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

Abstract

ANITA's fourth long-duration balloon flight in late 2016 detected 29 cosmic-ray (CR)-like events on a background of $0.37_{-0.17}^{+0.27}$ 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.

6 pages, 4 figures, to appear in Physical Review Letters. Supplemental material (reference 17) available from corresponding author