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When ultrahigh energy tau neutrinos skim the Earth, they can generate tau leptons that then decay in the atmosphere, forming upgoing extensive air showers. The Beamforming Elevated Array for COsmic Neutrinos (BEACON) is a novel detector concept that utilizes a mountaintop radio interferometer to search for the radio emission due to these extensive air showers. The prototype, located at the White Mountain Research Station in California, consists of 4 custom crossed-dipole antennas operating in the 30-80 MHz range and uses a directional interferometric trigger to achieve reduced thresholds and background rejection. The prototype will first be used to detect extensive air showers from down-going cosmic rays to validate the detector model. In this talk, we give an overview of the BEACON concept and the status of its prototype. We also discuss the ongoing cosmic ray search which utilizes both data analysis and simulation. 

A dedicated search for upward-going air showers at zenith angles exceeding 110° and energies $E>0.1\mathrm{EeV}$has been performed using the Fluorescence Detector of the Pierre Auger Observatory. The search is motivated by two “anomalous” radio pulses observed by the ANITA flights I and III that appear inconsistent with the standard model of particle physics. Using simulations of both regular cosmic-ray showers and upward-going events, a selection procedure has been defined to separate potential upward-going candidate events and the corresponding exposure has been calculated in the energy range [0.1–33] EeV. One event has been found in the search period between January 1, 2004, and December 31, 2018, consistent with an expected background of $0.27\pm 0.12$events from misreconstructed cosmic-ray showers. This translates to an upper bound on the integral flux of $(7.2\pm 0.2)\times {10}^{-21}{\mathrm{cm}}^{-2}{\mathrm{sr}}^{-1}{\mathrm{y}}^{-1}$and $(3.6\pm 0.2)\times {10}^{-20}{\mathrm{cm}}^{-2}{\mathrm{sr}}^{-1}{\mathrm{y}}^{-1}$for an $E^{-1}$and $E^{-2}$spectrum, respectively. An upward-going flux of showers normalized to the ANITA observations is shown to predict over 34 events for an $E^{-3}$spectrum and over 8.1 events for a conservative $E^{-5}$spectrum, in strong disagreement with the interpretation of the anomalous events as upward-going showers. Published by the American Physical Society2025