Abstract: Over 60 years after their discovery, the sources of ultra-high-energy cosmic rays (UHECRs), atomic nuclei from outer space with energies over 1 EeV, are still unknown. Deflections by intergalactic and Galactic magnetic fields prevent us from straightforwardly inferring the position of their sources from their arrival directions as can be done with neutral messengers, and interactions with extragalactic background photons alter their energy spectrum and mass composition making it nontrivial to infer properties of their sources from terrestrial observations. Nevertheless, the large numbers of events detected by the last-generation UHECR detectors have allowed us to answer a few of the longstanding questions about these particles, though many others remain. Furthermore, their extreme energies allow us to probe certain hypotheses about physics beyond the Standard Model in regimes not accessible via artificial particle accelerators. In this seminar, I will provide an overview of the field, with a special emphasis on relatively recent developments and on frequently misunderstood issues, and an outlook for the near- and medium-term future.
Ultra-high-energy cosmic rays: what we know and what we don't, and possible "new physics" implications