The propagation of ultra-high energy tau leptons and neutrinos while skimming the Earth

TL;DR

This paper examines the detailed propagation of ultra-high energy tau neutrinos and tau leptons through Earth, assessing the validity of common simplifications and their impact on neutrino detection strategies.

Contribution

It provides a comprehensive analysis of tau neutrino and tau lepton propagation, including effects often neglected, for both standard and non-standard interaction scenarios.

Findings

Neglecting multiple regenerations can lead to inaccuracies in flux predictions.

Stochastic energy losses significantly affect tau lepton propagation.

Simplified models may underestimate detection rates in certain scenarios.

Abstract

The detection of Earth-skimming tau neutrinos has turned into a very promising strategy for the observation of UHE cosmic neutrinos. The sensitivity of this channel crucially depends on the parameters of the propagation of the tau neutrino (and the tau lepton) through the terrestrial crust, which governs the flux of emerging tau leptons that can be detected. This propagation problem is usually treated in a simplified framework where several effects are neglected, e.g. the possibility of multiple regenerations of the tau neutrino, the weak interactions of the tau lepton, as well as the stochastic nature of its energy losses. We discuss here the validity of such approximations by studying the propagation in standard rock of tau leptons and neutrinos with both mono-energetic and power-law spectra. We also investigate the impact of such simplifications in non-standard scenarios for the neutrino-nucleon interactions as well as for the tau energy losses.