| Abstract: | 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 $
u_{e,mu, au}$ ($ar
u_{e,mu, au}$) or charged-current
$
u_{e}$ ($ar
u_{e}$) 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 two neutrino events are
observed over an expected atmospheric background of $0.082 pm 0.004
ext{(stat)}^{+0.041}_{-0.057} ext{(syst)}$. The resulting p-value for the
background-only hypothesis is $2.9 imes10^{-3}$ ($2.8sigma$) taking into
account the uncertainty on the expected number of background events. Though the
two events could be a first indication of an astrophysical neutrino flux, the
moderate significance and the uncertainties on the expected atmospheric
background from neutrinos produced in the decay of charmed mesons do not allow
for a firm conclusion at this point. |
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