How many of the observed neutrino events can be described by cosmic ray interactions in the Milky Way?

We report on results of an all-sky search for high-energy neutrino events interacting within the IceCube neutrino detector conducted between May 2010 and May 2012. The search follows up on the previous detection of two PeV neutrino events, with improved sensitivity and extended energy coverage down to approximately 30 TeV. Twenty-six additional events were observed, substantially more than expected from atmospheric backgrounds. Combined, both searches reject a purely atmospheric origin for the twenty-eight events at the \(4\sigma\) level. These twenty-eight events, which include the highest energy neutrinos ever observed, have flavors, directions, and energies inconsistent with those expected from the atmospheric muon and neutrino backgrounds. These properties are, however, consistent with generic predictions for an additional component of extraterrestrial origin.

We report on the observation of two neutrino-induced events which have an estimated deposited energy in the IceCube detector of 1.04 \(\pm\) 0.16 and 1.14 \(\pm\) 0.17 PeV, respectively, the highest neutrino energies observed so far. These events are consistent with fully contained particle showers induced by neutral-current \(\nu_{e,\mu,\tau}\) (\(\bar\nu_{e,\mu,\tau}\)) or charged-current \(\nu_{e}\) (\(\bar\nu_{e}\)) interactions within the IceCube detector. The events were discovered in a search for ultra-high energy neutrinos using data corresponding to 615.9 days effective livetime. The expected number of atmospheric background is \(0.082 \pm 0.004 \text{(stat)}^{+0.041}_{-0.057} \text{(syst)}\). The probability to observe two or more candidate events under the atmospheric background-only hypothesis is \(2.9\times10^{-3}\) (\(2.8\sigma\)) taking into account the uncertainty on the expected number of background events. These two events could be a first indication of an astrophysical neutrino flux, the moderate significance, however, does not permit a definitive conclusion at this time.