THE ETA-MESON MASS

The mass of the eta-meson has been measured through the study of the d p --> He-3 eta-reaction near threshold, the beam having been calibrated through comparison with three other nuclear reactions. The value obtained is m(eta) = 547.30 +/- 0.15 MeV/c2, where the error bar includes both systematic and statistical effects. This is much more precise and significantly lower than the Particle Data Group average, though it is consistent with a previous counter experiment.