Evidence for $\hat{I}{\cdot }_c$(2S)$\to\pipieta$ decay

Abstract

The decay $η_c(2S)\to\pipieta$ is searched for through the radiative transition $\ddot{I}ˆ(3686)\to {\hat{I}}^3\hat{I}{\cdot }_c(2S)$ using 448 million $\ddot{I}ˆ$(3686) events accumulated at the BESIII detector. The first evidence of $\hat{I}{\cdot }_c(2S)\to \ddot{I}{€}^{+}\ddot{I}{€}^{-}\hat{I}\cdot$ is found with a statistical significance of 3.5$\ddot{I}ƒ$. The product of the branching fractions of $\ddot{I}ˆ(3686)\to {\hat{I}}^3\hat{I}{\cdot }_c(2S)$ and $η_c(2S)\to\pipieta$ is measured to be $Br(ψ(3686)\toγη_c(2S))\times Br(η_c(2S)\to\pipieta)=(2.97\pm0.81\pm0.26)\times10^{-6}$, where the first uncertainty is statistical and the second one is systematic. The branching fraction of the decay $η_c(2S)\to\pipieta$ is determined to be $Br(η_c(2S)\to\pipieta)=(42.4\pm11.6\pm3.8\pm30.3)\times10^{-4}$, where the third uncertainty is transferred from the uncertainty of the branching fraction of $\ddot{I}ˆ(3686)\to {\hat{I}}^3\hat{I}{\cdot }_c(2S)$.