The η-meson mass

Abstract The mass of the η meson has been measured through the study of the d p→ 3 He η reaction near threshold, the beam having been calibrated through comparison with three other nuclear reactions. The value obtained is m η =547.30±0.15 MeV/ c 2 , where the error bar includes both systematic and statistical effects. This is much more precise and significantly lower than the Particle Data Group average, through it is consistent with a previous counter experiment.