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Colloquium
Challenges and Opportunities in Ultra-High Energy Cosmic Rays

Abstract

Ultra-High Energy Cosmic Rays (UHECRs) consist of protons and nuclei that possess energies exceeding about 10 exaelectronvolt (10^19 eV) and interact with Earth’s atmosphere. With the establishment of the Pierre Auger Observatory (covering about 3600 sq. km) in the Southern hemisphere and the Telescope Array Project (800 -> 3200 sq. km) in the North, the research community aimed to pioneer "charged-particle astronomy" to probe sources capable of accelerating particles to such extreme energies. However, prevailing evidence suggests a dominance of nuclei over protons at these energy levels, which implies greater deflections away from the source direction by galactic and extragalactic magnetic fields at a given energy. Subsequently, there has been remarkable advancement in modeling the transport of UHECR nuclei through the intergalactic medium and within potential sources like Gamma-Ray Bursts, AGN, or kpc-scale jets. This has led to sophisticated models linking source and propagation, affirming that populations of “similar” sources align with the observed, spatially averaged UHECR spectrum and mass composition. These models also bridge the gap to high-energy neutrino observations by IceCube, utilizing the advantages of multimessenger studies. The current challenge lies in pinpointing individual UHECR sources from the data accumulated over more than a decade. To achieve this, advanced models and inference techniques are required to disentangle the effects of UHECR propagation from present and future the spatial and spectral observations, utilizing all available modeling insights.