Upper Limit on the Diffuse Flux of Ultrahigh Energy Tau Neutrinos from the Pierre Auger Observatory

The surface detector array of the Pierre Auger Observatory is sensitive to Earth-skimming tau neutrinos that interact in Earth's crust. Tau leptons from ν_τ charged-current interactions can emerge and decay in the atmosphere to produce a nearly horizontal shower with a significant electromagnetic component. The data collected between 1 January 2004 and 31 August 2007 are used to place an upper limit on the diffuse flux of ν_τ at EeV energies. Assuming an E_ν^-2 differential energy spectrum the limit set at 90% C.L. is ${E}_{{ν}}²d{N}_{{{ν}}_{{τ}}}/d{E}_{{ν}}<1.3×{}{10}^{{-}7} GeV {cm}^{{-}2} {s}^{{-}1} {sr}^{{-}1}$ in the energy range $2×{}{10}¹⁷ eV<{E}_{{ν}}<2×{}{10}¹⁹ eV$.