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.
We report the observation of sixteen cosmic ray events of mean energy of 1.5 × 10 19 eV, via radio pulses originating from the interaction of the cosmic ray air shower with the Antarctic geomagnetic field, a process known as geosynchrotron emission. We present the first ultra-wideband, far-field measurements of the radio spectral density of geosynchrotron emission in the range from 300-1000 MHz. The emission is 100% linearly polarized in the plane perpendicular to the projected geomagnetic field. Fourteen of our observed events are seen to have a phase-inversion due to reflection of the radio beam off the ice surface, and two additional events are seen directly from above the horizon.The origin of ultra-high energy cosmic rays (UHECR) remains a mystery decades after their discovery [1,2]. Key to the solution will be increased statistics on events of high enough energy (≥ 3 × 10 19 eV) to elucidate the endpoint of the UHECR energy spectrum as seen at Earth. The primary difficulty is the extreme rarity of events at these energies. Despite steady progress with experiments such as the Pierre Auger Observatory, there remains room for new methodologies. Cosmic rays have been detected for decades via impulsive radio geosynchrotron emission [3,[5][6][7][8][9][10][11][12][13][14][15][16]] but until now not in this crucial energy range, which offers the possibility of pointing the UHECRs back to their sources. We present data from the Antarctic Impulsive Transient Antenna (ANITA) [21] which represents the first entry of radio techniques into this energy range. We find 16 UHECR events, at least 40% of which are above 10 19 eV, and we show compelling evidence of their origin as geosynchrotron emission from cosmic-ray showers. Our results indicate degree-scale precision for reconstruction of the UHECR arrival direction, lending strong credence to efforts to develop radio geosynchrotron detection as a competitive method of UHECR particle astronomy.Geosynchrotron emission arises when the electron-positron particle cascade initiated by a primary cosmic ray encounters the Lorentz force in the geomagnetic field. The resulting acceleration deflects the electrons and positrons and they begin to spiral in opposite directions around the field lines [17,18]. In air, the particles' radiation length is of order 40 g cm −2 , a kilometer or less at the altitudes of air shower maximum development. Particle trajectories form partial arcs around the field lines before they lose enough energy to drop out of the shower. The meter-scale longitudinal thickness of the shower particle 'pancake' is comparable to radio wavelengths below several hundred MHz; thus the ensemble behavior of all of the cascade particles yields forward-beamed synchrotron emission which is partially or fully coherent in the radio regime. Therefore, the resulting radio impulse power grows quadratically with primary particle energy, and at the highest energies, yields radio pulses that are detectable at large distances. Current systems under development for detection of thes...
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