Based on a data sample of $(27.08 \pm 0.14 ) \times 10^8~\psi(3686)$ events collected with the BESIII detector at the BEPCII collider, the M1 transition $\psi(3686) \to \gamma \eta_{ c}(2S)$ with $\eta_{ c}(2S) \to K\bar{ K}\pi$ is studied, where $K\bar{ K}\pi$ is $K^{ +} K^{ -} \pi^{ 0}$ or $K_{ S}^{ 0}K^{ \pm}\pi^{ \mp}$. The mass and width of the $\eta_{ c}(2S)$ are measured to be $(3637.8 \pm 0.8 (\rm { stat}) \pm 0.2 (\rm { syst}))$ MeV/$c^{ 2}$ and $(10.5 \pm 1.7 (\rm { stat}) \pm 3.5 (\rm { syst}))$ MeV, respectively. The product branching fraction $\mathcal{ B}\left(\psi(3686) \rightarrow \gamma \eta_{ c}(2 S)\right) \times \mathcal{ B}(\eta_{ c}(2 S) \rightarrow K \bar{ K} \pi)$ is determined to be $(0.97 \pm 0.06 (\rm { stat}) \pm 0.09 (\rm { syst})) \times 10^{ -5}$. Using $\mathcal{ BR}(\eta_{ c}(2S)\to K\bar{ K}\pi)=(1.86^{ +0.68}_{ -0.49})\%$, we obtain the branching fraction of the radiative transition to be $\mathcal{ BR}(\psi(3686) \to \gamma \eta_{ c}(2S)) = (5.2 \pm 0.3 (\rm { stat}) \pm 0.5 (\rm { syst}) ^{ +1.9}_{ -1.4} (extr)) \times 10^{ -4}$, where the third uncertainty is due to the quoted $\mathcal{ BR}(\eta_{ c}(2S) \to K\bar{ K}\pi)$.
Categories: hep-ex