Antiparticles

Nearly a half century after the discovery of the antiproton the study of cosmic-ray antimatter continues to be an exciting and fertile field. Sensitive searches for heavy cosmic-ray antimatter continue, although in recent years their value as a probe of universal baryon symmetry has all but evaporated. Antiprotons and positrons have opened new windows on the origin and history of cosmic rays. The rarity of antimatter as compared to ordinary cosmic-ray species has posed substantial experimental challenges. Early reports of significant enhancements of antiprotons and high-energy positrons fueled speculation that non-baryonic dark matter had been found. A new generation of balloon-borne magnetic spectrometers employing powerful particle identification techniques to eliminate background have finally managed to uncover the true antimatter signal. These new measurements support simple models of secondary production but also suggest the possibility of a small yet interesting primary component.