Analysis of a Tau Neutrino Origin for the Near-Horizon Air Shower Events Observed by the Fourth Flight of the Antarctic Impulsive Transient Antenna (ANITA)

TL;DR

This paper investigates whether four near-horizon air shower events observed by ANITA could originate from tau neutrinos, analyzing their spectral and directional properties and comparing with other neutrino limits, highlighting challenges and future prospects.

Contribution

It provides a detailed analysis of ANITA's sensitivity to tau neutrino point sources and assesses the compatibility of observed events with tau neutrino origin hypotheses.

Findings

Four upward-going events are consistent with tau neutrino-induced air showers.

The required neutrino fluence for these events conflicts with limits from Auger.

Technical challenges exist in simulating and analyzing near-horizon events.

Abstract

We study in detail the sensitivity of the Antarctic Impulsive Transient Antenna (ANITA) to possible $\nu_\tau$ point source fluxes detected via $\tau$-lepton-induced air showers. This investigation is framed around the observation of four upward-going extensive air shower events very close to the horizon seen in ANITA-IV. We find that these four upgoing events are not observationally inconsistent with $\tau$-induced EASs from Earth-skimming $\nu_\tau$, both in their spectral properties as well as in their observed locations on the sky. These four events, as well as the overall diffuse and point source exposure to Earth-skimming $\nu_\tau$, are also compared against published ultrahigh-energy neutrino limits from the Pierre Auger Observatory. While none of these four events occurred at sky locations simultaneously visible by Auger, the implied fluence necessary for ANITA to observe these events is in strong tension with limits set by Auger across a wide range of energies and is additionally in tension with ANITA's Askaryan in-ice neutrino channel above $10^{19}$ eV. We conclude by discussing some of the technical challenges with simulating and analyzing these near horizon events and the potential for future observatories to observe similar events.