-ray coverage, offering a new window to probe cosmic-ray accelerators in the Galaxy.

Abstract

The Tibet AS$γ$ experiment successfully detected sub-PeV $γ$-rays from the Crab nebula using a Surface Array and underground muon detector. Considering this, we are building in Bolivia a new experiment to explore the Southern Hemisphere, looking for the origins of cosmic rays in our Galaxy. The name of this project is Andes Large area PArticle detector for Cosmic ray physics and Astronomy (ALPACA). A prototype array called ALPAQUITA, with $1/4$ the total area of the full ALPACA, started observations in September $2022$. In this paper we introduce the status of ALPAQUITA and the plans to extend the array. We also report the results of the observation of the moon shadow in cosmic rays.

Figures (8)

• Figure 1: Schematic diagram of the ALPAQUITA array and principle of function
• Figure 2: Schematic diagram of one underground MD pool in the ALPACA experiment and principle of function. Figure taken from anzorena25.
• Figure 3: Distribution of the $\sum N_{\mu}$ against particle density $\sum\rho$ for different types of showers. Distribution in the left corresponds to $\gamma$-ray events and the one on the right is for CR. Events with $m_{k=0}<0.1$ are accumulated at the bin e-2
• Figure 4: Sensitivity curve of ALPAQUITA with the $\gamma$-ray spectra of $3$ sources in the field view that can be detected within one year of observation.
• Figure 5: Construction plan of the ALPACA experiment. Left panel: layout of the ALPAQUITA array. Right panel: After the construction of the first MD unit, the surface array will be extended as enclosed by the solid-line octagon and will be operated as ALPAQUITA + MD. Full coverage array ALPACA will be eventually achieved