Og 7 . 1 . 3 Cosmic - Ray Positrons : Are There Primary Sources ?

The HEAT instrument has detected cosmic-ray electrons and positrons in two balloon ights, 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 ?) cosmic-ray ux, amounting to 1-2% of all cosmic rays, is dominated by negative electrons, produced presumably at primary acceleration sites. A small additional component exists, amounting to roughly 10% of the total electronic ux, and consisting of equal numbers of electrons and positrons. The origin of this component lies possibly in the decay of secondaries produced in collisions of hadronic cosmic rays with the dilute interstellar gas. Several measurements of the cosmic-ray positron fraction e + =(e + + e ?) have been made. Below about 1 GeV, the eeects of solar modulation on the interstellar electron and positron uxes and large atmospheric corrections render the interpretation of the measurements diicult. At intermediate energies, between about 1 and 10 GeV, new measurements (Barbiellini et al. 1996, Barwick et al. 1997) of the positron fraction indicate a slight excess over and above the predictions of Protheroe 1982, where all positrons are assumed to be of secondary origin and to propagate according to the prescription of the leaky-box model. This eeect has been predicted (Clem et al. 1996) as a consequence of charge sign-dependent solar modulation. At energies around 10 GeV and above, a signiicant excess of positrons over the fraction expected from secondary sources has been reported