The Pierre Auger Observatory is the world leading science project for the exploration of cosmic rays. More than 500 scientists from 16 countries work together to study the highest-energy cosmic rays by measuring the properties of the showers produced in the atmosphere.
The Observatory is located near Malargüe (Argentina) and has been detecting ultra-high energy cosmic rays for more than ten years. An international agreement has been signed in November 2015 by the science funding agency representatives to continue the operation until 2025.
The essential feature of the Observatory is its hybrid design, a combination of a large surface array and a fluorescence detector. The surface detector (SD) is composed of 1600 water Cherenkov units, spaced by 1500 m, covering a total area of 3000 km2. The fluorescence detector consists of 24 telescopes overlooking the surface array.
The simultaneous observation of the showers with different techniques enables high-statistics and high-precision studies and the huge extension of the SD array allows to detect the very rare events at ~10^20 eV, whose flux is ~1 particle/km^2/century. The baseline configuration of the detectors has been enhanced with a smaller and denser array of SD units and with high elevation fluorescence telescopes to reduce the minimum detectable shower energy down to ~10^17 eV.
The Pierre Auger Observatory has yielded dramatic advances in the measurements of cosmic rays. The abrubt suppression of the energy spectrum above 5 x 10^19 eV has been unequivocally observed. This was predicted since a long time as a consequence of the interaction of cosmic-ray particles with the low energy photons of Cosmic Microwave Background.
The so called ankle, that is the flattening of the spectrum at 5 x 10^18 eV, has been measured with an unprecedented precision. The FD measurements of the longitudinal shower profile has confirmed that the mass composition is mainly made by light primaries around the ankle and have provided evidence of an unexpected shift towards heavier primaries at the highest energies. A result that is consistent with the no evidence of anisotropy or of association with astrophysical sources of the arrival direction of cosmic rays.
Very stringent limits on the flux of photons and neutrinos have allowed to exclude most of the so-called top-down models, in which the cosmic rays are generated by decay of super heavy dark matter or topological defects or similar exotic particles, favouring scenarios in which the acceleration of the primaries occurs in astrophysical sources.
The three dimensional nature of the SD units has allowed to study the showers inclined at large zenith angles. In these showers only muons arrive at ground and the comparison of the measurements with the Monte Carlo simulations has provided a powerful test of the hadronic interaction models extrapolated at energies order of magnitudes larger than the one reachable at LHC.
Due to the reduced duty cycle of the FD, the mass composition of the cosmic rays into the suppression region remains unexplored.
To solve this problem the Auger collaboration has planned an upgrade of the SD called AugerPrime. Plastic scintillator detectors will be installed on the top of each SD units. Combining the different responses of the scintillators and of the water Cherenkov detectors to the electromagnetic and muonic component of the shower, it will be possible to estimate the mass composition at the very high energies and to make anisotropy studies of the light primaries.