Observation of the Dalitz Decay ${\eta }^{\prime }\to \gamma e^{+}{e}^{-}$

Abstract

We report the first observation of the Dalitz decay ${\eta }^{\prime }\to \gamma e^{+}{e}^{-}$, based on a data sample of 1.31 billion $J/\psi$ events collected with the BESIII detector. The ${\eta }^{\prime }$ mesons are produced via the $J/\psi \to \gamma {\eta }^{\prime }$ decay process. The ratio $\Gamma ({\eta }^{\prime }\to \gamma e^{+}{e}^{-})/\Gamma ({\eta }^{\prime }\to \gamma \gamma )$ 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 $\mathcal{B}({\eta }^{\prime }\to \gamma e^{+}{e}^{-})=(4.69\pm 0.20(\text{stat.})\pm 0.23(\text{sys.}))\times 10^{-4}$. 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

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