Search for neutrino-induced cascades with 5 years of the AMANDA-II data

The Antarctic Muon And Neutrino Detector Array (AMANDA) is a Cherenkov detector deployed in the Antarctic ice cap at the South Pole. We present the analysis of the AMANDA data collected during 1001 effective days of the detector lifetime between the years 2000 and 2004. We focus our search on electromagnetic and hadronic cascades which are produced in charged-current interactions of high-energy electron or tau neutrinos and in neutral-current interactions of neutrinos of any flavor. There are several advantages associated with the cascade channel in the search for a "diffuse" flux of astrophysical neutrinos. The AMANDA’s energy resolution allows us to distinguish between a hard astrophysical spectrum and a soft atmospheric spectrum. In addition, the flux of atmospheric electron neutrinos is lower than that of atmospheric muon neutrinos by one order of magnitude, and the background from downward-going atmospheric muons can be suppressed due to their track-like topology. The low background in this channel allows us to attain a 4π acceptance above energies of about 50 TeV. The number of events observed in this analysis is consistent with the background expectations. Therefore, we calculate an upper limit on the diffuse all-flavor neutrino flux assuming a flavor ratio 1:1:1 at the detection site. A flux of neutrinos with a spectrum dΦ/dE falling as E−2 is limited to EΦ90%CL ≤ 3.96 ·10−7GeV s−1 sr−1 cm−2 at 90% C.L. for a neutrino energy range spanning from 40TeV to 9PeV. This upper limit is currently the most sensitive limit on the diffuse all-flavor astrophysical neutrino flux.