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The goal of the High Resolution Fly's Eye (HiRes) experiment is to measure the energy spectrum and the composition of cosmic rays above 1018 eV. The existence of particles at these energies continues to challenge our imagination: where do they come from, what are the acceleration mechanisms and how can they travel astronomical distances without substantial loss of energy?
At EeV energies, the flux of particles has become so extremely low that the large effective area necessary for a sufficient event rate requires earthbound experiments. Consequently, the high energy particle is detected indirectly as cosmic ray primaries entering the Earth's atmosphere interact with atmospheric nuclei to produce large cascades of relativistic secondary particles. Air fluorescence detectors like HiRes make use of the fact that the particle cascade dissipates much of its energy while exciting and ionizing air molecules.
Located in the dry atmosphere of the Utah desert, the HiRes detector consists of a large number of focusing mirrors that image fluorescence light from distant air shower cascades onto arrays of photomultiplier tubes.
HiRes has been operating in various configurations since 1997. Since Fall 1999, HiRes comprises two detector sites that provide stereoscopic observations of cosmic ray induced air showers. The talk gives a description of the detector and the techniques used to observe and measure events at EeV energies, and presents recent results on the energy spectrum and the arrival directions.
