Barber, K.Bellido Caceres, J.Clay, R.Dawson, B.Holmes, V.Sorokin, J.Wahrlich, P.Whelan, B.Winnick, M.2010-03-022010-03-022009Physical Review D: Particles, Fields, Gravitation and Cosmology, 2009; 79(10):102001-01-102001-151550-79981550-2368http://hdl.handle.net/2440/56491Data 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 ντ may interact in the Earth’s crust and produce a τ lepton by means of charged-current interactions. The τ 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 τ 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 τ neutrino candidates have been found. For neutrinos in the energy range 2×1017  eV<Eν<2×1019  eV, assuming a diffuse spectrum of the form Eν-2, data collected between 1 January 2004 and 30 April 2008 yield a 90% confidence-level upper limit of Eν2dNντ/dEν<9×10-8  GeV  cm-2 s-1  sr-1.en©2009 American Physical SocietyJournal article002011123410.1103/PhysRevD.79.1020010002665019000072-s2.0-6564908759028418Bellido Caceres, J. [0000-0002-0833-9194]Clay, R. [0000-0002-9040-9648]Dawson, B. [0000-0002-4271-3055]