Cosmic-Ray Positrons: Are There Primary Sources?

The HEAT instrument has detected cosmic-ray electrons and positrons in two balloon flights, at energies between 1 and ¸50 GeV. The combined data set indicates that the positron fraction does not increase with energy above ¸10 GeV. However, our results suggest a slight overabundance of positrons at all energies compared with published predictions from secondary production sources, and possibly the hint of a feature in the positron fraction in the energy range 7--20 GeV. We discuss the implications of the observations on the origin and propagation of electrons and positrons, and investigate the possibility that some positrons are from primary, possibly exotic sources. INTRODUCTION The question of the origin and propagation of cosmic-ray electrons and positrons has been an interesting issue in cosmic-ray research for well over 30 years. The all-electron (e + +e \Gamma ) cosmicray flux, amounting to 1-2% of all cosmic rays, is dominated by negative electrons, produced presumably at pri..