Observation and Characterization of a Cosmic Muon Neutrino Flux from the Northern Hemisphere using six years of IceCube data

TL;DR

High-energy cosmic neutrinos probe astrophysical accelerators, and IceCube's prior all-flavor detection motivates complementary muon-neutrino analyses. This work leverages six years of data with external-interaction muons to boost the effective area in the Northern Hemisphere, using a two-dimensional likelihood in energy and zenith to isolate an astrophysical component. The results reveal a significant isotropic astrophysical flux with a hard spectrum, characterized by a power-law index of 2.13 and a normalization around 0.9×10^-18 GeV^-1 cm^-2 s^-1 sr^-1 at 100 TeV, with the highest-energy event at ~4.5 PeV and little atmospheric likelihood. No correlation with gamma-ray sources is found, and the analysis sets the strongest limits to date on prompt atmospheric muon neutrinos from charm decays.

Abstract

The IceCube Collaboration has previously discovered a high-energy astrophysical neutrino flux using neutrino events with interaction vertices contained within the instrumented volume of the IceCube detector. We present a complementary measurement using charged current muon neutrino events where the interaction vertex can be outside this volume. As a consequence of the large muon range the effective area is significantly larger but the field of view is restricted to the Northern Hemisphere. IceCube data from 2009 through 2015 have been analyzed using a likelihood approach based on the reconstructed muon energy and zenith angle. At the highest neutrino energies between 191 TeV and 8.3 PeV a significant astrophysical contribution is observed, excluding a purely atmospheric origin of these events at $5.6\,σ$ significance. The data are well described by an isotropic, unbroken power law flux with a normalization at 100 TeV neutrino energy of $\left(0.90^{+0.30}_{-0.27}\right)\times10^{-18}\,\mathrm{GeV^{-1}\,cm^{-2}\,s^{-1}\,sr^{-1}}$ and a hard spectral index of $γ=2.13\pm0.13$. The observed spectrum is harder in comparison to previous IceCube analyses with lower energy thresholds which may indicate a break in the astrophysical neutrino spectrum of unknown origin. The highest energy event observed has a reconstructed muon energy of $(4.5\pm1.2)\,\mathrm{PeV}$ which implies a probability of less than 0.005% for this event to be of atmospheric origin. Analyzing the arrival directions of all events with reconstructed muon energies above 200 TeV no correlation with known $γ$-ray sources was found. Using the high statistics of atmospheric neutrinos we report the currently best constraints on a prompt atmospheric muon neutrino flux originating from charmed meson decays which is below $1.06$ in units of the flux normalization of the model in Enberg et al. (2008).

Figures (2)

• Figure 11: Distribution of the expected neutrino energy (left) and zenith angle (right) for the data selection of this analysis. Shown are the distributions of conventional atmospheric neutrinos Conv:Honda2007, prompt atmospheric neutrinos Prompt:ERS where both are corrected for the cosmic-ray spectrum in Gaisser:2012zz and a benchmark astrophysical signal $10^{-18}\,\mathrm{GeV^{-1}\,cm^{-2}\,sr^{-1}\,s^{-1}} (E_\nu/100\,\mathrm{TeV})^{-2}$.
• Figure 22: Exposure. Top: Individual contributions to the total exposure from the different detector configurations. Bottom: Total exposure for different zenith regions for the combined dataset. Note that the exposure is based on the sum of the effective areas of $\nu_\mu$ and $\bar{\nu}_\mu$. Therefore, the total number of events is obtained by integrating the product of exposure and averaged neutrino flux $\phi_{\nu_\mu+\bar{\nu}_\mu} / 2$ over the neutrino energy and solid angle.