Cosmic-ray positrons: are there primary sources?

Galactic cosmic rays consist of primary and secondary particles. Primary co smic rays are thought to be energized by first order Fermi acceleration pro cesses at supernova shock fronts within our Galaxy. The cosmic rays that ev entually reach the Earth from this source are mainly protons and atomic nuc lei, but also include electrons. Secondary cosmic rays are created in colli sions of primary particles with the diffuse interstellar gas. They are rela tively 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 an d antiprotons may also come from unusual sources and possibly provide insig ht 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 Antima tter Telescope (HEAT) balloon-borne instrument, we have measured the abunda nces 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 w e describe the signature of the effect and discuss its possible origin. (C) 1999 Elsevier Science B.V. All rights reserved.