Abstract
The overall picture of the highest energy particles produced in the Universe is changing because of measurements made with the Pierre Auger Observatory. Composition studies of cosmic rays point towards an unexpected mixed composition of intermediate mass nuclei, more isotropic than anticipated, which is reshaping the future of the field and underlining the priority to understand composition at the highest energies. The Observatory is competitive in the search for neutrinos of all flavors above about 100 PeV by looking for very inclined showers produced deep in the atmosphere by neutrinos interacting either in the atmosphere or in the Earth’s crust. It covers a large field of view between −85◦ and 60◦ declination in equatorial coordinates. Neutrinos are expected because of the existence of ultra high energy cosmic rays. They provide valuable complementary information, their fluxes being sensitive to the primary cosmic ray masses and their directions reflecting the source positions. We report the results of the neutrino search providing competitive bounds to neutrino production and strong constraints to a number of production models including cosmogenic neutrinos due to ultra high energy protons. We also report on two recent contributions of the Observatory to multimessenger studies by searching for correlations of neutrinos both with cosmic rays and with gravitational waves. The correlations of the directions of the highest energy astrophysical neutrinos discovered with IceCube with the highest energy cosmic rays detected with the Auger Observatory and the Telescope Array revealed an excess that is not statistically significant and is being monitored. The targeted search for neutrinos correlated with the discovery of the gravitational wave events GW150914 and GW151226 with advanced LIGO has led to the first bounds on the energy emitted by black hole mergers in Ultra-High Energy Neutrinos.
Highlights
The overall picture of the highest energy particles produced in the Universe is changing because of measurements made with the Pierre Auger Observatory
We report the results of the neutrino search providing competitive bounds to neutrino production and strong constraints to a number of production models including cosmogenic neutrinos due to ultra high energy protons
We report on two recent contributions of the Observatory to multimessenger studies by searching for correlations of neutrinos both with cosmic rays and with gravitational waves
Summary
1. The spectrum measurements made at the Observatory have revealed a supression of the flux at energies above 40 EeV [4] which cannot be unambiguosly attributed to photoproduction with the Cosmic Microwave Background (CMB), the Greisen-Zatsepin-Kuz’min cut-off, because composition measurements indicate a transition to increasing mass primaries as the energy increases above about 2 EeV [1] which is suggestive of an intrinsic maximum acceleration energy in the sources which is proportional to the nucleus charge, Z. As extraterrestrial neutrinos of several PeV have already been detected [9], extending the spectrum measurement to the EeV region is most natural to effectively constrain the production mechanisms of UHECR Their detection would probe neutrino interactions at energies orders of magnitude above accelerator experiments.
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