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Measurement of the<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mi>η</mml:mi></mml:math>-Meson Mass Using<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mi>ψ</mml:mi><mml:mo stretchy="false">(</mml:mo><mml:mn>2</mml:mn><mml:mi>S</mml:mi><mml:mo stretchy="false">)</mml:mo><mml:mo>→</mml:mo><mml:mi>η</mml:mi><mml:mi>J</mml:mi><mml:mo>/</mml:mo><mml:mi>ψ</mml:mi></mml:math>
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 modes η→γγ, 3π0, π+π−π0, and π+π−γ, we find Mη=547.785±0.017±0.057 MeV, in which the first uncertainty is statistical and the second systematic. This result has …