Multi-PeV Signals from a New Astrophysical Neutrino Flux Beyond the Glashow Resonance

Abstract

The IceCube neutrino discovery was punctuated by three showers with $E_ν$ ~ 1-2 PeV. Interest is intense in possible fluxes at higher energies, though a marked deficit of $E_ν$ ~ 6 PeV Glashow resonance events implies a spectrum that is soft and/or cutoff below ~few PeV. However, IceCube recently reported a through-going track event depositing 2.6 $\pm$ 0.3 PeV. A muon depositing so much energy can imply $E_{ν_μ} \gtrsim$ 10 PeV. We show that extending the soft $E_ν^{-2.6}$ spectral fit from TeV-PeV data is unlikely to yield such an event. Alternatively, a tau can deposit this much energy, though requiring $E_{ν_τ}$ ~10x higher. We find that either scenario hints at a new flux, with the hierarchy of $ν_μ$ and $ν_τ$ energies suggesting a window into astrophysical neutrinos at $E_ν$ ~ 100 PeV if a tau. We address implications, including for ultrahigh-energy cosmic-ray and neutrino origins.