A new calculation of Earth-skimming very- and ultra-high energy tau neutrinos

Cosmic neutrinos above a PeV are produced either within astrophysicalsources or when ultra-high energy cosmic raysinteract in transit through the cosmic background radiation. Detection of these neutrinos will be essential for understanding cosmic ray acceleration, composition and source evolution. By using the Earth as a tau neutrinoconverter for upward-going extensive air showersfrom tau decays, balloon-borne and space-based instruments can take advantage of a large volume and mass of the terrestrial neutrino target. The theoretical inputs and uncertainties in determining the tau lepton exit probabilities and their translation to detection acceptance will be discussed in the context of a new calculation we have performed. We quantify the experimental detection capability based on our calculation, including using the Probe of Extreme Multi-Messenger Astrophysics(POEMMA) concept study response parameters for optical air Cherenkov detection. These case studies are used to illustrate the features and uncertainties in upward tau air showerdetection.