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

• Main Authors: ANITA Collaboration, Gorham, P. W., Ludwig, A., Deaconu, C., Cao, P., Allison, P., Banerjee, O., Batten, L., Bhattacharya, D., Beatty, J. J., Belov, K., Binns, W. R., Bugaev, V., Chen, C. H., Chen, P., Chen, Y., Clem, J. M., Cremonesi, L., Dailey, B., Dowkontt, P. F., Fox, B. D., Gordon, J. W. H., Hast, C., Hill, B., Hsu, S. Y., Huang, J. J., Hughes, K., Hupe, R., Israel, M. H., Liu, T. C., Macchiarulo, L., Matsuno, S., McBride, K., Miki, C., Nam, J., Naudet, C. J., Nichol, R. J., Novikov, A., Oberla, E., Olmedo, M., Prechelt, R., Prohira, S., Rauch, B. F., Roberts, J. M., Romero-Wolf, A., Rotter, B., Russell, J. W., Saltzberg, D., Seckel, D., Schoorlemmer, H., Shiao, J., Stafford, S., Stockham, J., Stockham, M., Strutt, B., Sutherland, M. S., Varner, G. S., Vieregg, A. G., Wang, S. H., Wissel, S. A.
• Format: Article in Journal/Newspaper
• Language: unknown
• Published: arXiv 2020
• Subjects: High Energy Astrophysical Phenomena astro-ph.HE; FOS Physical sciences; Antarctic; The Antarctic; Antarc*
• Online Access: https://dx.doi.org/10.48550/arxiv.2008.05690; https://arxiv.org/abs/2008.05690

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