Ultra High Energy Cosmic Rays: the Present Position and the Need for Mass Composition Measurements* 1. Motivation for Studying the Highest Energy Cosmic Rays

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. Efforts to discover the origin of the highest energy cosmic rays have been on going for many years. Since the recognition in 1966, by Greisen and by Zatsepin and Kuzmin, that protons with energies above 4 × 10 19 eV would interact with the cosmic microwave radiation, there has been great interest in measuring the spectrum, arrival direction distribution and mass composition of ultra high-energy cosmic rays (UHECR). UHECR may be defined as those cosmic rays having energies above 10 19 eV. Specifically, it was pointed out that if the sources of the highest energy protons were universally distributed, then there should be a sharp steepening of the energy spectrum in the range from 4 to 10 × 10 19 eV. This predicted feature has become known as the GZK cutoff. If the UHECR were mainly Fe nuclei then there would also be a steepening of the spectrum. However, it is harder to predict this feature accurately as the relevant diffuse infrared photon field is poorly known: the steepening is expected to set in at higher energy.