UC Berkeley

IceCube is an all-flavor, cubic kilometer neutrino telescope currently under construction

in the deep glacial ice at the South Pole. Its embedded optical sensors detect Cherenkov

light from charged particles produced in neutrino interactions in the ice. For several years

IceCube has been detecting muon tracks from charged-current muon neutrino interactions.

However, IceCube has yet to observe the electromagnetic or hadronic particle showers or

"cascades" initiated by charged-current or neutral-current neutrino interactions. The first

detection of such an event signature is expected to come from the known flux of atmospheric

electron and muon neutrinos.

A search for atmospheric neutrino-induced cascades was performed using 275.46

days of data from IceCube's 22-string configuration. Reconstruction and background rejection

techniques were developed to reach, for the first time, a signal-to-background ratio

~1. Above a reconstructed energy of 5 TeV, 12 candidate events were observed in the full

dataset. The signal expectation from the canonical Bartol atmospheric neutrino flux model

is 5.63 +- 2.25 events, while the expectation from the atmospheric neutrino flux as measured

by IceCube's predecessor array AMANDA is 7.48 +- 1.50 events. Quoted errors include the

uncertainty on the flux only.

While a conclusive detection can not yet be claimed because of a lack of background

Monte Carlo statistics, the evidence that we are at the level of background suppression

needed to see atmospheric neutrino-induced cascades is strong. In addition, one extremely

interesting candidate event of energy 133 TeV survives all cuts and shows an intriguing

double pulse structure in its waveforms that may signal the "double bang" of a tau neutrino

interaction.