Unusual Near-Horizon Cosmic-Ray-like Events Observed by ANITA-IV

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 reflection off the Antarctic ice sheets, creating characteristic phase-inverted waveform polarity. However, four below-horizon CR-like show anomalous non-inverted polarity, $p = 5.3 \times 10^{-4}$ chance if due to background. All from locations near horizon; ANITA-IV observed no steeply-upcoming similar two such prior flights.