Cosmic Ray Positrons at High Energies: A New Measurement

We present a new measurement of the cosmic-ray positron fraction $e^{+}/\left(e^{+}{+e}^{-}\right)$ 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 $\sim 10\mathrm{GeV}$ previously reported.