Measurement of the η-meson mass using ψ(2s) →ηj/ψ

We measure the mass of the η meson using ψ(2S) →ηJ/ψ events acquired with the CLEO-c detector operating at the CESR e + e - collider. Using the four decay modesη→ yy, 3π 0 , π + π - π 0 , and π + π - γ, we find M η = 547.785 ± 0.017 ± 0.057 MeV, in which the first uncertainty is statistical and the second systematic. This result has an uncertainty comparable to the two most precise previous measurements and is consistent with that of NA48, but is inconsistent at the level of 6.5σ with the much smaller mass obtained by GEM.