First search for extremely high energy cosmogenic neutrinos with the IceCube Neutrino Observatory

We report on the results of the search for extremely-high energy neutrinos with energies above ${10}^7\mathrm{GeV}$ obtained with the partially ($\sim 30\%$) constructed IceCube in 2007. From the absence of signal events in the sample of 242.1 days of effective live time, we derive a 90% C.L. model independent differential upper limit based on the number of signal events per energy decade at $E^2{\varphi }_{{\nu }_e+{\nu }_{\mu }+{\nu }_{\tau }}\simeq 1.4\times {10}^{-6}\mathrm{GeV}{\mathrm{cm}}^{-2}{sec}^{-1}{\mathrm{sr}}^{-1}$ for neutrinos in the energy range from $3\times {10}^7$ to $3\times {10}^9\mathrm{GeV}$.

Figure 1

Event distribution from Monte Carlo simulations of single muons with the IC-22 detector configurations in a plane of NPE and simulated true energy. The muon energy is given when the muon is 880 meters from the IceCube center (in-ice energy). The 80 DOM multiplicity cut (level-1 cut) is applied. The charged lepton energy distribution is assumed to follow $E^{-1}$ in this plot for illustrative purposes. Only particles with trajectories intersecting within 880 m from the center of IceCube array are considered in the plots. More distant events do not contribute to the data sample.

Figure 2

Event distributions for NPE, cosine of reconstructed zenith angle, and the NPE-weighted mean depth of event ($z_{\mathrm{COG}}$) for observational and the background Monte Carlo simulation data. The black dots represent observational data after the $\mathrm{NPE}>{10}^4$ cut, red for CORSIKA proton (SIBYLL), magenta for CORSIKA iron (SIBYLL). Green shaded regions represent distributions obtained with the empirical model with the size of shade expressing the uncertainty of the model. See text for the details.

Figure 3

Event distributions from the level-3 samples as functions of NPE (left) and $z_{\mathrm{COG}}$ (right). The black dots represent observational data, green boxes represent the empirical model including uncertainty. Red and magenta lines are CORSIKA samples with SIBYLL interaction model and proton and iron primaries, respectively. The left panel also includes the expected NPE distribution of events induced by the cosmogenic neutrinos [6] shown by the blue line for reference.

Figure 4

Event number distributions passing the level-2 selection cut ($\mathrm{NPE}>{10}^4$) of the experimental data (left), the background from the empirical model (middle left), the background from CORSIKA-SIBYLL with iron primaries (middle right), and the signal (right) on the NZ plane at the IceCube depth. The upper (lower) panels show the distributions in the region A (B). The GZK neutrino flux [6] determines the event intensity in the signal Monte Carlo plot, adding all three flavors of neutrinos. The series of thick lines in each panel indicates the level-3 ($\cos (\theta )<0.8$) and the final level-4 cuts.

Figure 5

The effective area of IC-22 for EHE neutrino search. The left panel shows the $4\pi$ solid angle averaged area as a function of neutrino energy at the Earth surface. The right panel shows the corresponding effective area for particles at 880 m from the IceCube center entering into the IC-22 fiducial volume. Muons and taus in this plot are secondary particles produced by neutrinos before reaching the neighborhood of the detector array. The energy here are defined as in-ice energy.

Figure 6

The all flavor neutrino flux differential limit from the IC-22 EHE analysis (filled circles). The systematic errors are included. Also the various model predictions are shown for comparison: GZK model 1 [6] (short dashed line), GZK model 2 [22] (dotted line), GZK model 3 [23] (long dashed line), $Z$-burst model [24] (dashed dot line). The model independent differential upper limits by other experiments are also shown for Auger [32] (open triangles), RICE [31] (crosses), ANITA [34] (open squares), AMANDA [35] (rhombi). Limits from other experiments are converted to the all flavor limit assuming full mixing neutrino oscillations and 90% C.L when necessary.