Probing the PeV Region in the Astrophysical Neutrino Spectrum using $\nu_\mu$ from the Southern Sky

TL;DR

This paper investigates the PeV energy range of astrophysical neutrinos using IceCube data, employing novel background rejection techniques, and finds no significant spectral cutoff or source correlation in the analyzed data.

Contribution

It introduces combined methods for atmospheric muon rejection and provides the first constraints on the neutrino spectrum between 1 and 10 PeV using 9 years of IceCube data.

Findings

Detected two neutrino candidates in the 1-10 PeV range, consistent with background expectations.

No significant spectral cutoff or source correlation was observed.

The SPL+cutoff model was not statistically favored over the single power-law.

Abstract

IceCube has observed a diffuse astrophysical neutrino flux over the energy region from a few TeV to a few PeV. At PeV energies, the spectral shape is not yet well measured due to the low statistics of the data. This analysis probes the gap between 1 PeV and 10 PeV by using high-energy downgoing muon neutrinos. To reject the large atmospheric muon background, two complementary techniques are combined. The first technique selects events with high stochasticity to reject atmospheric muon bundles whose stochastic energy losses are smoothed due to high muon multiplicity. The second technique vetoes atmospheric muons with the IceTop surface array. Using 9 years of data, we found two neutrino candidate events in the signal region, consistent with expectation from background, each with relatively high signal probabilities. A joint maximum likelihood estimation is performed using this sample and an independent 9.5-year sample of tracks to measure the neutrino spectrum. A likelihood ratio test is done to compare the single power-law (SPL) vs. SPL+cutoff hypothesis; the SPL+cutoff model is not significantly better than the SPL. High-energy astrophysical objects from four source catalogs are also checked around the direction of the two events. No significant coincidence was found.