In-ice Askaryan Emission from Air Showers: Implications for Radio Neutrino Detectors

TL;DR

This paper investigates radio emissions from air showers in ice caused by cosmic rays, compares them to neutrino signals, and assesses their impact as backgrounds for ultra-high energy neutrino detectors, proposing mitigation strategies.

Contribution

It introduces novel calculations of air-shower radio emissions using CORSIKA 8 and evaluates their implications as backgrounds in radio neutrino detection, including reflection effects.

Findings

Air-shower radio signals are similar to neutrino-induced signals.

Detection rates of 10-100 per year for deep antennas are feasible.

Reflection layers in ice significantly affect background rates and require mitigation.

Abstract

One of the most promising techniques for detecting ultra-high energy neutrinos involves the use of radio antennas to observe the 10-1000 MHz radiation generated by the showers that neutrinos induce in large volumes of ice. The expected neutrino detection rates of one neutrino or less per detector station per 10 years make the characterization of backgrounds a priority. The largest natural background comes from ultra-high energy cosmic rays which are orders of magnitude more abundant than neutrinos. Particularly crucial is the understanding of geometries in which substantial energy of the cosmic-ray-induced air shower is deposited in the ice giving rise to a compact in-ice shower close to the ice surface. We calculated the radio emission of air-shower cores using the novel CORSIKA 8 code and found it to be similar to the predictions for neutrino-induced showers. For the first time, we calculated the detection rates for O(100m) deep antennas yielding 10-100 detections per year and detector station, which makes this a useful calibration source as these downward-going signals can be differentiated from neutrino-induced showers based on the signal arrival direction. However, the presence of reflection layers in the ice confuses the arrival directions, which makes this a potentially important background. We review the existing information on reflecting layers in the South Pole glacier and, for the first time, quantify the corresponding rate of reflected air-shower signals for the proposed IceCube-Gen2 radio array and discuss mitigation strategies. The reflectivity of the layers is the dominant uncertainty resulting in rate predictions of much less than one detection to several detections per year for IceCube-Gen2 if not mitigated.