On the transition radiation interpretation of anomalous ANITA events

Abstract

The Antarctic Impulsive Transient Antenna (ANITA) detector has observed several radio pulses coming from the surface of the ice cap at the South Pole. These pulses were attributed to upward-going atmospheric particle showers instead of the downward-going showers induced by cosmic rays that exhibit a characteristic polarity inversion of the radio signal due to reflection in the ice. Coherent transition radiation from cosmic-ray showers developing in the atmosphere and intercepting the ice surface has been suggested as a possible and alternative explanation of these so-called “anomalous” events. To test this interpretation, we have developed an extension of ZHS, a program to calculate coherent pulses from electromagnetic showers, to deal with showers that transit a planar interface between two homogeneous and dielectric media, including transition radiation. By considering different geometries, it is found that all pulses from air showers intercepting the ice surface and detected at the height of ANITA, display the same polarity as pulses emitted by ultra-high-energy cosmic-ray showers that fully develop in the atmosphere and are reflected on the ice. We find that transition radiation is disfavored as a possible explanation of the anomalous ANITA events.