Book Description

This dissertation presents the results of a measurement of the branching ratio for the rare decay eta → pi0gammagamma. The experiment was carried out in the A2 hall of the Mainz Microtron facility at the Institut fur Kernphysik, on the campus of Johannes Gutenberg Universitat Mainz, in Mainz, Germany. The experiment used the Glasgow-Mainz Tagger, which is a recoil-electron spectrometer, to determine the energy of the incident photons. The principal detector is the Crystal Ball, a highly segmented multiphoton spectrometer covering nearly 4pi steradians surrounding the experimental target. There is also a forward detector, TAPS, which is a multiphoton spectrometer arranged as a downstream wall of detectors. Furthermore, our setup includes an instrument used to differentiate between charged and neutral particles called the Particle Identification Detector, and a liquid hydrogen target. The kinematic-fit technique was used to select the eta → pi 0gammagamma events. The major backgrounds, namely eta → 3pi0 and eta → gammagamma decay and 2pi 0 production were measured simultaneously. The result for the branching ratio is BR(eta → pi0gammagamma) = (2.0+/-0.7) x 10-4. This corresponds to a partial width of Gamma (eta → pi0gammagamma) = 0.26 +/- 0.10 eV. This result is somewhat smaller than the result of recent experiments, it is consistent with theoretical calculations based on Chiral Perturbation Theory.