Cosmic Ray Positrons Research Articles

REVIEW OF PRECISION MEASUREMENTS OF HIGH ENERGY ELECTRONS

An accurate measurement of the intensity and energy spectra of Cosmic Ray electrons and positrons represents a major experimental challenge. Long exposure times and excellent particle identification capabilities are needed in order to cope with the low intensity of the electron and positron fluxes and the overwhelming background from protons and nuclei in cosmic rays. The motivations for such an experimental effort will be briefly discussed and the most recent results revieweved together with the perspectives of future experiments.

Supersymmetry and the cosmic ray positron excess

We explore several supersymmetric alternatives to explain predictions for the cosmic ray positron excess. Light sneutrino or neutralino LSP's, and a fine-tuned model designed to provide a δ-function input, can give adequate statistical descriptions of the reported HEAT data if non-thermal production of the relic cold dark matter density dominates and/or if “boost factors” (that could originate in uncertainties from propagation or local density fluctuations) to increase the size of the signal are included. All the descriptions can be tested at the Tevatron or LHC, and some in other WIMP detecting experiments.

Positron Abundance in Galactic Cosmic Rays

On 2000 August 25 from Lynn Lake, Manitoba, we conducted a balloon flight of the LEE/AESOP (Low Energy Electrons/Anti-Electron Sub Orbital Payload) payload to measure the spectrum of cosmic-ray electrons (resolved into negatrons and positrons) from 500 MeV to 3 GeV. Analysis of the data from that flight reveals a significant decrease in the cosmic-ray positron abundance from a level that remained relatively stable throughout the decade of the 1990s. Errors on the new determination are comparatively large due to the low particle fluxes at solar maximum. Nevertheless, the magnitude of the effect is consistent with predictions based on the assumption that cosmic-ray modulation effects with 22 yr periodicity are related simply and directly to charge sign and large-scale structure of the magnetic field embedded in the solar wind.

Cosmic ray positron excess and neutralino dark matter

Using a new instrument, the HEAT Collaboration has confirmed the excess of cosmic ray positrons that they first detected in 1994. We explore the possibility that this excess is due to the annihilation of neutralino dark matter in the galactic halo. We confirm that neutralino annihilation can produce enough positrons to make up the measured excess only if there is an additional enhancement to the signal. We quantify the ``boost factor'' that is required in the signal for various models in the Minimal Supersymmetric Standard Model parameter space, and study the dependence on various parameters. We find models with a boost factor $g~30.$ Such an enhancement in the signal could arise if we live in a clumpy halo. We discuss what part of supersymmetric parameter space is favored (in that it gives the largest positron signal), and the consequences for other direct and indirect searches of supersymmetric dark matter.

Supersymmetry and the positron excess in cosmic rays

Recently the HEAT balloon experiment has confirmed an excess of high-energy positrons in cosmic rays. They could come from annihilation of dark matter in the galactic halo. We discuss expectations for the positron signal in cosmic rays from the lightest superpartner (LSP). The simplest interpretations are incompatible with the size and shape of the excess if the relic LSPs evolved from thermal equilibrium. Nonthermal histories can describe a sufficient positron rate. Reproducing the energy spectrum is more challenging, but perhaps possible. The resulting light superpartner spectrum is compatible with collider physics, the muon anomalous magnetic moment, Z-pole electroweak data, and other dark matter searches.

Fate of theBball

The gauge-mediated SUSY-breaking (GMSB) model needs entropy production at a relatively low temperature in the thermal history of the Universe for the unwanted relics to be diluted. This requires a mechanism for the baryogenesis after the entropy production, and the Affleck and Dine (AD) mechanism is a promising candidate for it. The AD baryogenesis in the GMSB model predicts the existence of the baryonic Q ball, that is the B ball, and this may work as the dark matter in the Universe. In this article, we discuss the stability of the B ball in th presence of baryon-number violating interactions. We find that the evaporation rate increases monotonically with the B-ball charge because the large field value inside the B ball enhances the effect of the baryon-number-violating operators. While there are some difficulties to evaluate the evaporation rate of the B ball, we derive the evaporation time (lifetime) of the B ball for the mass-to-charge ratio $\omega_0\gsim 100 \MEV$. The lifetime of the B ball and the distortion of the cosmic ray positron flux and the cosmic background radiation from the B ball evaporation give constraints on the baryon number of the B ball and the interaction, if the B ball is the dark matter. We also discuss some unresolved properties of the B ball.

