Ultrahigh energy neutrinos are interesting messenger particles since, if detected, they can transmit exclusive information about ultrahigh energy processes in the Universe. These particles, with energies above 10 16 eV, interact very rarely. Therefore, detectors that instrument several gigatons of matter are needed to discover them. The ARA detector is currently being constructed at the South Pole. It is designed to use the Askaryan effect, the emission of radio waves from neutrino-induced cascades in the South Pole ice, to detect neutrino interactions at very high energies. With antennas distributed among 37 widely separated stations in the ice, such interactions can be observed in a volume of several hundred cubic kilometers. Currently three deep ARA stations are deployed in the ice, of which two have been taking data since the beginning of 2013. In this article, the ARA detector "as built" and calibrations are described. Data reduction methods used to distinguish the rare radio signals from overwhelming backgrounds of thermal and anthropogenic origin are presented. Using data from only two stations over a short exposure time of 10 months, a neutrino flux limit of 1.5 × 10 −6 GeV=cm 2 =s=sr is calculated for a particle energy of 10 18 eV, which offers promise for the full ARA detector.

Ultra-high energy neutrinos are detectable through impulsive radio signals generated through interactions in dense media, such as ice. Subsurface in-ice radio arrays are a promising way to advance the observation and measurement of astrophysical high-energy neutrinos with energies above those discovered by the IceCube detector (≥ 1 PeV) as well as cosmogenic neutrinos created in the GZK process (≥ 100 PeV). Here we describe the NuPhase detector, which is a compact receiving array of low-gain antennas deployed 185 m deep in glacial ice near the South Pole. Signals from the antennas are digitized and coherently summed into multiple beams to form a low-threshold interferometric phased array trigger for radio impulses. The NuPhase detector was installed at an Askaryan Radio Array (ARA) station during the 2017/18 Austral summer season. In situ measurements with an impulsive, point-source calibration instrument show a 50% trigger efficiency on impulses with voltage signal-to-noise ratios (SNR) of ≤2.0, a factor of ∼1.8 improvement in SNR over the standard ARA combinatoric trigger. Hardware-level simulations, validated with in situ measurements, predict a trigger threshold of an SNR as low as 1.6 for neutrino interactions that are in the far field of the array. With the already-achieved NuPhase trigger performance included in ARASim, a detector simulation for the ARA experiment, we find the trigger-level effective detector volume is increased by a factor of 1.8 at neutrino energies between 10 and 100 PeV compared to the currently used ARA combinatoric trigger. We also discuss an achievable near term path toward lowering the trigger threshold further to an SNR of 1.0, which would increase the effective single-station volume by more than a factor of 3 in the same range of neutrino energies. * Corresponding Author

The Askaryan Radio Array (ARA) is an ultrahigh energy (UHE, > 10 17 eV) neutrino detector designed to observe neutrinos by searching for the radio waves emitted by the relativistic products of neutrinonucleon interactions in Antarctic ice. In this paper, we present constraints on the diffuse flux of ultrahigh energy neutrinos between 10 16 and 10 21 eV resulting from a search for neutrinos in two complementary analyses, both analyzing four years of data (2013-2016) from the two deep stations (A2, A3) operating at that time. We place a 90% CL upper limit on the diffuse all flavor neutrino flux at 10 18 eV of EFðEÞ ¼ 5.6 × 10 −16 cm −2 s −1 sr −1. This analysis includes four times the exposure of the previous ARA result and represents approximately 1=5th the exposure expected from operating ARA until the end of 2022.

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We present a new radio array experiment on the top of the highest mountains in Antarctica, to detect upward moving air-showers similar to those recently reported by the ANITA experiment. The detection concept follows ANITA, but has significantly greater livetime and extendibility. We propose to install 10 stations over the next 5 years to obtain sufficient sensitivity to test ANITA's `mystery events'. As the first step, we installed a prototype station (TAROGE-M) consisting of 5 LPDA antennas, atop Mt. Melbourne, Antarctica at an elevation of 2730 m, in Feb-Mar 2019. We present the scientific potential, detector design and construction, as well as the initial performance of the prototype station.