Characteristics of Four Upward-pointing Cosmic-ray-like Events Observed with ANITA

TL;DR

This paper reports four unusual cosmic-ray-like radio events detected by ANITA, including three nearly horizontal events and one upward-pointing event potentially caused by tau-lepton decay, challenging standard neutrino interaction models.

Contribution

It provides detailed analysis of four rare cosmic-ray-like events, including the first observation of an upward-pointing event possibly linked to tau-lepton decay, suggesting new physics implications.

Findings

Three nearly horizontal cosmic-ray events detected at high altitude.

One upward-pointing event consistent with tau-lepton decay origin.

Potential need for new physics to explain suppressed neutrino interactions.

Abstract

We report on four radio-detected cosmic-ray (CR) or CR-like events observed with the Antarctic Impulsive Transient Antenna (ANITA), a NASA-sponsored long-duration balloon payload. Two of the four were previously identified as stratospheric CR air showers during the ANITA-I flight. A third stratospheric CR was detected during the ANITA-II flight. Here we report on characteristics these three unusual CR events, which develop nearly horizontally, 20-30~km above the surface of the Earth. In addition, we report on a fourth steeply upward-pointing ANITA-I CR-like radio event which has characteristics consistent with a primary that emerged from the surface of the ice. This suggests a possible $\tau$-lepton decay as the origin of this event, but such an interpretation would require significant suppression of the Standard Model ${\tau}$-neutrino cross section.