We report results from 120 hours of livetime with the Goldstone Lunar Ultra-high energy neutrino Experiment (GLUE). The experiment searches for ≤ 10 ns microwave pulses from the lunar regolith, appearing in coincidence at two large radio telescopes separated by 22 km and linked by optical fiber. Such pulses would arise from subsurface electromagnetic cascades induced by interactions of ≥ 100 EeV neutrinos in the lunar regolith. No candidates are yet seen, and the implied limits constrain several current models for ultra-high energy neutrino fluxes.In 1962, G. Askaryan predicted that electromagnetic cascades in dense media should produce strong coherent pulses of microwave Cherenkov radiation [1]. Recent confirmation of this hypothesis at accelerators [2] strengthens the motivation to search for such emission from cascades induced by predicted high energy neutrino fluxes, closely related to the measured fluence of ≃ 10 20 eV cosmic rays in many models.Two such models, the Z-burst model [3], and a generic class known as Topological Defect (TD) models [4], predict ultrahigh energy (UHE) neutrinos with either monoenergetic or very hard energy spectra. In the Z-burst model, UHE neutrinos annihilate with relic cosmic background neutrinos via the νν → Z 0 channel. The Z 0 then decays rapidly in a burst of hadronic secondaries which create the observed ∼ 10 20 eV cosmic rays. The need to match the observed UHE cosmic ray fluxes and satisfy the current constraints on neutrino masses (which modify the annihilation resonance energy) then lead to a requirement on minimal neutrino fluxes at the resonance energy near 10 22−23 eV. The Z-burst model thus formally requires only neutrinos at a single energy, with no specification for how such a flux might be produced.The Z-burst model is also significant in that it is a variation on an earlier idea [5] in which the νν annihiliation process could be used as a probe of the cosmic background neutrinos, one of the few viable ways ever proposed for detection of these relic cosmological neutrinos-it requires only a sufficient flux of UHE neutrinos and a detector with the sensitivity to measure them. Constraints on these UHE ν fluxes thus can rule out this potential detection channel for the relics, in addition to excluding their role in UHE cosmic ray production.TD models, in contrast, postulate a very massive relic particle from the early universe which is decaying in the current epoch and producing secondaries observed as UHE cosmic rays. The required masses approach the Grand-Unified Theory (GUT) scale at ∼ 10 24 eV, and the decay products thus have a very hard spectrum extending up to the rest mass energy of the particles. Because of these very hard spectra, detectors optimized for lower-energy neutrinos, even up to PeV energies, do not yet constrain these models, and new approaches, such as the experiment we report on here, are required.Neutrinos with energies above 100 EeV (1 EeV = 10 18 eV) can produce cascades in the upper 10 m of the lunar regolith resulting in pulses that are...

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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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...

We report new limits on cosmic neutrino fluxes from the test flight of the Antarctic Impulsive Transient Antenna (ANITA) experiment, which completed an 18.4 day flight of a prototype long-duration balloon payload, called ANITA-lite, in early 2004. We search for impulsive events that could be associated with ultrahigh energy neutrino interactions in the ice and derive limits that constrain several models for ultrahigh energy neutrino fluxes and rule out the long-standing -burst model.