J. J. Beatty

DISCREPANT HARDENING OBSERVED IN COSMIC-RAY ELEMENTAL SPECTRA

The balloon-borne Cosmic Ray Energetics And Mass experiment launched five times from Antarctica has achieved a cumulative flight duration of about 156 days above 99.5% of the atmosphere. The instrument is configured with complementary and redundant particle detectors designed to extend direct measurements of cosmic-ray composition to the highest energies practical with balloon flights. All elements from protons to iron nuclei are separated with excellent charge resolution. Here, we report results from the first two flights of ~70 days, which indicate hardening of the elemental spectra above ~200 GeV/nucleon and a spectral difference between the two most abundant species, protons and helium nuclei. These results challenge the view that cosmic-ray spectra are simple power laws below the so-called knee at ~1015 eV. This discrepant hardening may result from a relatively nearby source, or it could represent spectral concavity caused by interactions of cosmic rays with the accelerating shock. Other possible explanations should also be investigated.

Cosmic-ray Proton and Helium Spectra from the First CREAM Flight

Cosmic-ray proton and helium spectra have been measured with the balloon-borne Cosmic Ray Energetics And Mass experiment flown for 42 days in Antarctica in the 2004–2005 austral summer season. High-energy cosmic-ray data were collected at an average altitude of �38.5 km with an average atmospheric overburden of �3.9 g cm −2 . Individual elements are clearly separated with a charge resolution of �0.15 e (in charge units) and �0.2 e for protons and helium nuclei, respectively. The measured spectra at the top of the atmosphere are represented by power laws with a spectral index of 2.66 ± 0.02 for protons from 2.5 TeV to 250 TeV and –2.58 ± 0.02 for helium nuclei from 630 GeV nucleon −1 to 63 TeV nucleon −1 . They are harder than previous measurements

Constraints on Cosmic Neutrino Fluxes from the Antarctic Impulsive Transient Antenna Experiment

P. F. Dowkontt, 4 M. A. DuVernois,5 P. A. Evenson, 6 D. Goldstein, 1 P. W. Gorham, 9 C. L. Hebert, 9 M. H. Israel,4 J. G. Learned, 9 K. M. Liewer,10 J. T. Link,9 S. Matsuno, 9 P. Miočinović,9 J. Nam, 1 C. J. Naudet, 10 R. Nichol,2 K. Palladino, 2 M. Rosen, 9 D. Saltzberg, 7 D. Seckel, 6 A. Silvestri,1 B. T. Stokes, 9 G. S. Varner, 9 and F. Wu1 1Department of Physics and Astronomy, University of California at Irvine, Irvine, California 2Department of Physics, Ohio State University, Columbus, Ohio 3Department of Physics and Astronomy, University of Kansas, Lawrence, Kansas 4Department of Physics, Washington University in St. Louis, St. Louis, Missouri 5School of Physics and Astronomy, University of Minnesota, Minneapolis, Minnesota 6Bartol Research Institute, University of Delaware, Newark, Delaware 7Department of Physics and Astronomy, University of California at Los Angeles, Los Angeles, California 8Department of Astronomy and Astrophysics, Pennsylvania State University, University Park, Pennsylvania 9Department of Physics and Astronomy, University of Hawaii at Manoa, Honolulu, Hawaii 10Jet Propulsion Laboratory, Pasadena, California

Measurements of the cosmic ray positron fraction from 1-GeV to 50-GeV

Two measurements of the cosmic-ray positron fraction as a function of energy have been made using the High-Energy Antimatter Telescope (HEAT) balloon-borne instrument. The first flight took place from Fort Sumner, New Mexico, in 1994 and yielded results above the geomagnetic cutoff energy of 4.5 GeV. The second flight, from Lynn Lake, Manitoba, in 1995, permitted measurements over a larger energy interval, from 1 to 50 GeV. We present results on the positron fraction based on data from the Lynn Lake flight and compare these with the previously published results from the Fort Sumner flight. The results confirm that the positron fraction does not increase with energy above ≈ 10 GeV, although a small excess above purely secondary production cannot be ruled out. At low energies the positron fraction is slightly larger than that reported from measurements made in the 1960s. This effect could possibly be a consequence of charge dependence in the level of solar modulation.

