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 νe + νμ + ντ, predicted by several models of AGN-like neutrino sources, are derived. For an E−2 behavior of the neutrino spectrum, our limit is E2Φν(E) < 8.1 × 10−7 cm−2 s−1 sr−1 GeV over an neutrino energy range 2 × 104–5 × 107 GeV. The upper limit on the resonant ν¯e diffuse flux is Φν¯e<3.3×10-20cm-2s-1sr-1GeV-1.