Exploring the Energy Limit of Cosmic Rays

Abstract

Ultra-high energy cosmic rays (UHECRs) exceeding the energy of 10²⁰eV are entering the Earth and are observed as an intensive air shower by AGASA, H₁Res and other experiments Their origin is expected to be the high energy astronomical object out of our galaxy but no particular source is identified in their arrival direction. The origin of such UHECRs remains a mystery. Moreover, the measurement of AGASA indicates that the energy spectrum continues beyond the GZK. Cutoff expected from the interaction of cosmic ray protons with the microwave background The recent result of H₁Res air fluorescence telescope, however, does not support the AGASA's claim Two large hybrid detectors, the Pierre Auger Observatory and the Telescope Array, are being built in Argentina and the US in order to confirm or refute the high flux of UHECRs observed by AGASA An observation by the space-based telescope, EUSO, is also planned. We expect that new measurements will soon resolve the puzzles existing in the generation and propagation of UHECRs in the Universe. It may lead us to a new physics beyond the standard theory of elementary particles and the cosmology.