Positron Flux Research Articles

PEBS — Positron electron balloon spectrometer

The best measurement of the cosmic ray positron flux available today was performed by the HEAT balloon experiment more than 10 years ago. Given the limitations in weight and power consumption for balloon experiments, a novel approach was needed to design a detector which could increase the existing data by more than a factor of 100. Using silicon photomultipliers for the readout of a scintillating fiber tracker and of an imaging electromagnetic calorimeter, the PEBS detector features a large geometrical acceptance of 2500 cm 2 sr for positrons, a total weight of 1500 kg and a power consumption of 600 W . The experiment is intended to measure cosmic ray particle spectra for a period of up to 20 days at an altitude of 40 km circulating the North or South Pole. A full Geant4 simulation of the detector concept has been developed and key elements have been verified in a testbeam in October 2006 at CERN.

The galactic positron flux and dark matter substructures

In this paper we calculate the galactic positron flux from dark matter annihilation in theframework of supersymmetry, taking the enhancement of the flux by the existence of darkmatter substructures into account. The propagation of positrons in the galactic magneticfield is solved in a realistic numerical model GALPROP. The secondary positron flux isrecalculated in the GALPROP model. The total positron flux from secondary products anddark matter annihilation can fit the HEAT data well when taking a cuspy density profile ofthe substructures.

The negative ion of positronium: measurement of the decay rate and prospects for further experiments

The negative positronium (Ps–) ion consisting of two electrons and a positron (e+e–e–), represents the simplest three-body system with a bound state. Its constituents are stable, point-like particles, and it is essentially free from perturbations by strong interaction effects. Together with the rather unique mass ratio, these properties make the positronium ion an interesting object for studying the quantum-mechanical three-body problem. We present a new determination of the decay rate of Ps–, using a beam-foil method and a stripping-based detection technique. The measured value of Γ = 2.089(15) ns–1 is a factor of six times more precise than the previous experimental result, and there is excellent agreement both with the latter and with the theoretical value. With the new high-intensity positron source NEPOMUC at the FRM-II research reactor in Munich being available, a further improvement in precision seems possible. Moreover, the high flux of low-energy positrons at this facility brings other properties of this exotic system within reach of experiments. The prospects for such investigations are discussed.PACS No.: 36.10Dr

Cosmic positron signature from dark matter in the littlest Higgs model withTparity

We calculate the flux of cosmic positrons from the dark matter annihilation in the littlest Higgs model with $T$ parity. The dark matter annihilates mainly into weak gauge bosons in the halo, and high-energy positrons are produced through leptonic and hadronic decays of the bosons. We investigate a possibility to detect the positron signal in upcoming experiments such as PAMELA and AMS-02. We found that the dark matter signal can be distinguished from the background in the PAMELA experiment when the dark matter mass is less than 120 GeV and the signal flux is enhanced due to a small scale clustering of dark matter. Furthermore, the signal from the dark matter annihilation can be detected in the AMS-02 experiment, even if such enhancement does not exist. We also discuss the invisible width of the Higgs boson in this model.

The electron–hadron separation performance of the PAMELA electromagnetic calorimeter

A silicon-tungsten sampling imaging calorimeter has been designed and built for the PAMELA satellite-borne experiment. The main physics goals of the experiment are the measurement of the flux of antiprotons (80 MeV–190 GeV) and positrons (50 MeV–270 GeV) in the cosmic radiation. The calorimeter has been designed to identify antiprotons from an electron background and positrons in a background of protons with a high efficiency and rejection power. This work presents the electron–hadron separation capabilities of the calorimeter as obtained using both Monte Carlo and test beam data. The calorimeter is found to have sufficient performance to reach the primary scientific objectives of PAMELA, providing a proton rejection factor of ∼10 5 while keeping a ∼90% efficiency in selecting electrons and positrons. From simulations, an electron rejection factor of ∼10 5 in antiproton measurements (∼90% antiproton identification efficiency) is demonstrated.

