Measurement of the η′-Meson Mass using J/ψ→γη′

We measure the mass of the η^′ meson using ψ(2S)→π⁺π^-J/ψ, J/ψ→γη^′ events acquired with the CLEO-c detector operating at the CESR e⁺e^- collider. Using three decay modes, η^′→ρ⁰γ, η^′→π⁺π^-η with η→γγ, and η^′→π⁺π^-η with η→π⁺π^-π⁰, we find M_η^′=957.793±0.054±0.036   MeV, in which the uncertainties are statistical and systematic, respectively. This result is consistent with but substantially more precise than the current world average.