High-Energy Antimatter Telescope Research Articles

High energy cosmic rays from the decay of gravitino dark matter

We study high energy cosmic rays from the decay of the gravitino dark matter in the framework of supersymmetric model with $R$-parity violation. Even though $R$ parity is violated, the lifetime of the gravitino, which is assumed to be the lightest superparticle, can be longer than the present age of the Universe if $R$-parity violating interactions are weak enough. We have performed a detailed calculation of the fluxes of gamma ray and positron from the decay of the gravitino dark matter. We also discuss the implication of such a scenario to present and future observations of high energy cosmic rays. In particular, we show that the excess of the gamma-ray flux observed by Energetic Gamma Ray Experiment Telescope and the large positron fraction observed by High Energy Antimatter Telescope can be simultaneously explained by the cosmic rays from the decay of the gravitino dark matter.

Local dark matter clumps and the positron excess

It has been proposed that the excess in cosmic ray positrons at approximately 8 GeV, observed on both flights of the High-Energy Antimatter Telescope (HEAT) balloon experiment, may be associated with the annihilation of dark matter within the Milky Way halo. In this paper, we demonstrate how the self-annihilation of neutralino dark matter within local substructure can account for this excess, and we estimate the annihilation cross-section for several benchmark minimal supersymmetric (MSSM) models. We also demonstrate the changes in the permitted parameter space as the effects of tidal disruption become increasingly severe.

New measurement of the altitude dependence of the atmospheric muon intensity

We present a new measurement of atmospheric muons made during an ascent of the High Energy Antimatter Telescope balloon experiment. The muon charge ratio ${\ensuremath{\mu}}^{+}/{\ensuremath{\mu}}^{\ensuremath{-}}$ as a function of atmospheric depth in the momentum interval 0.3--0.9 GeV/c is presented. The differential ${\ensuremath{\mu}}^{\ensuremath{-}}$ intensities in the 0.3--50 GeV/c range and for atmospheric depths between $4--960\text{ }\text{ }\mathrm{g}/{\mathrm{c}\mathrm{m}}^{2}$ are also presented. We compare these results with other measurements and model predictions. We find that our charge ratio is $\ensuremath{\sim}1.1$ for all atmospheric depths and is consistent, within errors, with other measurements and the model predictions. We find that our measured ${\ensuremath{\mu}}^{\ensuremath{-}}$ intensities are also consistent with other measurements, and with the model predictions, except at shallow atmospheric depths.

Cosmic‐Ray Electrons and Positrons from 1 to 100 GeV: Measurements with HEAT and Their Interpretation

Measurements of the energy spectra of negative electrons and positrons have been performed with the High-Energy Antimatter Telescope (HEAT) in two balloon flights—1994 May from Fort Sumner, NM, and 1995 August from Lynn Lake, Manitoba. We present the combined data set from these two flights, covering the energy range 1-100 GeV. We compare our data with results from other groups and discuss the data in the context of diffusive propagation models. There is some evidence that primary electrons above 10 GeV and cosmic-ray nuclei exhibit the same energy spectrum at the source, but that the source spectrum becomes harder at lower energy. Within the experimental uncertainties, the intensity of positrons is consistent with a purely secondary origin, due to nuclear interactions in interstellar space.

Energy spectra, altitude profiles, and charge ratios of atmospheric muons

We present a new measurement of air shower muons made during atmospheric ascent of the High Energy Antimatter Telescope balloon experiment. The muon charge ratio ${\ensuremath{\mu}}^{+}/{\ensuremath{\mu}}^{\ensuremath{-}}$ is presented as a function of atmospheric depth in the momentum interval 0.3--0.9 $\mathrm{GeV}/c.$ The differential ${\ensuremath{\mu}}^{\ensuremath{-}}$ momentum spectra are presented between 0.3 and $\ensuremath{\sim}50$ $\mathrm{GeV}/c$ at atmospheric depths between 13 and 960 ${\mathrm{g}/\mathrm{c}\mathrm{m}}^{2}.$ We compare our measurements with other recent data and with Monte Carlo calculations of the same type as those used in predicting atmospheric neutrino fluxes. We find that our measured ${\ensuremath{\mu}}^{\ensuremath{-}}$ fluxes are smaller than the predictions by as much as 70% at shallow atmospheric depths, by $\ensuremath{\sim}20%$ at the depth of shower maximum, and are in good agreement with the predictions at greater depths. We explore the consequences of this on the question of atmospheric neutrino production.

Cosmic-ray positrons: are there primary sources?

