Study of an Isolated Double-pulse Cosmic Ray Candidate Recorded with the Askaryan Radio Array

Abstract: The radio-frequency emissions produced by particle showers on Earth, resulting from cosmic rays (CRs) and ultra-high energy neutrinos (UHE-$\nu$) originating from astrophysical sources share significant similarities, enabling radio detectors initially designed for UHE-$\nu$ searches to also study CRs. The Askaryan Radio Array (ARA), an experiment currently operating within the ice at the South Pole, is primarily designed to detect UHE-$\nu$s. To date, ARA has deployed five stations, with each station equipped with antennas installed at depths up to 200 meters in the ice. Data recorded by ARA Station-2 (A2) suggest a potential CR origin for a subset of events identified in a UHE-$\nu$ search. This subset includes a double-pulse event potentially from a downward propagating CR-induced air shower, with in-air geomagnetic emission followed by in-ice Askaryan emission producing the two pulses. A detailed investigation of this CR candidate event using comprehensive simulations has been conducted with the goal of identifying the parameters of a CR-induced air shower that best match the experimentally observed quantities. We simulate predicted CR signals in ARA by combining an impacting CR shower simulation framework (FAERIE) with a realistic detector simulation (AraSim). We determine the event topology based on the vertex reconstruction of both the putative geomagnetic and Askaryan signals. After inferring the event geometry, we show that the simulation matches the observed time structure of the event (channel-by-channel relative signal arrival times) for the recorded event.