Ultra-High-Energy Cosmic Rays

Dear Colleagues,

Ultra-high-energy cosmic rays (UHECRs) are particles from the Universe with energies above 1 EeV (10¹⁸ eV),extending up to beyond 100 EeV. These energies are much higher than those achieved by human-made accelerators. Such high energy particles, subsequently, are important probes to study acceleration physics, particle physics, and new physics beyond the standard models. However, how and where those particles are accelerated to such high energies still remains a mystery, even 60 years after they were discovered. As UHECRs travel to Earth, their paths are altered by magnetic fields, causing the observed UHECR directions to deviate from the directions of their sources. This discrepancy presents a challenge in pinpointing the origins of these cosmic rays. Decades of experimental and theoretical work have been dedicated to exploring this phenomenon. The world’s largest UHECR observatories, the Pierre Auger Observatory (PAO) and the Telescope Array (TA) experiments, have been measuring the spectrum, composition, and anisotropy of UHECRs for over a decade, providing us with more and more information to uncover the origin and acceleration mechanism of UHECRs. The development of next-generation experiments, such as the Giant Radio Array for Neutrino Detection (GRAND) project and the Square Kilometre Array (SKA), would further increase the statistics of observed UHECRs. Furthermore, the progress in multi-messenger astronomy enriches our knowledge further on features of source candidates, the extra-galactic background light, and the intergalactic and galactic magnetic field, developing our understanding of the acceleration and escape of UHECRs in the source, and their propagation from the source to Earth. This marks an exciting time, bringing us closer to understanding this scientific marvel.

The aim of this Special Issue is to collect the current knowledge from theories and observations on the physics of UHECRs and their source candidates, understanding our position in relation to, and what kind of efforts are needed to solve, this 60-year-old mystery.

Dr. Haoning He
Guest Editor

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• high energy neutrino
• high energy gamma-ray
• ultra-high-energy cosmic rays
• particle acceleration
• hadronic interactions
• anisotropy
• composition
• multi-messenger astronomy
• magnetic fields
• Gamma-Ray Bursts (GRBs)
• galaxy clusters
• Active Galactic Nuclei (AGN)
• Tidal Disruption Events (TDEs)
• radio galaxies
• starburst galaxies
• pulsars