We report on the observation of two neutrino-induced events which have an estimated deposited energy in the IceCube detector of $1.04\ifmmode\pm\else\textpm\fi{}0.16$ and $1.14\ifmmode\pm\else\textpm\fi{}0.17\text{ }\text{ }\mathrm{PeV}$, respectively, the highest neutrino energies observed so far. These events are consistent with fully contained particle showers induced by neutral-current ${\ensuremath{\nu}}_{e,\ensuremath{\mu},\ensuremath{\tau}}$ (${\overline{\ensuremath{\nu}}}_{e,\ensuremath{\mu},\ensuremath{\tau}}$) or charged-current ${\ensuremath{\nu}}_{e}$ (${\overline{\ensuremath{\nu}}}_{e}$) interactions within the IceCube detector. The events were discovered in a search for ultrahigh energy neutrinos using data corresponding to 615.9 days effective live time. The expected number of atmospheric background is $0.082\ifmmode\pm\else\textpm\fi{}0.004(\mathrm{stat}{)}_{\ensuremath{-}0.057}^{+0.041}(\mathrm{syst})$. The probability of observing two or more candidate events under the atmospheric background-only hypothesis is $2.9\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}3}$ ($2.8\ensuremath{\sigma}$) 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.
First Observation of PeV-Energy Neutrinos with IceCube | Litlas
