Detection of Atmospheric Muon Neutrinos with the IceCube 9-String Detector

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The IceCube Neutrino Detector is a cubic kilometer ice-Cherenkov detector being constructed in the deep ice under the geographic South Pole. The full detector will consist of 4800 light-sensitive Digital Optical Modules (DOMs) arranged on 80 strings of 60 DOMs, each deployed at depths between 1400 and 2400 meters from the surface. In addition to the detector deep in the ice, there will be an array of 320 DOMs paired in tanks of frozen water at the surface named IceTop. The deep detector and the surface array are being deployed during the austral summers of 2004 through 2011. In 2006, the detector includes 9 strings of 60 DOMs each. IceCube is sensitive to high-energy muon neutrinos and muon anti-neutrinos by detecting Cherekov light from the secondary muon produced when the neutrino interacts in or near the instrumented volume. The principal background to the observation of these neutrinos is muons generated in cosmic-ray air-showers in the atmosphere above the detector. The separation of neutrino-induced muons from air-shower-induced muons proceeds by looking only for muons moving upward through the detector. This separation is possible since up-going muons could not have resulted from anything other than a neutrino interaction; muons cannot penetrate more than a few kilometers in the Earth. The principal source of neutrino-induced muons in the detector are from atmospheric neutrinos generated in cosmic-ray air-showers in the northern hemisphere. In order to establish the IceCube detector as a neutrino detector, a search for high-quality up-going muon events was conducted using the 9-string detector. The data was compared to predictions from neutrino and cosmic-ray simulations. Theoretical and experimental systematic errors have been estimated. A total of 156 neutrino-candidate events were detected in 90.0 days of livetime consistent with the prediction of 139.1 atmospheric neutrino events and a contamination of 9.5 non-neutrino background events. The ratio R between the experimental neutrino population and the prediction of simulation was measured at R = 1.05 +/- 0.24(syst) +/- 0.09(stat). This is consistent with the ~30% error expected from current neutrino flux modeling.