Measurement of $J/\psi \to \gamma {\eta }_{\mathrm{c}}$ decay rate and ${\eta }_{\mathrm{c}}$ parameters at KEDR

Abstract

Using the inclusive photon spectrum based on a data sample collected at the $J/\psi$ peak with the KEDR detector at the VEPP-4M $e^{+}{e}^{-}$ collider, we measured the rate of the radiative decay $J/\psi \to \gamma {\eta }_{\mathrm{c}}$ as well as ${\eta }_{\mathrm{c}}$ mass and width. Taking into account an asymmetric photon lineshape we obtained ${\Gamma }_{\gamma {\eta }_{\mathrm{c}}}^0=2.98\pm 0.18{}_{-0.33}^{+0.15}$ keV, $M_{{\eta }_{\mathrm{c}}}=2983.5\pm 1.4{}_{-3.6}^{+1.6}$ MeV/$c^2$, ${\Gamma }_{{\eta }_{\mathrm{c}}}=27.2\pm 3.1{}_{-2.6}^{+5.4}$ MeV.

6 pages, 3 figures

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