Experiments of Crystal Ball Collaboration at Bnl

The experimental program "Baryon spectroscopy with the Crystal Ball" was under way in 1998–2002 at the Alternating Gradient Synchrotron AGS of Brookhaven National Laboratory, USA. The main goal of this program was to map up the existence of all baryon resonances and to study their characteristics with a high precision. The program has been carried out with the team, the Crystal Ball Collaboration, including 14 institutions from 5 countries. Physicists of PNPI participated in this collaboration playing an essential role. Almost all our information on the N and Δ resonances comes from πN data, mainly from elastic scattering, in the form of cross section and polarization measurements. The data are synthesized by a partialwave analysis. The end products are the various partial waves; the Argand diagram and speed plot of each wave are used to identify the resonances. Till starting the experiments at BNL, the πN data sets in certain energy regions were incomplete and it was easy for a resonance with a small elasticity to be missed. One of the most evident shortcomings of the existed data-base was a lack of precise and systematic data on πp reaction with neutral particles in the final state: πp → γn, πp → πn, πp → ππn, πp → ηn. The experiment E913 at the AGS was concerned with the systematic measurement of neutral final states in πp interactions in the region of low-lying πN resonances; the experiment covered the N and Δ resonances using π-beams. The multi-photon Crystal Ball detector was used for registering photons and neutral mesons in this experiment. This detector has a good energy and angular resolution. The Crystal Ball made it possible to study neutral baryon spectroscopy on a scale not seen before.