ANITA’s fourth long-duration balloon flight in 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 phase-inverted waveform polarity. However, four of the below-horizon CR-like events show anomalous noninverted 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.

• Received 19 August 2020
• Revised 20 October 2020
• Accepted 12 November 2020

DOI:https://doi.org/10.1103/PhysRevLett.126.071103