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 $J/\psi\to\gamma\eta_{ \rm c}$ as well as $\eta_{ \rm c}$ mass and width. Taking into account an asymmetric photon lineshape we obtained $\Gamma^0_{ \gamma\eta_{ \rm c}}=2.98\pm0.18 \phantom{ |}^{ +0.15}_{ -0.33}$ keV, $M_{ \eta_{ \rm c}} = 2983.5 \pm 1.4 \phantom{ |}^{ +1.6}_{ -3.6}$ MeV/$c^2$, $\Gamma_{ \eta_{ \rm c}} = 27.2 \pm 3.1 \phantom{ |}^{ +5.4}_{ -2.6}$ MeV.
Categories: hep-ex