A precision determination of the mass of the η meson

Several processes of meson production in proton-deuteron collisions have been measured simultaneously using a calibrated magnetic spectrograph. Among these processes, the η meson is seen clearly as a sharp missing-mass peak on a slowly varying background in the p+d→He3+X reaction. Knowing the kinematics of the other reactions with well determined masses, it is possible to deduce a precise mass for the η meson. The final result, m(η)=547.311±0.028(stat)±0.032(syst)MeV/c2, is significantly lower than that found by the recent NA48 measurement, though it is consistent with values obtained in earlier counter experiments.