Indirect detection of Kaluza–Klein dark matter from latticized universal dimensions

We consider Kaluza–Klein dark matter from latticized universal dimensions. We investigatetwo different lattice models, where the models differ in the choice of boundary conditions.The models reproduce relevant features of the continuum model for Kaluza–Klein darkmatter. For the model with simple boundary conditions, this is the case even for a modelwith only a few lattice sites. We study the effects of the latticization on the differential fluxof positrons from annihilation of Kaluza–Klein dark matter in the galactic halo.We find that for different choices of the compactification radius, the differentialpositron flux rapidly converges to the continuum model results as a function ofthe number of lattice sites. In addition, we consider the prospects for upcomingspace-based experiments such as PAMELA and AMS-02 to probe the latticization effect.

Heavy wino-like neutralino dark matter annihilation into antiparticles

The lightest neutralino is a viable dark matter (DM) candidate. In this paper we study indirect detection of the wino-like neutralino DM using positrons and antiprotons from the annihilation in the galactic halo. When the mass is around 2 TeV, which is favored from the thermal relic abundance, the non-perturbation effect significantly enhances the annihilation cross sections into positrons and antiprotons. We find that the positron and antiproton fluxes with energies larger than 100 GeV may become larger than the expected backgrounds. Since the positron flux is less sensitive to the astrophysical parameters, the detection may be promising in the upcoming experiments such as PAMELA and AMS-02. We also find the wino-like neutralino DM with mass around 2 TeV is compatible with the HEAT anomaly.

Suppression of the neutralino relic density with supersymmetricCPviolation

We study pair annihilations of the neutralino dark matter in the minimal supersymmetric standard model with $CP$ violation. We consider the case that the Higgsino mass and the trilinear scalar couplings have $CP$-violating phases of order unity, taking a scenario that the scalar fermions in the first two generations are much heavier than those in the third generation to avoid a severe constraint from experimental limits on electric dipole moments. It is found that, when the lightest neutralino ($\ensuremath{\chi}$) is binolike, the cross sections of the $W$-boson pair production $\ensuremath{\chi}\ensuremath{\chi}\ensuremath{\rightarrow}{W}^{+}{W}^{\ensuremath{-}}$ and the lightest Higgs boson pair production $\ensuremath{\chi}\ensuremath{\chi}\ensuremath{\rightarrow}{H}_{2}^{0}{H}_{2}^{0}$ for nonrelativistic neutralinos can be significantly enhanced by the phase of the Higgsino mass. The relic density of the neutralino can be considerably suppressed by this effect. However, even this suppression is not enough to make binolike dark matter consistent with a cosmological constraint. We also discuss the effect of $CP$ violation on the positron flux from neutralino pair annihilations in the galactic halo.

HINTS ON PULSAR ELECTROMAGNETIC ENERGY LOSSES FROM COSMIC-RAY POSITRON OBSERVATIONS

We present preliminary results of the estimate of isolated pulsar contribution to the positron and electron interstellar fluxes when a polar cap model is assumed. Pulsars of all ages outside their host remnants have been considered. Cosmic-ray positron observations above a few GeV seem to indicate that an outer gap model is favoured over a polar cap model when the contribution of young pulsars only is taken into account. Here we show that pulsars with ages ranging between 10 kyr and 600 kyr, escaped from their remnants, give a contribution to the interstellar positron flux larger than that proposed in literature for the whole sample of pulsars younger than 10 kyr. Consequently, this result also indicates that outer gap electromagnetic energy losses overcome those at the polar cap. Future, low error e+/(e+ + e-) ratio observations as well as high energy pulsed gamma-ray measurements will allow us to verify this possibility.

Searching for dark matter with future cosmic positron experiments

Dark matter particles annihilating in the Galactic halo can provide a flux of positrons potentially observable in upcoming experiments, such as PAMELA and AMS-02. We discuss the spectral features which may be associated with dark matter annihilation in the positron spectrum and assess the prospects for observing such features in future experiments. Although we focus on some specific dark matter candidates, neutralinos and Kaluza-Klein states, we carry out our study in a model independent fashion. We also revisit the positron spectrum observed by HEAT.

