First observation of PeV-energy neutrinos with IceCube

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.