Unusual Near-Horizon Cosmic-Ray-like Events Observed by ANITA-IV
P. W. Gorham, Andrew Ludwig, Cosmin Deaconu, Pinlu Cao, P. Allison, O. Banerjee, L. Batten, D. Bhattacharya, J. J. Beatty, K. Belov, W. R. Binns, V. Bugaev, C. H. Chen, Pisin Chen, Y. Chen, J. M. Clem, L. Cremonesi, B. Dailey, P. F. Dowkontt, B. D. Fox, J. Gordon, C. Hast, B. Hill, Shih‐Ying Hsu, J.J. Huang, K. Hughes, R. Hupe, M. H. Israel, T. C. Liu, L. Macchiarulo, S. Matsuno, K. McBride, C. Miki, J. W. Nam, C. J. Naudet, R. J. Nichol, Alexander Novikov, Eric Oberla, Manuel Olmedo, R. Prechelt, B. F. Rauch, Jason L. Roberts, A. Romero‐Wolf, Björn Rotter, John W. Russell, D. Saltzberg, D. Seckel, Harm Schoorlemmer, Jen‐Chieh Shiao, S. Stafford, J. Stockham, M. Stockham, B. Strutt, M. S. Sutherland, G. S. Varner, A. G. Vieregg, S. H. Wang, Stephanie Wissel
Abstract
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=5.3×10^{-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.