TauRunner: A Monte Carlo for Very-High-Energy Tau Neutrino Propagation

Very-High-Energy (VHE) neutrinos are expected to be produced by cosmic-ray interactions with the Cosmic Microwave Background (CMB). In these photo-hadronic interactions, $\nu_{\mu}$ and $\nu_e$ are produced. As these neutrinos traverse the cosmic void, they morph from one flavor to another, yielding, in the standard scenario, a democratic flavor composition at their arrival on Earth. This so-called cosmogenic flux of VHE neutrinos is a target of the next generation neutrino observatories: IceCube-Gen2, TAMBO, RNO, GRAND, POEMMA, and CHANT. In a recent publication, a novel detection strategy for these neutrinos has been put forward. This new technique relies on the observation of Earth-throughgoing $\nu_{\tau}$ at PeV energies. By measuring the flux at this energy, we can indirectly observe the flux at EeV energies since these two are related by the cascading down of the neutrinos. However, such a link demands an accurate simulation of the VHE $\nu_{\tau}$ transport. TauRunner is a Python Monte Carlo (MC) package specialized in EeV $\nu_{\tau}$ transport with the limitation of not accounting for secondary flavors produced in some $\tau$ decay channels. In this contribution, I will present the newest version of this MC, which now incorporates all neutrino flavors in the propagation