Observation of an Unusual Upward-going Cosmic-ray-like Event in the Third Flight of ANITA

TL;DR

This paper reports the detection of an unusual upward-going cosmic-ray-like event by ANITA, which may be caused by tau lepton decay from tau neutrinos, challenging standard neutrino interaction models.

Contribution

First observation of an upward-going cosmic-ray-like event in ANITA's third flight, suggesting potential new physics or transient neutrino sources.

Findings

Event consistent with extensive air shower characteristics

Background estimate for the event is less than 1%

Implications for tau neutrino cross sections at EeV energies

Abstract

We report on an upward traveling, radio-detected cosmic-ray-like impulsive event with characteristics closely matching an extensive air shower. This event, observed in the third flight of the Antarctic Impulsive Transient Antenna (ANITA), a NASA-sponsored long-duration balloon payload, is consistent with a similar event reported in a previous flight. These events may be produced by the atmospheric decay of an upward-propagating $\tau$-lepton produced by a $\nu_{\tau}$ interaction, although their relatively steep arrival angles create tension with the standard model (SM) neutrino cross section. Each of the two events have $a~posteriori$ background estimates of $\lesssim 10^{-2}$ events. If these are generated by $\tau$-lepton decay, then either the charged-current $\nu_{\tau}$ cross section is suppressed at EeV energies, or the events arise at moments when the peak flux of a transient neutrino source was much larger than the typical expected cosmogenic background neutrinos.