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

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 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 depositing 2.6 ± 0.3 PeV. A muon depositing so much energy can imply E_νμ ≳ 10 PeV. Alternatively, we find a tau can deposit this much energy, requiring E_ντ ~ 10× higher. Here, we show that extending soft spectral fits from TeV-PeV data is unlikely to yield such an event, while an ~ E_v^-2 flux predicts excessive Glashow events. These instead hint at a new flux, with the hierarchy of ν_μ and ν_τ energies implying astrophysical neutrinos at E_ν ~ 100 PeV if a tau. We address implications for ultrahigh-energy cosmic-ray and neutrino origins.