An improved limit to the diffuse flux of ultra-high energy neutrinos from the Pierre Auger Observatory

TL;DR

This paper reports a search for ultra-high energy neutrinos using the Pierre Auger Observatory data from 2004 to 2013, setting new limits on their diffuse flux that challenge existing theoretical bounds.

Contribution

It introduces an improved method for detecting ultra-high energy neutrinos and provides the most stringent flux limits to date in the specified energy range.

Findings

No neutrino candidates were detected.

The 90% CL flux limit is below the Waxman-Bahcall bound.

The analysis covers both downward-going and Earth-skimming neutrinos.

Abstract

Neutrinos in the cosmic ray flux with energies near 1 EeV and above are detectable with the Surface Detector array of the Pierre Auger Observatory. We report here on searches through Auger data from 1 January 2004 until 20 June 2013. No neutrino candidates were found, yielding a limit to the diffuse flux of ultra-high energy neutrinos that challenges the Waxman-Bahcall bound predictions. Neutrino identification is attempted using the broad time-structure of the signals expected in the SD stations, and is efficiently done for neutrinos of all flavors interacting in the atmosphere at large zenith angles, as well as for "Earth-skimming" neutrino interactions in the case of tau neutrinos. In this paper the searches for downward-going neutrinos in the zenith angle bins $60^\circ-75^\circ$ and $75^\circ-90^\circ$ as well as for upward-going neutrinos, are combined to give a single limit. The $90\%$ C.L. single-flavor limit to the diffuse flux of ultra-high energy neutrinos with an $E^{-2}$ spectrum in the energy range $1.0 \times 10^{17}$ eV - $2.5 \times 10^{19}$ eV is $E_\nu^2 dN_\nu/dE_\nu < 6.4 \times 10^{-9}~ {\rm GeV~ cm^{-2}~ s^{-1}~ sr^{-1}}$.