Search for neutral primary cosmic-rays at the Pierre Auger Observatory

The large majority of high energy cosmic rays reaching Earth and producing air showers are charged nuclei which are difficult to use to identify the source of high energy particles in the Universe. Indeed the galactic and extra-galactic magnetic fields bend their trajectory due to their charge. Ideally the detection of neutral particles will resolve this problem. Both photons and neutrinos are expected to be produced by the same source of cosmic rays but high energy photons have a very limited mean free path due to a high cross-section with background radiation but can be produced by exotic particle decay. Neutrinos are very difficult to detect on Earth due to the very low interaction cross-section. Nevertheless, it is possible to use the largest cosmic ray experiment, namely the Pierre Auger Observatory, to try to detect these type of particles. It is the most sensitive detector to primary photons with energies above ~0.2 EeV. It measures extensive air showers using a hybrid technique that combines a fluorescence detector (FD) with a surface detectors (SD) array composed of 1660 water-Cherenkov detectors. The signatures of a photon-induced air shower are a larger atmospheric depth at the shower maximum (Xmax) and a steeper lateral distribution function, along with a lower number of muons with respect to the bulk of hadron-induced background. ... mehrUsing observables measured by the FD and SD, three photon searches in different energy bands are performed giving the most stringent limit on the photon flux at high energy. The SD of the Pierre Auger Observatory is sensitive to neutrinos at energies in the 100 PeV to 100 EeV range. This sensitivity, together with its large acceptance, makes it a complementary detector to other neutrino telescopes, which have their peak sensitivities at lower energies. The neutrino-induced air showers that the SD of the Pierre Auger Observatory is sensitive to can be divided into those induced by interactions of neutrinos of any flavor deep in the atmosphere, and those induced by charged-current interactions of tau neutrinos in the Earth crust. Both of these types can be efficiently distinguished from cosmic ray-induced air showers, provided that their zenith angles are larger than 60Ḟ. As no neutrino candidates were found in the performed searches, we present limits on the diffuse all-flavor neutrino flux.