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

The first direct, precision measurement of the branching ratio for the decay of the η meson into two photons, B(η→γγ)≡Γ(η→γγ)/${\mathrm{\Gamma }}_{\mathrm{tot}}$(η), is presented. The systematic undertainties are quite different than in previous indirect measurements. Using the SATURNE II proton beam and a liquid-deuterium target, the η mesons were produced in the reaction pd${\to }^3$He η at 1.5 MeV above threshold, and tagged by the detection of the recoil ${}_{}{}^3{}_{}{}^{}\mathrm{He}$ in the magnetic spectrometer SPES2. The two decay photons were detected in two BGO electromagnetic calorimeters located at 65° 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(η→γγ)=0.3949±0.0017(stat)±0.0030(syst). © 1995 The American Physical Society.