Updated measurements of the M1 transition $\psi (3686)\to \gamma {\eta }_c(2S)$ with ${\eta }_c(2S)\to K\bar{K}\pi$

Abstract

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 $(3637.8\pm 0.8(\mathrm{stat})\pm 0.2(\mathrm{syst}))$ MeV/$c^2$ and $(10.5\pm 1.7(\mathrm{stat})\pm 3.5(\mathrm{syst}))$ MeV, respectively. The product branching fraction $\mathcal{B}\left(\psi (3686)\to \gamma {\eta }_c(2S)\right)\times \mathcal{B}({\eta }_c(2S)\to K\bar{K}\pi )$ is determined to be $(0.97\pm 0.06(\mathrm{stat})\pm 0.09(\mathrm{syst}))\times 10^{-5}$. Using $\mathcal{B}\mathcal{R}({\eta }_c(2S)\to K\bar{K}\pi )=(1.86_{-0.49}^{+0.68})\%$, we obtain the branching fraction of the radiative transition to be $\mathcal{B}\mathcal{R}(\psi (3686)\to \gamma {\eta }_c(2S))=(5.2\pm 0.3(\mathrm{stat})\pm 0.5(\mathrm{syst}{)}_{-1.4}^{+1.9}(\mathrm{extr}))\times 10^{-4}$, where the third uncertainty is due to the quoted $\mathcal{B}\mathcal{R}({\eta }_c(2S)\to K\bar{K}\pi )$.

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