Ultra-High Energy Cosmic Rays: Results and Prospects

TL;DR

This paper reviews recent progress in understanding ultra-high energy cosmic rays, focusing on spectral features, composition, and the origin of flux suppression, highlighting current challenges and future prospects in the field.

Contribution

It summarizes recent experimental and theoretical developments in ultra-high energy cosmic ray research and discusses the prospects for resolving key questions in the next decade.

Findings

Observation of a knee of heavy primaries at 10^{17} eV

Detection of an ankle of light primaries at 4x10^{18} eV

Flux suppression at >4x10^{19} eV and its possible explanations

Abstract

Recent advances in measuring and interpreting cosmic rays from the spectral ankle to the highest energies are briefly reviewed. A knee of heavy primaries and an ankle of light primaries have been observed at about 10^{17} eV. The light component starts to dominate the flux at the ankle in the all particle spectrum at about 4x10^{18} eV and sheds light on the transition from galactic to extragalactic cosmic rays. The prime question at the highest energies is about the origin of the flux suppression observed at E > 4x10^{19} eV. Is this the long awaited GZK-effect or the exhaustion of sources? The key to answering this question is again the still largely unknown mass composition at the highest energies. Data from different observatories don't quite agree and common efforts have been started to settle that question. The high level of isotropy observed even at the highest energies challenges models of a proton dominated composition if extragalactic magnetic fields are on the order of a few nG or less. We will discuss the experimental and theoretical progress in the field and the prospects for the next decade.