We report on four radio-detected cosmic-ray (CR) or CR-like events observed with the Antarctic Impulsive Transient Antenna (ANITA), a NASA-sponsored long-duration balloon payload. Two of the four were previously identified as stratospheric CR air showers during the ANITA-I flight. A third stratospheric CR was detected during the ANITA-II flight. Here, we report on characteristics of these three unusual CR events, which develop nearly horizontally, 20-30 km above the surface of Earth. In addition, we report on a fourth steeply upward-pointing ANITA-I CR-like radio event which has characteristics consistent with a primary that emerged from the surface of the ice. This suggests a possible τ-lepton decay as the origin of this event, but such an interpretation would require significant suppression of the standard model τ-neutrino cross section.

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Recently, the ANITA collaboration reported on two upward-going extensive air shower events consistent with a primary particle that emerges from the surface of the Antarctic ice sheet. These events may be of ντ origin, in which the neutrino interacts within the Earth to produce a τ lepton that emerges from the Earth, decays in the atmosphere, and initiates an extensive air shower. In this paper we estimate an upper bound on the ANITA acceptance to a diffuse ντ flux detected via τ -lepton-induced air showers within the bounds of Standard Model uncertainties. By comparing this estimate with the acceptance of Pierre Auger Observatory and IceCube and assuming Standard Model interactions, we conclude that a ντ origin of these events would imply a neutrino flux at least two orders of magnitude above current bounds.

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 IceCube Neutrino Observatory at the South Pole has measured the diffuse astrophysical neutrino flux up to ∼PeV energies and is starting to identify first point source candidates. The next generation facility, IceCube-Gen2, aims at extending the accessible energy range to EeV in order to measure the continuation of the astrophysical spectrum, to identify neutrino sources, and to search for a cosmogenic neutrino flux. As part of IceCube-Gen2, a radio array is foreseen that is sensitive to detect Askaryan emission of neutrinos beyond ∼30 PeV. Surface and deep antenna stations have different benefits in terms of effective area, resolution, and the capability to reject backgrounds from cosmic-ray air showers and may be combined to reach the best sensitivity. The optimal detector configuration is still to be identified. This contribution presents the full-array simulation efforts for a combination of deep and surface antennas, and compares different design options with respect to their sensitivity to fulfill the science goals of IceCube-Gen2.

RNO is the mid-scale discovery instrument designed to make the first observation of neutrinos from the cosmos at extreme energies, with sensitivity well beyond current instrument capabilities. This new observatory will be the largest ground-based neutrino telescope to date, enabling the measurement of neutrinos above 10 16 eV, determining the nature of the astrophysical neutrino flux that has been measured by IceCube at higher energies, similarly extending the reach of multi-messenger astrophysics to the highest energies, and enabling investigations of fundamental physics at energies unreachable by particle accelerators on Earth.

The balloon-borne HiCal radio-frequency (RF) transmitter, in concert with the ANITA radio-frequency receiver array, is designed to measure the Antarctic surface reflectivity in the RF wavelength regime. The amplitude of surface-reflected transmissions from HiCal, registered as triggered events by ANITA, can be compared with the direct transmissions preceding them by Oð10Þ microseconds, to infer the surface power reflection coefficient R. The first HiCal mission (HiCal-1, Jan. 2015) yielded a sample of 100 such pairs, resulting in estimates of R at highly glancing angles (i.e., zenith angles approaching 90°), with measured reflectivity for those events which exceeded extant calculations [P. W. Gorham et al., Journal of Astronomical Instrumentation, 1740002 (2017)]. The HiCal-2 experiment, flying from December 2016-January 2017, provided an improvement by nearly 2 orders of magnitude in our event statistics, allowing a considerably more precise mapping of the reflectivity over a wider range of incidence angles. We find general agreement between the HiCal-2 reflectivity results and those obtained with the earlier HiCal-1 mission, as well as estimates from Solar reflections in the radio-frequency regime [D. Z. Besson et al., Radio Sci. 50, 1 (2015)]. In parallel, our calculations of expected reflectivity have matured; herein, we use a plane-wave expansion to estimate the reflectivity R from both a flat, smooth surface (and, in so doing, recover the Fresnel reflectivity equations) and also a curved surface. Multiplying our flat-smooth reflectivity by improved Earth curvature and surface roughness corrections now provides significantly better agreement between theory and the HiCal-2 measurements. DOI: 10.1103/PhysRevD.98.042004 PHYSICAL REVIEW D 98, 042004 (2018) 2470-0010=2018=98(4)=042004 (19) 042004-1 © 2018 American Physical Society I. OVERVIEWThe NASA-sponsored ANITA project [1][2][3][4] has the goal of detecting the highest-energy particles incident on the Earth. Although designed for measurement of ultra-highenergy neutrinos interacting in-ice, the first ANITA flight also demonstrated (unexpectedly) excellent sensitivity to primary ultra-high-energy cosmic rays (UHECR) with energies exceeding 1 EeV (10 18 eV) [5] interacting in the Earth's atmosphere. These are assumed to be charged nuclei (likely protons), given the lack of efficient acceleration mechanisms for electrically uncharged particles, and the long lifetimes required to traverse megaparsec-scale distances. Through interactions with terrestrial matter, both neutrinos and charged cosmic-rays produce observable radio-frequency (RF) emissions via the Askaryan effect [6][7][8], with three important distinctions between the two experimental signatures:(1) as viewed from the airborne ANITA gondola, charged primary cosmic ray interactions in the atmosphere generally produce down-coming signals, which subsequently reflect off the surface and up to the gondola, whereas neutrinos interacting in-ice produce up-coming signals which refract through the surf...

The Radio Neutrino Observatory Greenland (RNO-G) is planned to be the first large-scale implementation of the in-ice radio detection technique. It targets astrophysical as well as cosmogenic neutrinos with energies above 10 PeV. The deep component of a single RNO-G station consists of three strings with antennas to capture horizontal as well as vertical polarization. This contribution shows a model-based approach to reconstruct the arrival direction of the neutrinos with an RNO-G station. The timing of the waveforms is used to reconstruct the vertex position. The shape and amplitude of the waveform are used to reconstruct the viewing angle. Together with the signal polarization it will add up to the neutrino arrival direction. We present the method used and the achieved angular resolution using the deep component of an RNO-G station.