New Measurement of the Cosmic-Ray Positron Fraction from 5 to 15 GeV

We present a new measurement of the cosmic-ray positron fraction at energies between 5 and 15 GeV with the balloon-borne HEAT-pbar instrument in the spring of 2000. The data presented here are compatible with our previous measurements, obtained with a different instrument. The combined data from the three HEAT flights indicate a small positron flux of nonstandard origin above 5 GeV. We compare the new measurement with earlier data obtained with the HEAT-e(+/-) instrument, during the opposite epoch of the solar cycle, and conclude that our measurements do not support predictions of charge sign dependent solar modulation of the positron abundance at 5 GeV.

Kaluza-Klein dark matter and the positron excess

The excess of cosmic positrons observed by the HEAT experiment may be the result of Kaluza-Klein dark matter annihilating in the galactic halo. Kaluza-Klein dark matter annihilates dominantly into charged leptons that yield a large number and hard spectrum of positrons per annihilation. Given a Kaluza-Klein dark matter particle with a mass in the range of 300-400 GeV, no exceptional substructure or clumping is needed in the local distribution of dark matter to generate a positron flux that explains the HEAT observations. This is in contrast to supersymmetric dark matter that requires unnaturally large amounts of dark substructure to produce the observed positron excess. Future astrophysical and collider tests are outlined that will confirm or rule out this explanation of the HEAT data.

High‐energy protons, electrons, and positrons trapped in Earth's radiation belts

Accurate measurements of under cutoff proton, electron, and positron fluxes in the energy range 0.07 ÷ 9.1 GeV have been performed with the Alpha Magnetic Spectrometer at altitudes of 370 ÷ 390 km in the geographic latitude interval ±51.7°. The flux maps as a function of the canonical adiabatic variables L, αo, and energy E are presented and discussed.

PAMELA: a satellite experiment for antiparticles measurement in cosmic rays

PAMELA is a satellite-borne experiment that will study the antiproton and positron fluxes in cosmic rays in a wide range of energy (from 80 MeV up to 190 GeV for antiprotons and from 50 MeV up to 270 GeV for positrons) and with high statistics, and that will measure the antihelium/helium ratio with a sensitivity of the order of 10/sup -8/. The detector will fly on-board a polar orbiting Resurs DK1 satellite, which will be launched into space by a Soyuz rocket in 2004 from Baikonur cosmodrome in Kazakhstan, for a 3-year-long mission. Particle identification and energy measurements are performed in the PAMELA apparatus using the following subdetectors: a magnetic spectrometer made up of a permanent magnet equipped with double-sided microstrip silicon detectors, an electromagnetic imaging calorimeter composed of layers of tungsten absorber and silicon detectors planes, a transition radiation detector made of straw tubes interleaved with carbon fiber radiators, a plastic scintillator time-of-flight and trigger system, a set of anticounter plastic scintillator detectors, and a neutron detector. The features of the detectors and the main results obtained in beam test sessions are presented.

Can supersymmetry naturally explain the positron excess?

It has often been suggested that the cosmic positron excess observed by the HEAT experiment could be the consequence of supersymmetric dark matter annihilating in the galactic halo. Although it is well known that evenly distributed dark matter cannot account for the observed excess, if substantial amounts of local dark matter substructure are present, the positron flux would be enhanced, perhaps to the observed magnitude. In this paper, we attempt to identify the nature of the substructure required to match the HEAT data, including the location, size and density of any local dark matter clump(s). Additionally, we attempt to assess the probability of such substructure being present. We find that if the current density of neutralino dark matter is the result of thermal production, very unlikely ($\sim 10^{-4}$ or less) conditions must be present in local substructure to account for the observed excess.

