J/ψ and ψ(2S) Radiative Transitions to ηc

Using 2.45 x 10(7) psi(2S) decays collected with the CLEO-c detector at the Cornell Electron Storage Ring we present the most precise measurements of magnetic dipole transitions in the charmonium system. We measure B(psi(2S) -> gamma eta(c)) = (4.32 +/- 0.16 +/- 0.60) x 10(-3), B(J/psi -> gamma eta(c))/B(psi(2s) -> gamma eta(c)) = 4.59 +/- 0.23 +/- 0.64, and B(J/psi -> gamma eta(c)) = (1.98 +/- 0.09 +/- 0.30)%. We observe a distortion in the eta(c) line shape due to the photon-energy dependence of the magnetic dipole transition rate. We find that measurements of the eta(c) mass are sensitive to the line shape, suggesting an explanation for the discrepancy between measurements of the eta(c) mass in radiative transitions and other production mechanisms.