Search for a diffuse flux of high-energy extraterrestrial neutrinos with the NT200 neutrino telescope

We present the results of a search for high-energy extraterrestrial neutrinos with the Baikal underwater Cherenkov detector NT200, based on data taken in 1998–2003. Upper limits on the diffuse fluxes of ν e + ν μ + ν τ , predicted by several models of AGN-like neutrino sources, are derived. For an E −2 behavior of the neutrino spectrum, our limit is E 2 Φ ν ( E) < 8.1 × 10 −7 cm −2 s −1 sr −1 GeV over an neutrino energy range 2 × 10 4–5 × 10 7 GeV. The upper limit on the resonant ν ¯ e diffuse flux is Φ ν ¯ e < 3.3 × 10 - 20 cm - 2 s - 1 sr - 1 GeV - 1 .