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We report on the first observations of the Askaryan effect in ice: coherent impulsive radio Cherenkov radiation from the charge asymmetry in an electromagnetic (EM) shower. Such radiation has been observed in silica sand and rock salt, but this is the first direct observation from an EM shower in ice. These measurements are important since the majority of experiments to date that rely on the effect for ultra-high energy neutrino detection are being performed using ice as the target medium. As part of the complete validation process for the Antarctic Impulsive Transient Antenna (ANITA) experiment, we performed an experiment at the Stanford Linear Accelerator Center (SLAC) in June 2006 using a 7.5 metric ton ice target, yielding results fully consistent with theoretical expectations.Very large scale optical Cherenkov detectors such as the Antarctic Muon and Neutrino Detector Array (AMANDA) and its successor IceCube have demonstrated the excellent utility of Cherenkov radiation in detection of neutrino interactions at >TeV energies [1, 2] with ice as a target medium. However, at neutrino energies above 100 PeV, the cubic-km scale of such detectors is inadequate to detect more than a handful of events from the predicted cosmogenic neutrino fluxes [3] which represent the most compelling models at these energies. The relevant detector volume for convincing detection and characterization of these neutrinos is in the range of hundreds to thousands of cubic km of water equivalent mass, and the economic constraints of scaling up the optical Cherenkov technique almost certainly preclude extending it much beyond the size of the current IceCube detector, which will be completed early in the next decade.Given the need for an alternative technique with a more tractable economy of scale to reach into the EeV (=1000 PeV) energy regime, a new method which we denote the radio Cherenkov technique, has emerged within the last decade. This method relies on properties of electromagnetic cascades in a dielectric medium. It was first hypothesized by Askaryan [4] and confirmed in 2001 at SLAC [5]. High energy processes such as Compton, Bhabha, and Moller scattering, along with positron annihilation rapidly lead to a ∼ 20% negative charge asymmetry in the electron-photon part of a cascade. In dense media the shower charge bunch is compact, largely contained within a several cm radius. At wavelengths of 10 cm or more, much larger than the characteristic shower bunch size, the relativistic shower bunch appears as a single charge moving through the dielectric over a distance of several meters or more. As an example, a typical shower with mean Bjorken inelasticity y = 0.2, initiated by a E ν = 100 PeV neutrino will create a total number of charged particles at shower maximum of order n e+ +n e− = y E ν /1 GeV ∼ 2 × 10 7 . The net charge is thus n e+ − n e− − ∼ 4 × 10 6 e. Since the radiated power for Cherenkov emission grows quadratically with the charge of the emitter, the coherent power in the cm-to-m wavelength regime is ∼ 10 13 times gre...

The first flight of the Antarctic Impulsive Transient Antenna (ANITA) experiment recorded 16 radio signals that were emitted by cosmic-ray induced air showers. The dominant contribution to the radiation comes from the deflection of positrons and electrons in the geomagnetic field, which is beamed in the direction of motion of the air shower. For 14 of these events, this radiation is reflected from the ice and subsequently detected by the ANITA experiment at a flight altitude of ∼36 km. In this paper, we estimate the energy of the 14 individual events and find that the mean energy of the cosmic-ray sample is 2.9 × 10 18 eV, which is significantly lower than the previous estimate. By simulating the ANITA flight, we calculate its exposure for ultra-high energy cosmic rays. We estimate for the first time the cosmic-ray flux derived only from radio observations and find agreement with measurements performed at other observatories. In addition, we find that the ANITA data set is consistent with Monte-Carlo simulations for the total number of observed events and with the properties of those events.

There has been a recent surge in interest in the detection of τ lepton-induced air showers from detectors at altitude. When a τ neutrino (ντ ) enters the Earth it produces τ leptons as a result of nuclear charged current interactions. In some cases, this process results in a τ lepton exiting the surface of the Earth, which can subsequently decay in the atmosphere and produce an extensive air shower. These upward-going air showers can be detected via fluorescence, optical Cherenkov, or geomagnetic radio emission. Several experiments have been proposed to detect these signals. We present a comprehensive simulation of the production of τ leptons by ντ 's propagating through Earth to aid the design of future experiments. These simulations for ντ 's and leptons in the energy range from 10 15 eV to 10 21 eV treat the full range of incidence angles from Earth-skimming to diametrically-traversing. Propagation of ντ 's and leptons include the effects of rock and an ocean or ice layer of various thicknesses. The interaction models include ντ regeneration and account for uncertainties in the Standard Model neutrino cross-section and in the photo-nuclear contribution to the τ energy loss rate.

