Observation of $\hat{I}{\cdot }_c$(1S, 2S) and $\ddot{I}{‡}_{cJ}$ decays to 2$(\ddot{I}{€}^{+}\ddot{I}{€}^{-})\hat{I}\cdot$ via $\ddot{I}ˆ$(3686) radiative transitions

Abstract

Based on $2.7\times 10^9\ddot{I}ˆ(3686)$ decays collected with the BESIII detector, the radiative decay $\ddot{I}ˆ(3686)\to \gamma 2(\ddot{I}{€}^{+}\ddot{I}{€}^{-})\hat{I}\cdot$ is investigated to measure properties of S- and P-wave charmonium states. The branching fraction of the decay $\hat{I}{\cdot }_c(1S)\to 2(\ddot{I}{€}^{+}\ddot{I}{€}^{-})\hat{I}\cdot$, which is found to have a strong dependence on the interference pattern between $\hat{I}{\cdot }_c(1S)$ and non-$\hat{I}{\cdot }_c(1S)$ processes, is measured in both destructive and constructive interference scenarios for the first time. The mass and width of the $\hat{I}{\cdot }_c(1S)$ are measured to be $M=(2984.14\pm 0.13\pm 0.38)$ MeV/$c^2$ and $\hat{I}“=(28.82\pm 0.11\pm 0.82)$ MeV, respectively. Clear signals for the decays of the $\ddot{I}{‡}_{cJ}(J=0,1,2)$ and the $\hat{I}{\cdot }_c(2S)$ to $2(\ddot{I}{€}^{+}\ddot{I}{€}^{-})\hat{I}\cdot$ are also observed for the first time, and the corresponding branching fractions are measured. The ratio of the branching fractions between the $\hat{I}{\cdot }_c(2S)$ and $\hat{I}{\cdot }_c(1S)$ decays is significantly lower than the theoretical prediction, which might suggest different dynamics in their decays.