Spectral analysis of the high-energy IceCube neutrinos

TL;DR

This paper performs a detailed spectral and flavor analysis of three-year IceCube high-energy neutrino data, revealing how energy range choices influence flavor ratios and spectral properties, and suggesting possible spectral features like a broken power-law.

Contribution

It provides a comprehensive, flavor- and energy-dependent analysis of IceCube neutrino data, highlighting the impact of energy thresholds on spectral and flavor ratio reconstructions.

Findings

Including spectral information improves fit to canonical flavor ratios.

Higher energy thresholds lead to a harder spectrum and more muon neutrinos.

Extending the cutoff to the Glashow resonance suggests tau neutrino dominance.

Abstract

A full energy and flavor-dependent analysis of the three-year high-energy IceCube neutrino events is presented. By means of multidimensional fits, we derive the current preferred values of the high-energy neutrino flavor ratios, the normalization and spectral index of the astrophysical fluxes, and the expected atmospheric background events, including a prompt component. A crucial assumption resides on the choice of the energy interval used for the analyses, which significantly biases the results. When restricting ourselves to the ~30 TeV - 3 PeV energy range, which contains all the observed IceCube events, we find that the inclusion of the spectral information improves the fit to the canonical flavor composition at Earth, (1:1:1), with respect to a single-energy bin analysis. Increasing both the minimum and the maximum deposited energies has dramatic effects on the reconstructed flavor ratios as well as on the spectral index. Imposing a higher threshold of 60 TeV yields a slightly harder spectrum by allowing a larger muon neutrino component, since above this energy most atmospheric tracklike events are effectively removed. Extending the high-energy cutoff to fully cover the Glashow resonance region leads to a softer spectrum and a preference for tau neutrino dominance, as none of the expected electron antineutrino induced showers have been observed so far. The lack of showers at energies above 2 PeV may point to a broken power-law neutrino spectrum. Future data may confirm or falsify whether or not the recently discovered high-energy neutrino fluxes and the long-standing detected cosmic rays have a common origin.