Measurement Of The Anisotropy Of Cosmic-Ray Arrival Directions With Icecube

We report the first observation of an anisotropy in the arrival direction of cosmic rays with energies in the multi-TeV region in the Southern sky using data from the IceCube detector. Between 2007 June and 2008 March, the partially deployed IceCube detector was operated in a configuration with 1320 digital optical sensors distributed over 22 strings at depths between 1450 and 2450 m inside the Antarctic ice. IceCube is a neutrino detector, but the data are dominated by a large background of cosmic-ray muons. Therefore, the background data are suitable for high-statistics studies of cosmic rays in the southern sky. The data include 4.3 billion muons produced by downward-going cosmic-ray interactions in the atmosphere; these events were reconstructed with a median angular resolution of 3 degrees and a median energy of similar to 20 TeV. Their arrival direction distribution exhibits an anisotropy in right ascension with a first-harmonic amplitude of (6.4 +/- 0.2 stat. +/- 0.8 syst.) x 10(-4).

ENERGY SPECTRA OF COSMIC-RAY NUCLEI AT HIGH ENERGIES

We present new measurements of the energy spectra of cosmic-ray (CR) nuclei from the second flight of the balloon-borne experiment Cosmic-Ray Energetics And Mass (CREAM). The instrument included different particle detectors to provide redundant charge identification and measure the energy of CRs up to several hundred TeV. The measured individual energy spectra of C, O, Ne, Mg, Si, and Fe are presented up to ~1014 eV. The spectral shape looks nearly the same for these primary elements and it can be fitted to an E –2.66 ± 0.04 power law in energy. Moreover, a new measurement of the absolute intensity of nitrogen in the 100-800 GeV/n energy range with smaller errors than previous observations, clearly indicates a hardening of the spectrum at high energy. The relative abundance of N/O at the top of the atmosphere is measured to be 0.080 ± 0.025 (stat.)±0.025 (sys.) at ~800 GeV/n, in good agreement with a recent result from the first CREAM flight.

Observations of the Askaryan Effect in Ice

We report on observations of coherent, impulsive radio Cherenkov radiation from electromagnetic showers in solid ice. This is the first observation of the Askaryan effect in ice. As part of the complete validation process for the ANITA experiment, we performed an experiment at the Stanford Linear Accelerator Center in June 2006 using a 7.5 metric ton ice target. We measure for the first time the large-scale angular dependence of the radiation pattern, a major factor in determining the solid-angle acceptance of ultrahigh-energy neutrino detectors.

The Antarctic Impulsive Transient Antenna ultra-high energy neutrino detector: Design, performance, and sensitivity for the 2006–2007 balloon flight

Abstract We present a comprehensive report on the experimental details of the Antarctic Impulsive Transient Antenna (ANITA) long-duration balloon payload, including the design philosophy and realization, physics simulations, performance of the instrument during its first Antarctic flight completed in January of 2007, and expectations for the limiting neutrino detection sensitivity.

New Limits on the Ultrahigh Energy Cosmic Neutrino Flux from the ANITA Experiment

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(nu) approximately 3 x 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 ultrahigh energy extensive air showers.

Observation of ultrahigh-energy cosmic rays with the ANITA balloon-borne radio interferometer

We report the observation of 16 cosmic ray events with a mean energy of 1.5 × 10¹⁹ 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 measurements in the 300-900 MHz range, which are the first self-triggered, first ultrawide band, first far-field, and the highest energy sample of cosmic ray events collected with the radio technique. Their properties are inconsistent with current ground-based geosynchrotron models. The emission is 100% polarized in the plane perpendicular to the projected geomagnetic field. Fourteen 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. Based on a likelihood analysis, we estimate angular pointing precision of order 2° for the event arrival directions.