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Abstract:
The Pierre Auger Observatory has been collecting experimental data over the recent 16 years. The high accuracy of air shower measurements, combined with large statistics accumulated, enables determination of ultra-high energy cosmic ray (UHECR) properties with accuracy not attainable until now. A large-scale anisotropy of UHECR arrival directions was measured over more than three decades in energy. The energy spectrum was determined up to beyond 10^20 eV, confirming the flux suppression above ~5 x 10^19 eV. The UHECR mass composition determined using both fluorescence and surface array measurements indicates a mixed composition, rather than pure light or heavy nuclei. Such a result suggests that the spectrum suppression may be related to a limit of cosmic ray acceleration at sources, rather than to a propagation effect. However, the composition measurement is heavily influenced by uncertainties in hadronic interaction properties at ultra-high energies. Precise measurement of the muon component of air showers is the key to disentangling the cosmic ray composition from the influence of interaction properties, and thus to understanding the origin of the spectrum suppression. This is the main motivation for the ongoing upgrade of the Observatory.