The surface detector array of the Pierre Auger Observatory is sensitive to Earth-skimming tau-neutrinos \nu _ { \tau } that interact in the Earth ’ s crust .
Tau leptons from \nu _ { \tau } 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 \nu _ { \tau } at EeV energies .
Assuming an E _ { \nu } ^ { -2 } differential energy spectrum the limit set at 90 \% C.L .
is E _ { \nu } ^ { 2 } ~ { } \mathrm { d } N _ { \nu _ { \tau } } / \mathrm { d } E _ { \nu } < 1.3 \times 10 ^ { -7 } GeV cm ^ { -2 } s ^ { -1 } sr ^ { -1 } in the energy range 2 \times 10 ^ { 17 } \mathrm { eV } < E _ { \nu } < 2 \times 10 ^ { 19 } eV .