Limit on the diffuse flux of ultra-high energy tau neutrinos with the surface detector of the Pierre Auger Observatory

The Pierre Auger Collaboration

Published 2009-03-19Version 1

Data collected at the Pierre Auger Observatory are used to establish an upper limit on the diffuse flux of tau neutrinos in the cosmic radiation. Earth-skimming $\nu_{\tau}$ may interact in the Earth's crust and produce a $\tau$ lepton by means of charged-current interactions. The $\tau$ lepton may emerge from the Earth and decay in the atmosphere to produce a nearly horizontal shower with a typical signature, a persistent electromagnetic component even at very large atmospheric depths. The search procedure to select events induced by $\tau$ decays against the background of normal showers induced by cosmic rays is described. The method used to compute the exposure for a detector continuously growing with time is detailed. Systematic uncertainties in the exposure from the detector, the analysis and the involved physics are discussed. No $\tau$ neutrino candidates have been found. For neutrinos in the energy range $2\times10^{17}$ eV $< E_{\nu}$ $<$ $2\times10^{19}$ eV, assuming a diffuse spectrum of the form $E_{\nu}^{-2}$, data collected between 1 January 2004 and 30 April 2008 yield a 90% confidence-level upper limit of $E_\nu^{2} \mathrm{d}N_{\nu_\tau}/\mathrm{d}E_{\nu} < 9 \times 10^{-8}$ GeV cm$^{-2}$ s$^{-1}$ sr$^{-1}$.