Limit on the diffuse flux of ultrahigh energy tau neutrinos with the surface detector of the Pierre Auger Observatory

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Abstract

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 ${\ensuremath{\nu}}_{\ensuremath{\tau}}$ may interact in the Earth's crust and produce a $\ensuremath{\tau}$ lepton by means of charged-current interactions. The $\ensuremath{\tau}$ 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 $\ensuremath{\tau}$ 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 $\ensuremath{\tau}$ neutrino candidates have been found. For neutrinos in the energy range $2\ifmmode\times\else\texttimes\fi{}{10}^{17}\text{ }\text{ }\mathrm{eV}<{E}_{\ensuremath{\nu}}<2\ifmmode\times\else\texttimes\fi{}{10}^{19}\text{ }\text{ }\mathrm{eV}$, assuming a diffuse spectrum of the form ${E}_{\ensuremath{\nu}}^{\ensuremath{-}2}$, data collected between 1 January 2004 and 30 April 2008 yield a 90% confidence-level upper limit of ${E}_{\ensuremath{\nu}}^{2}d{N}_{{\ensuremath{\nu}}_{\ensuremath{\tau}}}/d{E}_{\ensuremath{\nu}}<9\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}8}\text{ }\text{ }\mathrm{GeV}\text{ }\text{ }{\mathrm{cm}}^{\ensuremath{-}2}\text{ }{\mathrm{s}}^{\ensuremath{-}1}\text{ }\text{ }{\mathrm{sr}}^{\ensuremath{-}1}$.

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