Cosmogenic Neutrinos at the Pierre Auger Observatory
Ines Valino (left) reported on the studies of ultra-high-energy neutrinos from cosmic rays. Cosmogenic neutrinos come from the interaction of ultra high-energy cosmic ray (UHECR) protons above 50 EeV with CMB photons via the GZK mechanism. They can travel in a straight line undeflected by magnetic field, so they point back directly to their originating source. They can thus reveal the sources of UHECRs at cosmological distances.
Two journal articles: PRD 9, 092008 (2015) and PRD 94, 12207 (2016) describe the latest results on the matter by the Pierre Auger Collaboration. The observatory is located in Malargue, Argentina. It is an array of 1600 water CHerenkov stations spanning a surface of 3000 km^2. The area is overlooked by 4 fluorescence telescopes. 10% of the cosmic rays are measured simultaneously by the two sets of devices.
Pierre Auger (see map, right) is not a dedicated neutrino observatory, as the main aim is directly to study UHECRs, but UHE neutrinos induce showers that can be distinguished from backgrounds. The identification concept is simple: it exploits the small cross section of neutrinos with matter. While protons, nuclei, and photons interact shortly after they enter the atmosphere, neutrinos can interact at a bigger depth, and can initiate deep showers close to the ground. The electromagnetic component is thus larger. So the neutrino-induced events are characterized by inclined showers with EM component, so-called “young showers”.
Mainly two types of neutrinos are searched in these showers: ones traveling slightly upward, when they skim the earth and interact, producing a shower close to the detector. The other ones are downward neutrinos (see figure above). Additionally, they also consider double-bang showers initiated by tau neutrinos, and down-going showers initiated by tau neutrinos as well.
The reconstruction of showers is not optimized for horizontal showers, so this needs a dedicated treatment. Observables are the elongated footprint, the apparent velocity of propagation of the shower front at the ground, along the major shower axis; and the reconstructed incident angle.
They applied identification criteria blindly to their data, and found no neutrino candidate events in any of the analyses so far. So they can put a limit to the flux. At 90% CL they exclude 2.39 or more events for the Poisson mean of the rate in the analyzed data after selection cuts.
Finally the speaker discussed the gravitational wave signals observed by Ligo. General consensus is that binary black-hole mergers do not produce an electromagnetic or neutrino counterpart; however there was a signal reported by Fermi coincident with the Ligo signal. So there was some interest to look for neutrinos matching those signals. Auger selected a 500 second window around the two events, plus a one-day window for an “afterglow” search.
As the sensitivity of the Auger observatory is limited to large zenith angles, the region of the sky to which there is sensitivity varies in time. In the case of the first merger event there was little overlap in field of view; in the second case there was good overlap. No events were seen in coincidence in either time window for the second GW signal however. They obtained constraints in the flux which are declination-dependent. These translate in a limit of less than 0.5 to 3.0 solar masses emitted as neutrinos as a function of the source declination. This is based on neutrinos of energy above 10^17 eV.