Particle Physics with UHECRs at the Cosmic Frontier

Ultra-high energy cosmic rays (UHECRs) reach the Earth with energies up to around 10²⁰eV where they interact with air molecules, producing extensive air showers (EAS) in the atmosphere. These EAS can be measured with large detector arrays at the ground or with experiments observing the atmosphere from space. The interpretation of these measurements relies on sophisticated models of the EAS development which represents a challenge, as well as an opportunity to test quantum chromodynamics (QCD) under extreme conditions. The EAS development is driven by hadron-ion collisions under low momentum transfer in the non-perturbative regime of QCD. Under these conditions, hadron production cannot be described using first principles and these interactions cannot be probed with existing collider experiments. Thus, accurate measurements of the EAS development yield a unique probe of multi-particle production in hadronic interactions and provide a potential window into new particle physics.

We will present an overview of recent and proposed measurements probing particle physics with UHECRs at the cosmic frontier. In particular, we will discuss the prospects of existing EAS experiments for the next decade and highlight the unique opportunities for high-energy particle physics with proposed next-generation UHECR experiments.