Evidence for Astrophysical Muon Neutrinos from the Northern Sky with IceCube

TL;DR

The analysis presented here, a data sample of approximately 35,000 muon neutrinos from the Northern sky is extracted from data taken during 659.5 days of live time recorded between May 2010 and May 2012, and a fit for an astrophysical flux with an arbitrary spectral index is performed.

Abstract

Results from the IceCube Neutrino Observatory have recently provided compelling evidence for the existence of a high energy astrophysical neutrino flux utilizing a dominantly Southern Hemisphere dataset consisting primarily of nu_e and nu_tau charged current and neutral current (cascade) neutrino interactions. In the analysis presented here, a data sample of approximately 35,000 muon neutrinos from the Northern sky was extracted from data taken during 659.5 days of livetime recorded between May 2010 and May 2012. While this sample is composed primarily of neutrinos produced by cosmic ray interactions in the Earth's atmosphere, the highest energy events are inconsistent with a hypothesis of solely terrestrial origin at 3.7 sigma significance. These neutrinos can, however, be explained by an astrophysical flux per neutrino flavor at a level of Phi(E_nu) = 9.9^{+3.9}_{-3.4} times 10^{-19} GeV^{-1} cm^{-2} sr^{-1} s^{-1} ({E_nu / 100 TeV})^{-2}, consistent with IceCube's Southern Hemisphere dominated result. Additionally, a fit for an astrophysical flux with an arbitrary spectral index was performed. We find a spectral index of 2.2^{+0.2}_{-0.2}, which is also in good agreement with the Southern Hemisphere result.

Figures (6)

• Figure 1: The distribution of reconstructed zenith angles of events in the final sample, compared to the expected distributions for the fit of an $E^{-2}$ astrophysical neutrino spectrum. Only statistical errors are shown, though in almost all bins they are small enough to be hidden by the data markers.
• Figure 2: The distribution of reconstructed muon energy proxy for events in the final sample, compared to the expected distributions for the fit of an $E^{-2}$ astrophysical neutrino spectrum. Only statistical errors are shown. The energy proxy does not have a linear relationship to actual muon energy, but values $\sim 3 \times 10^3$ are roughly equivalent to the same quantity in GeV. Larger proxy values increasingly tend to underestimate muon energies, while smaller values tend to overestimate.
• Figure 3: Likelihood profile of the astrophysical flux power-law index and the flux normalization at $100\, \mathrm{TeV}$ in units of $10^{-18}\, \mathrm{GeV}^{-1}\, \mathrm{cm}^{-2}\, \mathrm{sr}^{-1}\, \mathrm{s}^{-1}$. While the $E^{-2}$ result is well within the 68% contour, it is not the overall best fit. Also shown are the best fits from various IceCube analyses of starting events, which generally have good agreement: Starting Events (HE) ref:HESE2, Starting Events (LE 1) ref:JvS, Starting Events (LE 2) ref:GB.
• Figure 4: Comparison of the best fit per-flavor astrophysical flux spectrum of $E^{-2}$ from this work, assuming a flavor ratio of 1:1:1, (shown in dark green with the 68% error range in lighter green) to other selected IceCube measurements (heavy lines) ref:IC59ref:HESE2 and theoretical model predictions (thin, dashed lines) ref:Hondaref:ERSref:WB_GRBref:WB2013ref:Stecker2005ref:LoebWaxman. The sensitivity of this analysis is also shown as the thin, green line.
• Figure 4: Each column in the figure has been independently normalized to form a PDF of possible true parameter values. The best fit result with an $E^{-2.2}$ astrophysical flux has been assumed. Fluctuations and missing data at the edges of the distributions are due to limited simulation statistics.