We report initial results of the first flight of the Antarctic Impulsive Transient Antenna (ANITA-1) 2006-2007 Long Duration Balloon flight, which searched for evidence of a diffuse flux of cosmic neutrinos above energies of E ν ≃ 3 × 10 18 eV. ANITA-1 flew for 35 days looking for radio impulses due to the Askaryan effect in neutrino-induced electromagnetic showers within the Antarctic ice sheets. We report here on our initial analysis, which was performed as a blind search of the data. No neutrino candidates are seen, with no detected physics background. We set model-independent limits based on this result. Upper limits derived from our analysis rule out the highest cosmogenic neutrino models. In a background horizontal-polarization channel, we also detect six events consistent with radio impulses from ultra-high energy extensive air showers.In all standard models for ultra-high energy cosmic ray (UHECR) propagation, their range is ultimately limited by the opacity of the cosmic microwave background radiation. The UHECR energy above which this becomes significant is about 6 × 10 19 eV in the current epoch. This cuts off their travel beyond distances of order 50 Mpc as first noted by Greisen [1], and Zatseptin and Kuzmin [2] (GZK). As a result of this absorption, the UHECR energy above this GZK cutoff is ultimately converted to photons, neutrinos, and lower energy hadrons. The resulting neutrinos were first described by Berezinsky and Zatsepin (BZ) [3]. In standard UHECR source models the BZ neutrino fluxes peak at energies about 2 orders of magnitude below the GZK energy. Thus a "guaranteed" flux of neutrinos at energies of E ν = 10 17−20 eV exists. Its detection is one of the clearest ways to reveal the nature and cosmic distribution of the UHECR sources [4], which is one of the longest-standing problems in high energy astrophysics.The ANITA-1 Long Duration Balloon experiment was designed specifically to search for this cosmogenic BZ neutrino flux. ANITA-1 exploits the Askaryan effect, in which strong coherent radio emission arises from electromagnetic showers in any dielectric medium [5]. The effect was first observed in 2000 [6], and has now been clearly confirmed and characterized for ice as the medium, as part of the pre-flight calibration of the ANITA-1 payload [7]. A prior flight of a prototype payload called ANITA-lite in 2003ANITA-lite in -2004 led to validation of the technique and initial neutrino flux limits that ruled out several UHE neutrino models [8].In a previous paper [9], we describe in detail the ANITA-1 instrument, payload, and flight system. Reference [9] also includes details of the instrument performance during the flight, estimates of the overall sensitivity of the instrument to neutrino fluxes, and discussions of possible backgrounds. Because of the complexity of the flight system and methodology, we refer the reader to ref.[9] for more detail when necessary.The ANITA-1 payload (Fig. 1) launched from Williams Field, Antarctica near McMurdo station, on December 15, 2006, and executed m...

The electric field of theČerenkov radio pulse produced by a single charged particle track in a dielectric medium is derived from first principles. An algorithm is developed to obtain the pulse in the time domain for numerical calculations. The algorithm is implemented in a Monte Carlo simulation of electromagnetic showers in dense media (specifically designed for coherent radio emission applications) as might be induced by interactions of ultra-high energy neutrinos. The coherenť Cerenkov radio emission produced by such showers is obtained simultaneously both in the time and frequency domains. A consistency check performed by Fourier-transforming the pulse in time and comparing it to the frequency spectrum obtained directly in the simulations yields, as expected, fully consistent results. The reversal of the time structure inside theČerenkov cone and the signs of the corresponding pulses are addressed in detail. The results, besides testing algorithms used for reference calculations in the frequency domain, shed new light into the properties of the radio pulse in the time domain. The shape of the pulse in the time domain is directly related to the depth development of the excess charge in the shower and its width to the observation angle with respect to theČerenkov direction. This information can be of great practical importance for interpreting actual data.PACS numbers: 95.85. Bh, 95.85.Ry,

An in-depth characterization of coherent radio Cherenkov pulses from particle showers in dense dielectric media, referred to as the Askaryan effect, is presented. The time-domain calculation developed in this article is based on a form factor to account for the lateral dimensions of the shower. It is computationally efficient and able to reproduce the results of detailed particle shower simulations with high fidelity in most regions of practical interest including Fresnel effects due to the longitudinal development of the shower. In addition, an intuitive interpretation of the characteristics of the Askaryan pulse is provided. We expect our approach to benefit the analysis of radio pulses in experiments exploiting the radio technique.PACS numbers: 95.85. Bh, 95.85.Ry,

Tau neutrinos are expected to comprise roughly one third of both the astrophysical and cosmogenic neutrino flux, but currently the flavor ratio is poorly constrained and the expected flux at energies above 10 17 eV is low. We present a detector concept aimed at measuring the diffuse flux of tau neutrinos in this energy range via a high-elevation mountaintop detector using the radio technique. The detector searches for radio signals from upgoing air showers generated by Earthskimming tau neutrinos. Signals from several antennas in a compact array are coherently summed at the trigger level, permitting not only directional masking of anthropogenic backgrounds, but also a low trigger threshold. This design takes advantage of both the large viewing area available at high-elevation sites and the nearly full duty cycle available to radio instruments. We present trade studies that consider the station elevation, frequency band, number of antennas in the array, and the trigger threshold to develop a highly efficient station design. Such a mountaintop detector can achieve a factor of ten improvement in acceptance over existing instruments with 100 independent stations. With 1000 stations and three years of observation, it can achieve a sensitivity to an integrated E −2 flux of < 10 −9 GeV cm −2 sr −1 s −1 , in the range of the expected flux of all-flavor cosmogenic neutrinos assuming a pure iron cosmic-ray composition.