Tau Appearance from High-Energy Neutrino Interactions

High-energy muon- and electron-neutrinos yield a non-negligible flux of tau neutrinos as they propagate through Earth. In this letter, we address the impact of this additional component in the PeV and EeV energy regimes for the first time. This contribution is predicted to be significantly larger than the atmospheric background above 300 TeV, and alters current and future neutrino telescopes' capabilities to discover a cosmic tau-neutrino flux. Further we demonstrate that Earthskimming neutrino experiments, designed to observe tau neutrinos, will be sensitive to cosmogenic neutrinos even in extreme scenarios without a primary tau-neutrino component.