0 5 0 5 0 0 6 v 2 7 J u n 2 0 0 5 A precision determination of the mass of the η meson The GEM Collaboration :

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 → He + 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/c, Preprint submitted to Elsevier Science 19 November 2013 is significantly lower than that found by the recent NA48 measurement, though it is consistent with values obtained in earlier counter experiments.