Physics with the Crystal Ball Detector

At the 1974 PEP Summer Study, one of the projects was to explore the possibilities and limitations of detectors optimized to measure photons produced in high energy e^+ e^- collisions. It was realized that a device that had high detection efficiency over a large solid angle and that could measure the energy of photons in the region above a few tens of MeV with high precision (in the range of a few percent) would provide a unique capability offered by no existing apparatus. Thus it could possibly yield important and otherwise unattainable information about these fundamental interactions. Furthermore, if it also measured the directions of both photons and charged particles well enough, even a nonmagnetic version of such a device would be able to compete with the large general-purpose magnetic spectrometers then in existence in the reconstruction of certain simple, few-particle final states. And finally, a device designed to absorb all the electromagnetic energy in an event would in fact quickly and directly measure a large fraction of its total energy. This prompt information could form the basis for an admirable trigger having very different biases from those used by the magnetic spectrometers. Thus such a device would be an interesting complementary technique for the investigation of e^+ e^- physics. In particular an efficient "all-neutral" trigger would be possible. Although the thrust of the summer's work had been directed toward

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