Ultra High Energy Cosmic Rays: the present position and the need for mass composition measurements

Abstract

The present situation with regard to experimental data on ultra high-energy cosmic rays is briefly reviewed. Whilst detailed knowledge of the shape of the energy spectrum is still lacking, it is clear that events above 10^20 eV do exist. Evidence for clustering of the directions of some of the highest energy events remains controversial. Clearly, more data are needed and these will come from the southern branch of the Pierre Auger Observatory in the next few years. What is evident is that our knowledge of the mass composition of cosmic rays is deficient at all energies above 10^18 eV. It must be improved if we are to discover the origin of the highest energy cosmic rays. The major part of the paper is concerned with this problem: it is argued that there is no compelling evidence to support the common assumption that cosmic rays of the highest energies are protons.