Cosmic Rays at the highest energies

TL;DR

This paper reviews the current understanding of ultra-high-energy cosmic rays, discussing observational evidence, potential sources, and the challenges in identifying their origins due to conflicting measurements and limited data.

Contribution

It provides a comprehensive overview of recent cosmic ray observations, highlighting the implications for acceleration models and the need for larger datasets to resolve existing ambiguities.

Findings

Confirmation of a GZK-like spectral feature narrows down possible sources.

Anisotropy observations suggest potential for particle astronomy.

Composition measurements present conflicting interpretations.

Abstract

After a century of observations, we still do not know the origin of cosmic rays. I will review the current state of cosmic ray observations at the highest energies, and their implications for proposed acceleration models and secondary astroparticle fluxes. Possible sources have narrowed down with the confirmation of a GZK-like spectral feature. The anisotropy observed by the Pierre Auger Observatory may signal the dawn of particle astronomy raising hopes for high energy neutrino observations. However, composition related measurements point to a different interpretation. A clear resolution of this mystery calls for much larger statistics than the reach of current observatories.