DIRECT MEASUREMENT OF THE BRANCHING RATIO FOR THE DECAY OF THE ETA-MESON INTO 2 PHOTONS

The first direct, precision measurement of the branching ratio for the decay of the eta meson into two photons, B(eta --> gamma gamma) = Gam ma(eta --> gamma gamma)/Gamma(tot)(eta), is presented. The systematic uncertainties are quite different than in previous indirect measuremen ts. Using the SATURNE II proton beam and a liquid-deuterium target, th e eta mesons were produced in the reaction pd --> He-3 eta at 1.5 MeV above threshold, and tagged by the defection of the recoil He-3 in the magnetic spectrometer SPES2. The two decay photons were detected in t wo EGO electromagnetic calorimeters located at 65 degrees left and rig ht of the beam axis. A lead collimator placed in front of the right ca lorimeter defined the detector acceptance. The analysis yielded B(eta --> gamma gamma) = 0.3949 +/- 0.0017(stat) +/- 0.0030(syst).