The Snowmass UHECR White Paper on Ultra-High-Energy Cosmic Rays

¹ Bartol Research Institute, Department of Physics and Astronomy, University of Delaware, Newark, DE 19716, USA
² Institute for Astroparticle Physics (IAP), Karlsruhe Institute of Technology (KIT), 76021 Karlsruhe, Germany
³ Department of Physics and Astronomy, Uppsala University, Uppsala, SE-752 37, Sweden
⁴ Department of Astronomy and Astrophysics, University of Chicago, Chicago, IL, USA
⁵ Department of Physics, Colorado School of Mines, Golden, CO, USA
⁶ Institute of Experimental Particle Physics (ETP), Karlsruhe Institute of Technology (KIT), 76021 Karlsruhe, Germany
⁷ Astroparticle Physics Laboratory, NASA Goddard Space Flight Center, Greenbelt, MD, USA

^* Corresponding author: fgs@udel.edu

Published online: 28 April 2023

Abstract

This proceeding summarizes the talk given at the opening of the UHECR 2022 conference in L’Aquila on the whitepaper ‘Ultra-High-Energy Cosmic Rays: The Intersection of the Cosmic and Energy Frontiers’ [Astroparticle Physics 149 (2023) 102819 - arXiv:2205.05845] that has been prepared for the Snowmass survey in the USA. The whitepaper provides an overview of recent progress and open questions regarding the particle physics and astrophysics related to ultra-high-energy cosmic rays (UHECR) and outlines the connections between the particle and astrophysics aspects of cosmic rays. It also discusses what instrumentation is needed to address the major scientific questions in ultra-high-energy cosmic-ray physics. While the upgraded Pierre Auger Observatory and Telescope Array will remain the workhorses at the highest energies in the current decade, new experiments with significantly higher exposure are needed in the coming decade. Ground arrays featuring simultaneous detection of the position of the shower maximum and the size of the muonic component will enable particle astronomy by measuring the rigidity of individual events. They should be complemented by other detectors maximizing the total exposure. This can be achieved by a few next-generation experiments using the latest developments in detection and analysis techniques: GRAND as a ground-based radio array, and POEMMA as a space-borne stereo fluorescence telescope will feature complementary approaches to provide maximum exposure; IceCube-Gen2 with its surface array, and GCOS aim at increased statistics with high accuracy for particle physics and rigidity-based galactic and extra-galactic astrophysics. While designed to discover the astrophysical cosmic-ray sources at the highest energies, the same experiments also contribute to particle physics, e.g., by studying the muon puzzle in cosmic-ray air showers, and by their discovery potential for exciting new physics, such as certain Dark Matter candidates. With the full whitepaper available as a reference, this proceeding will briefly present the science cases of the experiments, highlighting their individual strengths and outlining how they complement each other.