Galactic cosmic rays consist of primary and secondary particles. Primary cosmic rays are thought to be energized by first order Fermi acceleration processes at supernova shock fronts within our Galaxy. The cosmic rays that eventually reach the Earth from this source are mainly protons and atomic nuclei, but also include electrons. Secondary cosmic rays are created in collisions of primary particles with the diffuse interstellar gas. They are relatively rare but carry important information on the Galactic propagation of the primary particles. The secondary component includes a small fraction of antimatter particles, positrons and antiprotons. In addition, positrons and antiprotons may also come from unusual sources and possibly provide insight into new physics. For instance, the annihilation of heavy supersymmetric dark matter particles within the Galactic halo could lead to positrons or antiprotons with distinctive energy signatures. With the High-Energy Antimatter Telescope (HEAT) balloon-borne instrument, we have measured the abundances of positrons and electrons at energies between 1 and 50 GeV. The data suggest that indeed a small additional antimatter component may be present that cannot be explained by a purely secondary production mechanism. Here we describe the signature of the effect and discuss its possible origin.

The Energy Spectra and Relative Abundances of Electrons and Positrons in the Galactic Cosmic Radiation

Observations of cosmic-ray electrons and positrons have been made with a new balloon-borne detector, HEAT (the High-Energy Antimatter Telescope), first flown in 1994 May from Fort Sumner, NM. We describe the instrumental approach and the data analysis procedures, and we present results from this flight. The measurement has provided a new determination of the individual energy spectra of electrons and positrons from 5 GeV to about 50 GeV, and of the combined all-electron intensity (e+ + e-) up to about 100 GeV. The single power-law spectral indices for electrons and positrons are alpha = 3.09 +/- 0.08 and 3.3 +/- 0.2, respectively. We find that a contribution from primary sources to the positron intensity in this energy region, if it exists, must be quite small.

Cosmic ray reentrant electron albedo: High‐Energy Antimatter Telescope balloon measurements from Fort Sumner, New Mexico

The High‐Energy Antimatter Telescope (HEAT) balloon cosmic ray detector flew from Fort Sumner, New Mexico on May 3–5, 1994. The instrument measured electron and positron abundances and spectra from ∼1 to 100 GeV at a vertical geomagnetic cutoff rigidity that varied between 4.0 and 4.5 GV. The intensities of electrons and positrons have been measured as a function of atmospheric depth between 3.8 and 7.4 g cm−2 of overburden. At magnetic rigidities below cutoff, the intensity of downward moving e± consists of secondary (spallogenic) particles and the reentrant (or return) albedo. We determine the contribution of the reentrant electron albedo and compare it with earlier measurements and limits at similar geomagnetic cutoff levels. In the range of 1.0–2.4 GeV, the reentrant albedo component amounts to 40% of the total electron intensity observed.

The High-Energy Antimatter Telescope (HEAT): An instrument for the study of cosmic-ray positrons

The HEAT (High-Energy Antimatter Telescope) instrument has been developed for a series of observations in cosmic-ray astrophysics that require the use of a superconducting magnet spectrometer. This paper describes the first configuration of HEAT which is optimized for the detection of cosmic-ray electrons and positrons below 100 GeV. In addition to the spectrometer, a combination of time-of-flight scintillators, a transition radiation detector, and an electromagnetic shower counter, provides particle identification, energy measurement, and powerful discrimination against the large background of protons. The instrument was successfully flown aboard high-altitude balloons in 1994 and 1995. The design and construction of the spectrometer and of the detector systems are described, and the performance of the instrument is demonstrated with data obtained in flight.

Measurements of the Cosmic-Ray Positron Fraction from 1 to 50 G[CLC]e[/CLC

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.

Cosmic ray positrons at high energies: A new measurement.

We present a new measurement of the cosmic-ray positron fraction ${e}^{+}/({e}^{+}{+e}^{\ensuremath{-}})$ obtained from the first balloon flight of the High Energy Antimatter Telescope (HEAT). Using a magnet spectrometer combined with a transition radiation detector, an electromagnetic calorimeter, and time-of-flight counters we have achieved a high degree of background rejection. Our results do not indicate a major contribution to the positron flux from primary sources. In particular, we see no evidence for the significant rise in the positron fraction at energies above $\ensuremath{\sim}10\mathrm{GeV}$ previously reported.

High resolution drift tube hodoscopes for cosmic ray studies

Thin-walled drift tubes have been used in conjunction with a superconducting magnet for the rigidity spectrometer aboard two recent particle astrophysics experiments flown on high altitude balloons: PBAR (a low energy antiproton search) and SMILI (the superconducting magnet instrument for light isotopes). The HEAT (high energy antimatter telescope) experiment currently under construction will also employ this technology. This paper reviews the design, construction, and in-flight operation of the PBAR and SMILI systems, as well as the design of the HEAT system which will be used in conjunction with a new superconducting magnet aboard an upcoming series of balloon experiments to study high energy positrons and antiprotons in the cosmic radiation. In addition to a brief account of the scientific goals for these flights, the prospects for future application of this technology to long duration exposures aboard antarctic balloon flights and spacecraft are discussed.