The highest energy cosmic rays: the past, the present and the future

TL;DR

This paper reviews the historical development, current understanding, and future prospects of the highest-energy cosmic rays, highlighting recent findings on their mass composition and energy spectrum from major observatories.

Contribution

It provides a comprehensive historical overview and presents new insights into the mass composition at the highest energies, resolving a long-standing controversy.

Findings

Agreement between Telescope Array and Auger on depth of shower maximum

Indication that cosmic rays become heavier above ~4 EeV

Challenge to the pure proton dominance hypothesis at highest energies

Abstract

The greater part of this paper is concerned with a historical discussion of the development of the search for the origins of the highest-energy cosmic-rays together with a few remarks about future prospects. Additionally, in section 6, the situation with regard to the mass composition and energy spectrum at the highest energies is discussed. It is shown that the change of the depth of shower maximum with energy above 1 EeV, measured using the Telescope Array, is in striking agreement with similar results from the Auger Observatory. This implies that either the mean mass of cosmic rays is becoming heavier above ~4 EeV or that there is a change in details of the hadronic interactions in a manner such that protons masquerade as heavier nuclei. A long-standing controversy is thus resolved: the belief that pure protons dominate the mass distribution at the highest energies is no longer tenable.