HERA is Now Running with Longitudinally Polarized Positrons

Recently there have been some interesting developments at HERA, the Hadron‐Electron Ring Accelerator that wends its circular way for 6 kilometers beneath the streets and parks of Hamburg. Since the fall of 1992 this uniquely asymmetric pair of storage rings has been providing experimenters with collisions between 820‐GeV protons and 30‐GeV electrons. (See PHYSICS TODAY March 1992, page 21.) Since July, however, HERA has been running with positrons instead of electrons, and will continue to do so at least until the end of 1995. And more importantly, the circulating HERA positron (or electron) beam can now be longitudinally polarized at will. That's an important first: No other electron storage ring has ever achieved longitudinal polarization.