Direct measurement of the branching ratio for the decay of the eta meson into two photons

The first direct, precision measurement of the branching ratio for the decay of the eta meson into two photons, B(eta --> gamma gamma) = Gamma(eta --> gamma gamma)/Gamma(tot)(eta), is presented. The systematic uncertainties are quite different than in previous indirect measurements. Using the SATURNE II proton beam and a liquid-deuterium target, the 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 two EGO electromagnetic calorimeters located at 65 degrees left and right of the beam axis. A lead collimator placed in front of the right calorimeter defined the detector acceptance. The analysis yielded B(eta --> gamma gamma) = 0.3949 +/- 0.0017(stat) +/- 0.0030(syst).