Unusual Near-horizon Cosmic-ray-like Events Observed by ANITA-IV

TL;DR

ANITA-IV detected 29 cosmic-ray-like events, including four with unusual non-inverted polarity near the horizon, suggesting potential new physics or unknown sources beyond standard cosmic-ray reflections.

Contribution

This study reports the observation of anomalous near-horizon cosmic-ray-like events with non-inverted polarity, a novel finding not seen in previous flights.

Findings

Four anomalous events with non-inverted polarity near the horizon

Anomalous events have a low probability of being background

No similar steeply-upcoming anomalous events observed in this flight

Abstract

ANITA's fourth long-duration balloon flight in late 2016 detected 29 cosmic-ray (CR)-like events on a background of $0.37^{+0.27}_{-0.17}$ anthropogenic events. CRs are mainly seen in reflection off the Antarctic ice sheets, creating a characteristic phase-inverted waveform polarity. However, four of the below-horizon CR-like events show anomalous non-inverted polarity, a $p = 5.3 \times 10^{-4}$ chance if due to background. All anomalous events are from locations near the horizon; ANITA-IV observed no steeply-upcoming anomalous events similar to the two such events seen in prior flights.