Ultra high energy cosmic rays: the highest energy frontier

TL;DR

This paper reviews the detection and analysis of ultra-high energy cosmic rays, discussing experimental methods, key findings on energy spectrum, composition, and interactions, and implications for physics and astrophysics.

Contribution

It provides a comprehensive overview of the current state-of-the-art in UHECR detection, including recent experimental results and their significance.

Findings

Cosmic ray energy spectrum characterized up to 10^20 eV

Evidence of anisotropy in cosmic ray arrival directions

Insights into hadronic interactions at energies beyond current accelerators

Abstract

Ultra-high energy cosmic rays (UHECRs) are the highest energy messengers of the present universe, with energies up to $10^{20}$ eV. Studies of astrophysical particles (nuclei, electrons, neutrinos and photons) at their highest observed energies have implications for fundamental physics as well as astrophysics. The primary particles interact in the atmosphere and generate extensive air showers. Analysis of those showers enables one not only to estimate the energy, direction and most probable mass of the primary cosmic particles, but also to obtain information about the properties of their hadronic interactions at an energy more than one order of magnitude above that accessible with the current highest energy human-made accelerator. In this contribution we will review the state-of-the-art in UHECRs detection. We will present the leading experiments Pierre Auger Observatory and Telescope Array and discuss the cosmic ray energy spectrum, searches for directional anisotropy, studies of mass composition, the determination of the number of shower muons (which is sensitive to the shower hadronic interactions) and the proton-air cross section.