Demystifying the PeV Cascades in IceCube: Less (Energy) is More (Events)

TL;DR

This paper analyzes the origin of PeV neutrino cascade events in IceCube, suggesting astrophysical sources are plausible while conventional and cosmogenic neutrinos are unlikely, and emphasizes the importance of low-energy cascade searches.

Contribution

It provides a detailed analysis of potential neutrino sources for IceCube's PeV events and highlights the need for dedicated sub-PeV cascade searches to identify their origin.

Findings

Atmospheric and cosmogenic neutrinos are strongly disfavored as sources.

Prompt neutrinos are likely disfavored, astrophysical neutrinos remain plausible.

Low-energy cascade searches can significantly improve understanding of neutrino origins.

Abstract

The IceCube neutrino observatory has detected two cascade events with energies near 1 PeV. Without invoking new physics, we analyze the source of these neutrinos. We show that atmospheric conventional neutrinos and cosmogenic neutrinos (those produced in the propagation of ultra-high- energy cosmic rays) are strongly disfavored. For atmospheric prompt neutrinos or a diffuse background of neutrinos produced in astrophysical objects, the situation is less clear. We show that there are tensions with observed data, but that the details depend on the least-known aspects of the IceCube analysis. Very likely, prompt neutrinos are disfavored and astrophysical neutrinos are plausible. We demonstrate that the fastest way to reveal the origin of the observed PeV neutrinos is to search for neutrino cascades in the range below 1 PeV, for which dedicated analyses with high sensitivity have yet to appear, and where many more events could be found.