Cosmic-Ray Positrons Produced by Pulsar Winds from Mature Gamma-Ray Pulsars

In the frame of the γ-ray pulsar outer gap model, e±pairs in the pulsar magnetosphere are produced by the cascade of e±pairs through synchrotron radiation of the return current from theouter gap. These pairs are accelerated mono-energetically to relativisticenergies in the pulsar wind driven by low-frequency electromagnetic waves.Using Monte Carlo simulations, we generate a sample of the matureγ-ray pulsars in our Galaxy and calculate the positron productionrate from these pulsars. With a simple leaky box model, we calculate theratio of cosmic-ray positron to total electrons. Our result indicates that the pulsarcontribution to the cosmic-ray positron peaks at about 40 GeV and theobserved e+/(e- + e+) ratio can be explained by this model.

Cosmic-ray positrons from mature gamma-ray pulsars

We consider a possible contribution of mature γ -ray pulsars (with ages of yrs) to cosmic ray positrons. Within the framework of the γ -ray pulsar outer gap model, pairs in the pulsar magnetosphere are produced by the cascade of pairs through synchrotron radiation of the return current from the outer gap. A good fraction of these cascade pairs are reflected by the hard X-rays from the polar cap via resonant scattering and escape from the pulsar through the light cylinder. The escaped pairs are accelerated to relativistic energies in the pulsar wind driven by low-frequency electromagnetic waves. Using Monte Carlo simulations, we generate a sample of the mature γ -ray pulsars in our Galaxy and calculate the positron production rate from these pulsars. In a simple leaky box model, we calculate the ratio of cosmic-ray positrons to total electrons. Our result indicates that the pulsar contribution to the cosmic ray positrons peaks at about 60 GeV and the observed ratio can be explained in this model.

Models for galactic cosmic-ray propagation

A new numerical model of particle propagation in the Galaxy has been developed, which allows the study of cosmic-ray and gamma-ray production and propagation in 2D or 3D, including a full reaction network. This is a further development of the code which has been used for studies of cosmic ray reacceleration, Galactic halo size, antiprotons and positrons in cosmic rays, the interpretation of diffuse continuum gamma rays, and dark matter. In this paper we illustrate recent results focussing on B/C, sub-Fe/Fe, ACE radioactive isotope data, source abundances and antiprotons. From the radioactive nuclei we derive a range of 3–7 kpc for the height of the cosmic-ray halo.

Cosmic-ray electrons and positrons

Abstract Measurements of cosmic-ray electrons and positrons address a number of significant astrophysical questions concerning the nature and distribution of sources in the galaxy, and the characteristics of cosmic ray propagation in the galactic disk and halo. The abundance of positrons may also carry the signature of unusual pair production processes or dark matter particle decays. We shall review the body of information available, including recent results from the HEAT collaboration. We describe constraints on the source energy spectrum of electrons, which appears to have the same shape as that of nuclear cosmic rays, and we discuss the evidence that positrons are predominantly if not exclusively of interstellar secondary origin. Finally, we emphasize the need for several key observations that are required in the future in order to resolve the remaining questions.

Charge sign dependence of cosmic ray modulation near a rigidity of 1 GV

New observations of electron fluxes made in 1997 and 1998 extend our ongoing investigation of the relative modulation of positively and negatively charged particles. We compare electron fluxes measured on high‐altitude balloon flights with continuing observations of helium fluxes from the IMP 8 spacecraft and present new measurements of the primary cosmic ray positron abundance in 1997 and 1998. Electron fluxes during the 1984–1990 period show a flat topped distribution, whereas the positively charged He fluxes show a peaked distribution, with the peak in 1987. This is expected from modulation theory, including the role of drifts when the northern heliospheric magnetic field is inward, and the southern heliospheric field is outward. From 1990 to 1999, data are consistent with an inverse relationship, but electron data are too sparse to allow a definitive statement. Near a rigidity of 1 GV the relative abundance of electrons and helium nuclei is a weak function of the tilt angle of the heliospheric current sheet.

