On the acceleration of Ultra-High-Energy Cosmic Rays

TL;DR

This paper reviews the current understanding of how ultra-high-energy cosmic rays are accelerated and propagated, highlighting recent experimental findings, theoretical models, and the potential need for new physics to explain observations.

Contribution

It provides a comprehensive review of the theoretical and observational challenges in understanding UHECR acceleration and propagation, including recent experimental results and potential new physics implications.

Findings

Discovery of a high-energy cut-off in the UHECR spectrum

Clustering of highest energy events towards nearby active galactic nuclei

Constraints on acceleration models from recent observations

Abstract

Ultra High Energy Cosmic Rays (UHECRs) hit the Earth's atmosphere with energies exceeding $10^{18}$ eV. This is the same energy as carried by a tennis ball moving at 100 km/h, but concentrated on a sub-atomic particle. UHECRs are so rare (the flux of particles with $E > 10^{20}$ eV is 0.5/km$^2$/century) that only a few such particles have been detected over the past 50 years. Recently, the HiRes and Auger experiments have reported the discovery of a high-energy cut-off in the UHECR spectrum, and Auger has found an apparent clustering of the highest energy events towards nearby active galactic nuclei. Consensus is building that the highest energy particles are accelerated within the radio-bright lobes of these objects, but it remains unclear how this actually happens, and whether the cut-off is due to propagation effects or reflects an intrinsically physical limitation of the acceleration process. The low event statistics presently allows for many different plausible models; nevertheless observations are beginning to impose strong constraints on them. These observations have also motivated suggestions that new physics may be implicated. We present a review of the key theoretical and observational issues related to the processes of propagation and acceleration of UHECRs and proposed solutions.