ForewordThe Pierre Auger Observatory has begun a major Upgrade of its already impressive capabilities, with an emphasis on improved mass composition determination using the surface detectors of the Observatory. Known as AugerPrime, the upgrade will include new 4 m 2 plastic scintillator detectors on top of all 1660 water-Cherenkov detectors, updated and more flexible surface detector electronics, a large array of buried muon detectors, and an extended duty cycle for operations of the fluorescence detectors.This Preliminary Design Report was produced by the Collaboration in April 2015 as an internal document and information for funding agencies. It outlines the scientific and technical case for AugerPrime 1 . We now release it to the public via the arXiv server. We invite you to review the large number of fundamental results already achieved by the Observatory and our plans for the future.The Pierre Auger Collaboration 1 As a result of continuing R&D, slight changes have been implemented in the baseline design since this Report was written. These changes will be documented in a forthcoming Technical Design Report. ix x Executive Summary Present Results from the Pierre Auger ObservatoryMeasurements of the Auger Observatory have dramatically advanced our understanding of ultra-high energy cosmic rays. The suppression of the flux around 5×10 19 eV is now confirmed without any doubt. Strong limits have been placed on the photon and neutrino components of the flux indicating that "top-down" source processes, such as the decay of superheavy particles, cannot account for a significant part of the observed particle flux. A largescale dipole anisotropy of ∼7% amplitude has been found for energies above 8×10 18 eV. In addition there is also an indication of the presence of a large scale anisotropy below the ankle. Particularly exciting is the observed behavior of the depth of shower maximum with energy, which changes in an unexpected, non-trivial way. Around 3×10 18 eV it shows a distinct change of slope with energy, and the shower-to-shower variance decreases. Interpreted with the leading LHC-tuned shower models, this implies a gradual shift to a heavier composition. A number of fundamentally different astrophysical model scenarios have been developed to describe this evolution. The high degree of isotropy observed in numerous tests of the small-scale angular distribution of UHECR above 4×10 19 eV is remarkable, challenging original expectations that assumed only a few cosmic ray sources with a light composition at the highest energies. Interestingly, the largest departures from isotropy are observed for cosmic rays with E > 5.8×10 19 eV in ∼20 • sky-windows. Due to a duty cycle of ∼15% of the fluorescence telescopes, the data on the depth of shower maximum extend only up to the flux suppression region, i.e. 4×10 19 eV. Obtaining more information on the composition of cosmic rays at higher energies will provide crucial means to discriminate between the model classes and to understand the origin of the observed flux suppre...

he Pierre Auger Observatory, located on a vast, high plain in western\ud Argentina, is the world's largest cosmic ray observatory. The objectives\ud of the Observatory are to probe the origin and characteristics of cosmic\ud rays above 10(17) eV and to study the interactions of these, the most\ud energetic particles observed in nature. The Auger design features an\ud array of 1660 water Cherenkov particle detector stations spread over\ud 3000 km(2) overlooked by 24 air fluorescence telescopes. In addition,\ud three high elevation fluorescence telescopes overlook a 23.5 km(2),\ud 61-detector infilled array with 750 in spacing. The Observatory has been\ud in successful operation since completion in 2008 and has recorded data\ud from an exposure exceeding 40,000 km(2) sr yr. This paper describes the\ud design and performance of the detectors, related subsystems and\ud infrastructure that make up the Observatory

We report a study of the distributions of the depth of maximum, Xmax, of extensive air-shower profiles with energies above 10 17.8 eV as observed with the fluorescence telescopes of the Pierre Auger Observatory. The analysis method for selecting a data sample with minimal sampling bias is described in detail as well as the experimental cross-checks and systematic uncertainties. Furthermore, we discuss the detector acceptance and the resolution of the Xmax measurement and provide parameterizations thereof as a function of energy. The energy dependence of the mean and standard 4 deviation of the Xmax-distributions are compared to air-shower simulations for different nuclear primaries and interpreted in terms of the mean and variance of the logarithmic mass distribution at the top of the atmosphere.

The Giant Radio Array for Neutrino Detection (GRAND) 1 is a planned large-scale observatory of ultra-highenergy (UHE) cosmic particles -cosmic rays, gamma rays, and neutrinos with energies exceeding 10 8 GeV. Its ultimate goal is to solve the long-standing mystery of the origin of UHE cosmic rays. It will do so by detecting an unprecedented number of UHECRs and by looking with unmatched sensitivity for the undiscovered UHE neutrinos and gamma rays associated to them. Three key features of GRAND will make this possible: its large exposure at ultra-high energies, sub-degree angular resolution, and sensitivity to the unique signals made by UHE neutrinos.The strategy of GRAND is to detect the radio emission coming from large particle showers that develop in the terrestrial atmosphereextensive air showers -as a result of the interaction of UHE cosmic rays, gamma, rays, and neutrinos. To achieve this, GRAND will be the largest array of radio antennas ever built. The relative affordability of radio antennas makes the scale of construction possible. GRAND will build on years of progress in the field of radio-detection and apply the large body of technological, theoretical, and numerical advances, for the first time, to the radio-detection of air showers initiated by UHE neutrinos.The design of GRAND will be modular, consisting of several independent sub-arrays, each of 10 000 radio antennas deployed over 10 000 km 2 in radio-quiet locations. A staged construction plan ensures that key techniques are progressively validated, while simultaneously achieving important science goals in UHECR physics, radioastronomy, and cosmology early during construction.Already by 2025, using the first sub-array of 10 000 antennas, GRAND could discover the long-sought cosmogenic neutrinos -produced by interactions of ultra-high-energy cosmic-rays with cosmic photon fields -if their flux is as high as presently allowed, by reaching a sensitivity comparable to planned upgraded versions of existing experiments. By the 2030s, in its final configuration of 20 sub-arrays, GRAND will reach an unparalleled sensitivity to cosmogenic neutrino fluxes of 4 • 10 −10 GeV cm −2 s −1 sr −1 within 3 years of operation, which will guarantee their detection even if their flux is tiny. Because of its sub-degree angular resolution, GRAND will also search for point sources of UHE neutrinos, steady and transient, potentially starting UHE neutrino astronomy. Because of its access to ultra-high energies, GRAND will chart fundamental neutrino physics at these energies for the first time.GRAND will also be the largest detector of UHE cosmic rays and gamma rays. It will improve UHECR statistics at the highest energies ten-fold within a few years, and either discover UHE gamma rays or improve their limits ten-fold. Further, it will be a valuable tool in radioastronomy and cosmology, allowing for the discovery and follow-up of large numbers of radio transients -fast radio bursts, giant radio pulses -and for precise studies of the epoch of reionization.Following the disc...