Observation of the Dalitz Decay $\hat{I}{\cdot }^{\prime }\to {\hat{I}}^3{e}^{+}{e}^{-}$

Abstract

We report the first observation of the Dalitz decay $\hat{I}{\cdot }^{\prime }\to {\hat{I}}^3{e}^{+}{e}^{-}$, based on a data sample of 1.31 billion $J/\ddot{I}ˆ$ events collected with the BESIII detector. The $\hat{I}{\cdot }^{\prime }$ mesons are produced via the $J/\ddot{I}ˆ\to {\hat{I}}^3\hat{I}{\cdot }^{\prime }$ decay process. The ratio $\hat{I}“(\hat{I}{\cdot }^{\prime }\to {\hat{I}}^3{e}^{+}{e}^{-})/\hat{I}“(\hat{I}{\cdot }^{\prime }\to {\hat{I}}^3{\hat{I}}^3)$ is measured to be $(2.13\pm 0.09(\text{stat.})\pm 0.07(\text{sys.}))\times 10^{-2}$. This corresponds to a branching fraction $\mathcal{B}(\hat{I}{\cdot }^{\prime }\to {\hat{I}}^3{e}^{+}{e}^{-})=(4.69\pm 0.20(\text{stat.})\pm 0.23(\text{sys.}))\times 10^{-4}$. The transition form factor is extracted and different expressions are compared to the measured dependence on the $e^{+}{e}^{-}$ invariant mass. The results are consistent with the prediction of the Vector Meson Dominance model.

11 pages,7 figures