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1. Experimental tests of sub-surface reflectors as an explanation for the ANITA anomalous events

   https://doi.org/10.1088/1475-7516/2021/04/016  

   Smith, D.; Besson, D.Z.; Deaconu, C.; Prohira, S.; Allison, P.; Batten, L.; Beatty, J.J.; Binns, W.R.; Bugaev, V.; Cao, P.; et al (April 2021, Journal of Cosmology and Astroparticle Physics)

   Abstract The balloon-borne ANITA [1] experiment is designed to detect ultra-high energy neutrinos via radio emissions produced by in-ice showers. Although initially purposed for interactions within the Antarctic ice sheet, ANITA also demonstrated the ability to self-trigger on radio emissions from ultra-high energy charged cosmic rays [2] (CR) interacting in the Earth's atmosphere. For showers produced above the Antarctic ice sheet, reflection of the down-coming radio signals at the Antarctic surface should result in a polarity inversion prior to subsequent observation at the ∼35–40 km altitude ANITA gondola. Based on data taken during the ANITA-1 and ANITA-3 flights, ANITA published two anomalous instances of upcoming cosmic-rays with measured polarity opposite the remaining sample of ∼50 UHECR signals [3, 4]. The steep observed upwards incidence angles (25–30 degrees relative to the horizontal) require non-Standard Model physics if these events are due to in-ice neutrino interactions, as the Standard Model cross-section would otherwise prohibit neutrinos from penetrating the long required chord of Earth. Shoemaker et al. [5] posit that glaciological effects may explain the steep observed anomalous events. We herein consider the scenarios offered by Shoemaker et al. and find them to be disfavored by extant ANITA and HiCal experimental data. We note that the recent report of four additional near-horizon anomalous ANITA-4 events [6], at >3σ significance, are incompatible with their model, which requires significant signal transmission into the ice. 

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2. Overview of radio experiments for UHE cosmic particles detection

   Chiche, Simon; Decoene, Valentin (January 2022, 2022 Very High Energy Phenomena in the Universe)

   Radio-detection is a mature technique that has gained large momentum over the past decades. Its physical detection principle is mainly driven by the electromagnetic part of the shower, and is therefore not too sensitive to uncertainties on hadronic interactions. Furthermore its technical detection principle allows for a 100% duty cycle, and large surface coverage thanks to the low cost of antennas. Various detection methods of UHE particles now rely on the radio signal as main observable. For instance, ground based experiments such as AERA on the Pierre Auger Observatory or LOFAR detect the radio emission from air-showers induced by high-energy particles in the atmosphere; in-ice experiment such as ARA, IceCube, or ARIANNA benefits from a detection in denser media which reduces the interaction lengths; finally, balloon experiments such as ANITA allow for very sensitive UHE neutrino detection with only a few antennas. Radio-detection is now focused on building increasingly large-scale radio experiments to enhance the detector sensitivity and address the low fluxes at UHE. In this proceeding we give an overview of the past, current and future experiments for the detection of UHE cosmic particles using the radio technique in air (AERA, Auger-Prime, GRAND), in balloon (ANITA, PUEO) or in other media (IceCube-Gen2, BEACON, RNO-G) 

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3. The Beamforming Elevated Array for COsmic Neutrinos (BEACON): A Radio Detector for Earth-Skimming Tau Neutrinos

   Zeolla, A for (June 2022, 9th International Workshop on Acoustic and Radio EeV Neutrino Detection Activities - ARENA2022)

   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. 

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4. Radio detection of ultrahigh-energy cosmic-ray air showers

   https://doi.org/10.1140/epjs/s11734-025-01503-4  

   Schröder, Frank_G (February 2025, The European Physical Journal Special Topics)

   Abstract Radio antennas have become a standard tool for the detection of cosmic-ray air showers in the energy range above$$10^{16}\,$$ ${10}^{16}$eV. The radio signal of these air showers is generated mostly due to the deflection of electrons and positrons in the geomagnetic field, and contains information about the energy and the depth of the maximum of the air showers. Unlike the traditional air-Cherenkov and air-fluorescence techniques for the electromagnetic shower component, radio detection is not restricted to clear nights, and recent experiments have demonstrated that the measurement accuracy can compete with these traditional techniques. Numerous particle detector arrays for air showers have thus been or will be complemented by radio antennas. In particular when combined with muon detectors, the complementary information provided by the radio antennas can enhance the total accuracy for the arrival direction, energy and mass of the primary cosmic rays. Digitization and computational techniques have been crucial for this recent progress, and radio detection will play an important role in next-generation experiments for ultrahigh-energy cosmic rays. Moreover, stand-alone radio experiments are under development and will search for ultrahigh-energy photons and neutrinos in addition to cosmic rays. This article provides a brief introduction to the physics of the radio emission of air showers, an overview of air-shower observatories using radio antennas, and highlights some of their recent results. 

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5. Isolating cosmic ray candidates with the BEACON prototype

   Southall, Dan (January 2022, 2022 Very High Energy Phenomena in the Universe)

   The Beamforming Elevated Array for COsmic Neutrinos (BEACON) is a concept for a neutrino telescope designed to detect radio emission from upgoing air showers induced by tau leptons that are generated by ultra-high energy tau neutrino interactions in the Earth. This detection mechanism provides a pure measurement of the tau flavor of cosmogenic and astrophysical neutrinos, which could be used to set limits on the observed flavor ratios in a manner complimentary to the all-flavor neutrino flux measurements made by other experiments. A BEACON prototype has been installed at high elevation at Barcroft Field Station for several years and consists of 4 crossed-dipole antennas operating in the 30-80 MHz band and connected to a custom DAQ. The BEACON prototype is at high elevation to maximize effective volume and uses a directional beamforming trigger to reduce man-made background signals at the trigger level. This prototype system is expected to be capable of detecting downgoing cosmic ray air showers, a signal like the upgoing tau lepton air shower, but distinguishable chiefly by arrival direction. Here we give an overview of the BEACON experiment and present an ongoing cosmic ray search with data from the BEACON prototype. Cosmic ray candidates that are identified by this search will be used to experimentally determine the sensitivity of the BEACON concept to the known cosmic ray flux, which can then be used to predict the sensitivity of a full-scale BEACON array to the cosmogenic and astrophysical neutrino fluxes. 

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