Observation of an Unusual Upward-going Cosmic-ray-like Event in the Third Flight of ANITA

Understand

We report on an upward traveling, radio-detected cosmic-ray-like impulsive event with characteristics closely matching an extensive air shower.

• This event, observed in the third flight of the Antarctic Impulsive Transient Antenna (ANITA), a NASA-sponsored long-duration balloon payload, is consistent with a similar event reported in a previous flight.
• These events may be produced by the atmospheric decay of an upward-propagating $\tau$-lepton produced by a $\nu_{\tau}$ interaction, although their relatively steep arrival angles create tension with the standard model (SM) neutrino cross section.
• Each of the two events have $a~posteriori$ background estimates of $\lesssim 10^{-2}$ events.