THE PIERRE AUGER OBSERVATORY

The Pierre Auger Project is an international Collaboration involving over 400 scientists from 18 countries, with the objective of studying the highest energy cosmic rays. The construction of the Pierre Auger Observatory was started in 2002 and completed in 2008. The Auger Observatory in the Province of Mendoza, Argentina, has been taking data since 2004. What are cosmic rays? Most of the particles that compose Cosmic Rays are either the nuclei of atoms or electrons. Of the nuclei, most are single protons - the nuclei of Hydrogen atoms - but a few are much heavier, for instance Iron nuclei. Cosmic ray particles can have extremely high energies. Some of them, in fact, are the most energetic of any particles ever observed in nature, since a few instants after the Big Bang. The highest-energy cosmic rays have a hundred million times more energy than the particles produced in the world's most powerful particle accelerator.

Fig 1: Cosmic ray showers hitting the Earth surface

Hybrid detector The Auger Observatory is a “hybrid detector”, employing two independent methods to detect and study the particle produced by high-energy cosmic rays when they interact in the Earth's atmosphere (air showers). One technique detects high energy particles through their interaction with water placed in surface detector tanks. The other technique tracks the development of air showers by observing ultraviolet light emitted high in the Earth's atmosphere by the interaction of propagating secondary particles. The hybrid nature of the Pierre Auger Observatory provides for two independent ways to see cosmic rays. These two ways are an array of surface detectors (Fig. 2) and a collection of air fluorescence telescopes (Fig. 3). Fig 2: A surface detector (SD) of the Pierre Auger Observatory. The SD array consists of 1600 water-Cherenkov detectors on a regular triangular grid, covering a total area of 3000 km².

Fig 3: A view of a fluorescence detector (FD). The FD of the Pierre Auger Observatory comprises four observation sites located on the perimeter of the SD array. Six independent telescopes, each with field of view of 30° x 30° in azimuth and elevation, are located in each FD site. Highlights of science results The energy spectrum of cosmic rays has been measured with unprecedented precision [1] during Auger data taking since 2004. The measurement of the chemical composition of cosmic rays is also possible using data of the Auger Observatory, and could be interpreted as an evolution from light to heavier nuclei if current hadronic interaction models describe well the air shower physics [2]. Thanks to the high statistics of the SD data, a first harmonic analysis was performed in different energy ranges starting from 2.5 x10¹⁷ eV in a search for dipolar modulations in right ascension [3]. The upper limits in the dipole amplitude impose stringent constraints in astrophysical models. The Auger data provide evidence for a weak correlation between arrival direction of cosmic rays above 55 EeV and the positions of AGNs with z<0.018 in the VCV catalogue [4]. The Collaboration also has performed the measurement of the proton-air cross-section at 57 TeV [5] that favors a moderately slow rise of the cross-section towards higher energies, and inferred the proton-proton cross-section, whose value is within one sigma of the best extrapolation from the recent LHC data points. Upper limits have been obtained on the photon flux integrated above an energy threshold which impose stringent limits for top-down models [6]. Also, competitive neutrino limits were published [7], as well as searches for Galactic neutron signals [8]. The local Auger team (L'Aquila/LNGS) Denise Boncioli¹ Armando di Matteo² Aurelio F. Grillo¹ Sergio Petrera²³ Vincenzo Rizi² ¹INFN Laboratori Nazionali del Gran Sasso ²INFN and Department of Physical and Chemical Sciences, University of L'Aquila ³INFN Gran Sasso Science Institute

Bibliografia [1] A. Schulz, Pierre Auger Collaboration, Measurement of the Energy Spectrum of Cosmic Rays above 3x10¹⁷ eV with the Pierre Auger Observatory. [arXiv:1307.5059] [2] A. Aab, et al., Pierre Auger Collaboration, Depth of maximum of air-shower profiles at the Pierre Auger Observatory. Measurements at energies above 10 17.8 eV. Phys.Rev. D90 (2014) 122005 [3] I. Sidelnik, Pierre Auger Collaboration, Measurement of the first harmonic modulation in the right ascension distribution of cosmic rays detected at the Pierre Auger Observatory: towards the detection of dipolar anisotropies over a wide energy range, [arXiv:1307.5059] [4] J. Abraham, et al., Pierre Auger Collaboration, Correlation of the highest energy cosmic rays with nearby extragalactic objects, Science 318 (2007) 938–943. [5] P. Abreu, et al., Pierre Auger Collaboration, Measurement of the proton-air cross-section at √s = 57 TeV with the Pierre Auger Observatory, Phys. Rev. Lett. 109 (2012) 062002. [arXiv:1208.1520] [6] A. Aab, et al., Pierre Auger Collaboration, A search for point sources of EeV photons, Astrophys.J. 789 (2014) 160. [arXiv:1406.2912] [7] P. Abreu, et al., Pierre Auger Collaboration, Search for point-like sources of ultra-high energy neutrinos at the Pierre Auger Observatory and improved limit on the diffuse flux of tau neutrinos, Astrophys.J. 755 (2012) L4. [arXiv:1210.3143] [8] A. Aab, et al., Pierre Auger Collaboration, A Targeted Search for Point Sources of EeV Neutrons, Astrophys.J. 789 (2014) L34 Ulteriori informazioni sulla pagina web ufficiale: https://www.auger.org/