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.

Positrons from particle dark-matter annihilation in the Galactic halo: Propagation Green’s functions

We calculate the propagation of positrons from dark-matter particle annihilation in the Galactic halo in different models of the dark matter halo distribution using our three-dimensional code, and present fits to our numerical propagation Green's functions. We show that the Green's functions are not very sensitive to the dark matter distribution for the same local dark matter energy density. We compare our predictions with the computed cosmic ray positron spectra (``background'') for the ``conventional'' cosmic ray nucleon spectrum which matches the local measurements, and a modified spectrum which respects the limits imposed by measurements of diffuse Galactic $\ensuremath{\gamma}$ rays, antiprotons, and positrons. We conclude that significant detection of a dark matter signal requires favorable conditions and precise measurements unless the dark matter is clumpy which would produce a stronger signal. Although our conclusion qualitatively agrees with those of previous authors, it is based on a more realistic model of particle propagation and thus reduces the scope for future speculations. A reliable background evaluation requires new accurate positron measurements and further developments in modeling production and propagation of cosmic ray species in the Galaxy.

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.

Indirect detection of wimps using cosmic-ray positrons and antiprotons: Current status and future prospects

I present the history, current status and future prospects of using cosmic-ray particles to indirectly search for the secondary products of WIMP Dark Matter annihilation.

Lower limit on pulsar birthrate set by cosmic ray positron measurements

A recent measurement of thee+/(e++e−) ratio in cosmic rays between 5 and 50 GeV (HEAT experiment), is consistent with positron production theories via primary cosmic radiation interactions in the interstellar medium. This paper will show that atmospheric corrections result in a 50% level of uncertainty in thee+/(e++e−) ratio measurements carried out with balloon-borne experiments. In light of the current theories on electron-positron production in neutron stars and by using different calculations for atmospheric corrections, a lower limit on Milky Way pulsar birthrate of 30–60 years can be set on the basis of recent observations of the positron fraction in cosmic rays.

Electrons and positrons in the galactic cosmic rays.

The detection of primary cosmic ray electrons with energies above 1 TeV implies the existence of a nearby, r\ensuremath{\le}100 pc, and relatively young, t\ensuremath{\le}${10}^{5}$ yr, source(s) of accelerated electrons. Therefore a correct treatment of the formation of the spectra of electrons during their propagation in the interstellar medium requires a separate consideration of the contribution of one (or a few) nearby source(s) from the contribution of distant (R\ensuremath{\ge}1 kpc) sources. To implement this approach, the problem of energy-dependent diffusive propagation of relativistic particles from a single source is considered, and the analytical solution to the diffusion equation in the general case of arbitrary energy losses and injection spectrum of primary particles is found. We show that in the framework of the proposed two-component approach, i.e., separating the contribution of the local (discrete) source(s) from the contribution of distant sources, it is possible to explain all the locally observed features of the energy spectrum of cosmic ray electrons from sub-GeV to TeV energies. In addition, assuming that the local source produces electrons and positrons in equal amounts, the model allows us to explain also the reported increase of the positron content in the flux above 10 GeV.

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.

Cosmic ray positrons connected with galactic gamma radiation of high and very high energies

A model of high-energy positrons in cosmic rays is suggested according to which the positron flux detected at E>or=10 GeV is due to the interactions of very high energy (Egamma >100 GeV) gamma rays with optical and ultraviolet (UV) radiation in the vicinity of discrete sources. In the frames of the model it is possible to explain both the sharp increase of the ratio r+(E)=e+/(e++e-) in cosmic rays at energies E>or=10 GeV, and the total spectra of electrons and positrons at energies E<or=1 TeV.

Distinctive positron feature from particle dark-matter annihilations in the galactic halo.

If the dark matter in our galactic halo consists of weakly interacting massive particles (WIMP's) heavier than the W± boson which have a significant annihilation branch into W± and Z0 pairs, e.g., a Higgsino-like neutralino, a very distinctive feature in the cosmic-ray positron spectrum arises from W+ and Z0 decays. Because of inherent astrophysical uncertainties such a signal is by no means guaranteed even if heavy WIMP's do comprise the galactic halo. However, the positron signature is virtually a smoking gun for particle dark matter in the halo and thus worthy of note.