Formation of a high intensity low energy positron string

The possibility of a high intensity low energy positron beam production is discussed. The proposed Positron String Trap (PST) is based on the principles and technology of the Electron String Ion Source (ESIS) developed in JINR during the last decade. A linear version of ESIS has been used successfully for the production of intense highly charged ion beams of various elements. Now the Tubular Electron String Ion Source (TESIS) concept is under study and this opens really new promising possibilities in physics and technology. In this report, we discuss the application of the tubular-type trap for the storage of positrons cooled to the cryogenic temperatures of 0.05 meV. It is intended that the positron flux at the energy of 1–5 eV, produced by the external source, is injected into the Tubular Positron Trap which has a similar construction as the TESIS. Then the low energy positrons are captured in the PST Penning trap and are cooled down because of their synchrotron radiation in the strong (5–10 T) applied magnetic field. It is expected that the proposed PST should permit storing and cooling to cryogenic temperature of up to 5×109 positrons. The accumulated cooled positrons can be used further for various physics applications, for example, antihydrogen production.

Indirect evidence for neutralinos as dark matter

From the relic density measurement by WMAP the WIMP annihilation cross section can be determined in a model independent way. If the WIMPS are postulated to be the neutralinos of Supersymmetry, then only a limited region of the supersymmetric parameter space matches this annihilation cross section. It is shown that the resulting positrons, antiprotons and gamma rays from the neutralino annihilation (mainly into $b\overline{b}$ quark pairs) provide the correct shape and order of magnitude for the missing gamma and hard positron fluxes in the Galactic Models and are consistent with antiproton production.

Formation of the 255-keV annihilation line near the magnetic poles of pulsars

The possibility of existance is discussed for pulsars emitting soft γ radiation near their magnetic poles upon the annihilation of ultrarelativistic positrons from the magnetosphere and electrons from the surface of a star. With an increase in the energy of incident positrons, the photon energy of this backward radiation tends to a constant value m e c2/2=255 keV. This radiation is shown to be directed opposite to the positron flux direction.

Neutralino dark matter from minimal supersymmetric standard model flat directions in light of WMAP results

The minimal supersymmetric standard model has a truly supersymmetric way of explaining both the baryon asymmetry and cold dark matter in the present Universe: that is, ``Affleck-Dine baryo/DM genesis.'' The associated late-time decay of Q balls directly connects the origins of the baryon asymmetry and dark matter, and also predicts a specific nature of the LSP. In this paper, we investigate the prospects for indirect detection of these dark matter candidates observing the high-energy neutrino flux from the Sun and hard positron flux from the halo. We also update the previous analysis of the direct detection of Fujii and Hamaguchi [Phys. Rev. D 66, 083501 (2002)] by implementing the recent results from the WMAP satellite.

Hypernovae/Gamma-Ray Bursts in the Galactic Center as Possible Sources of Galactic Positrons

The observation of a strong and extended positron-electron line annihilation emission in the central regions of the Galaxy by the SPectrometer on the International Gamma-Ray Astrophysical Laboratory (INTEGRAL/SPI), consistent with the Galactic bulge geometry and without any counterpart in the gamma-ray range, neither at high energy nor in the 1809 keV 26Al decay line, is challenging. Leaving aside the geometrical question, we address the problem of the adequate positron sources, showing the potentiality of a new category of Type Ic supernovae (SNe Ic), exemplified by SN 2003dh, that is associated with a gamma-ray burst (GRB). This kind of supernova/hypernova/GRB event is interpreted as the result of a bipolar Wolf-Rayet explosion, which produces a large amount of 56Ni and ejects it at high velocity along the rotation axis. The bulk of positrons resulting from 56Co decay escapes in the surrounding medium as a result of the rapid thinning of the ejecta in the polar direction. We show that a rate of about 0.02 SN 2003dh-like events per century in the central region of the Galaxy is sufficient to explain the positron flux detected by INTEGRAL/SPI. In order to explain this flux by SN Ia events alone, a rate of 0.5 per century is necessary, much higher than indicated by Galactic evolutionary models applied to the bulge. Further observations of late light curves of SNe Ia and SNe Ic in the bulge of spiral galaxies, together with three-dimensional hydrodynamic calculations of anisotropic ejections of 56Ni in SN Ic/GRB events, will allow us to estimate the separate contributions of SNe Ia and SNe Ic to positron injection.