Evidence for High-Energy Extraterrestrial Neutrinos at the IceCube Detector

Observations suggest that \(\gamma\)-ray bursts (GRBs) are produced by the dissipation of the kinetic energy of a relativistic fireball. We show that a large fraction, \(\ge 10%\), of the fireball energy is expected to be converted by photo-meson production to a burst of \(\sim10^{14} eV\) neutrinos. A km^2 neutrino detector would observe at least several tens of events per year correlated with GRBs, and test for neutrino properties (e.g. flavor oscillations, for which upward moving \(\tau\)'s would be a unique signature, and coupling to gravity) with an accuracy many orders of magnitude better than is currently possible.

We calculate the prompt neutrino flux from atmospheric charm production by cosmic rays, using the dipole picture in a perturbative QCD framework, which incorporates the parton saturation effects present at high energies. We compare our results with the next-to-leading order perturbative QCD result and find that saturation effects are large for neutrino energies above 10^6 GeV, leading to a substantial suppression of the prompt neutrino flux. We comment on the range of prompt neutrino fluxes due to theoretical uncertainties.

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.