Evidence for Astrophysical Muon Neutrinos from the Northern Sky with IceCube

TL;DR

IceCube's analysis of 35,000 Northern sky muon neutrinos provides evidence for an astrophysical neutrino flux, with high-energy events inconsistent with purely terrestrial origins, supporting the existence of cosmic neutrino sources.

Contribution

This study presents the first detailed analysis of Northern sky muon neutrinos, confirming an astrophysical flux consistent with previous Southern Hemisphere findings.

Findings

High-energy neutrinos are inconsistent with terrestrial origin at 3.7 sigma

Detected neutrino flux aligns with a power-law spectrum with index ~2.2

Results support the existence of astrophysical neutrino sources in the Northern sky

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.