Tau depolarization at very high energies for neutrino telescopes

TL;DR

This paper investigates how tau lepton depolarization due to electromagnetic interactions in Earth affects ultra-high energy neutrino detection, finding minimal impact on neutrino energy and providing methods for simulation implementation.

Contribution

First quantification of tau depolarization effects in electromagnetic energy loss for high-energy neutrino interactions in Earth.

Findings

Tau depolarization has small effects on tau and neutrino energies.

Depolarization effects can be incorporated into existing Monte Carlo simulations.

Tau polarization impacts secondary particle energy distributions in neutrino detection.

Abstract

The neutrino interaction length scales with energy, and becomes comparable to Earth's diameter above 10's of TeV energies. Over terrestrial distances, the tau's short lifetime leads to an energetic regenerated tau neutrino flux, tau neutrino to tau to tau neutrino, within the Earth. The next generation of neutrino experiments aim to detect ultra-high energy neutrinos. Many of them rely on detecting either the regenerated tau neutrino, or a tau decay shower. Both of these signatures can be affected by the polarization of the tau through the energy distribution of the secondary particles produced from the tau's decay. While taus produced in weak interactions are nearly 100 percent polarized, it is expected that taus experience some depolarization due to electromagnetic interactions in the Earth. In this paper, for the first time we quantify the depolarization of taus in electromagnetic energy loss. We find that tau depolarization has only small effects on the final energy of tau neutrinos or taus produced by high energy tau neutrinos incident on the Earth. Tau depolarization can be directly implemented in Monte Carlo simulations such as nuPyProp and TauRunner.