Observation of the hindered electromagnetic Dalitz decay $\ddot{I}ˆ(3686)\to e^{+}{e}^{-}\hat{I}{\cdot }_c$

Abstract

Using a data sample of $(448.1\pm 2.9)\times 10^6\ddot{I}ˆ(3686)$ decays collected at an $e^{+}{e}^{-}$ center-of-mass energy of $3.686\mathrm{GeV}$ by the BESIII detector at BEPCII, we report an observation of the hindered electromagnetic Dalitz decay $\ddot{I}ˆ(3686)\to e^{+}{e}^{-}\hat{I}{\cdot }_c$ with a significance of $7.9\ddot{I}ƒ$. The branching fraction is determined to be $\mathcal{B}(\ddot{I}ˆ(3686)\to e^{+}{e}^{-}\hat{I}{\cdot }_c)=(3.77\pm 0.40_{stat.}\pm 0.18_{syst.})\times 10^{-5}$, agreeing well with the prediction of the vector meson dominance model. This is the first measurement of the electromagnetic Dalitz transition between the $\ddot{I}ˆ(3686)$ and the $\hat{I}{\cdot }_c$, which provides new insight into the electromagnetic properties of this decay, and offers new opportunities to measure the absolute branching fractions of $\hat{I}{\cdot }_c$ decays.

8 pages, 2 figures