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.
