Abstract
The Pierre Auger Observatory has been designed to measure cosmic rays above 1018 eV with unprecedented statistics and precision. After outlining why there is interest in such particles, recent measurements from the Observatory relating to the mass composition and energy spectrum above 1018 eV are described. From measurements of the variation of the depth of shower maximum with energy, there are indications - if models of high-energy interactions are correct - that the mass composition is not proton-dominated at the highest energies. A flattening of the slope of the energy spectrum from (-3.30 ± 0.06) to (-2.62 ± 0.02) is observed at 4.5 × 1018 eV while above 3.6 × 1019 eV the spectrum steepens to a slope of (-4.1 ± 0.4). Because of the composition result, caution needs to be exercised over interpretation of the steepening as the long-sought Greisen-Zatsepin-Kuzmin effect. The importance of data from the LHC for the more accurate interpretation of air-shower measurements is emphasised.
Published Version
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