First search for atmospheric and extraterrestrial neutrino-induced cascades with the IceCube detector

TL;DR

This paper reports the first search for atmospheric and diffuse astrophysical neutrino-induced cascades using IceCube data from 2007-2008, setting upper limits on neutrino fluxes and observing events consistent with background expectations.

Contribution

It presents the first analysis of cascade events in IceCube data, establishing upper limits on diffuse neutrino fluxes and demonstrating the detector's capability to identify neutrino-induced showers.

Findings

14 events above 16 TeV with expected background of 8.3±3.6

Upper limit on diffuse neutrino flux: E^2Φ<3.6×10^{-7} GeV·cm^{-2}·s^{-1}·sr^{-1}

12 events above 5 TeV consistent with background

Abstract

We report on the first search for atmospheric and for diffuse astrophysical neutrino-induced showers (cascades) in the IceCube detector using 257 days of data collected in the year 2007-2008 with 22 strings active. A total of 14 events with energies above 16 TeV remained after event selections in the diffuse analysis, with an expected total background contribution of $8.3\pm 3.6$. At 90% confidence we set an upper limit of $E^2\Phi_{90%CL}<3.6\times10^{-7} GeV \cdot cm^{-2} \cdot s^{-1}\cdot sr^{-1} $ on the diffuse flux of neutrinos of all flavors in the energy range between 24 TeV and 6.6 PeV assuming that $\Phi \propto E^{-2}$ and that the flavor composition of the $\nu_e : \nu_\mu : \nu_\tau$ flux is $1 : 1 : 1$ at the Earth. The atmospheric neutrino analysis was optimized for lower energies. A total of 12 events were observed with energies above 5 TeV. The observed number of events is consistent with the expected background, within the uncertainties.