Observation of the Dalitz decayη′→γe+e−

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Abstract

We report the first observation of the Dalitz decay ${\ensuremath{\eta}}^{\ensuremath{'}}\ensuremath{\rightarrow}\ensuremath{\gamma}{e}^{+}{e}^{\ensuremath{-}}$, based on a data sample of 1.31 billion $J/\ensuremath{\psi}$ events collected with the BESIII detector. The ${\ensuremath{\eta}}^{\ensuremath{'}}$ mesons are produced via the $J/\ensuremath{\psi}\ensuremath{\rightarrow}\ensuremath{\gamma}{\ensuremath{\eta}}^{\ensuremath{'}}$ decay process. The ratio $\mathrm{\ensuremath{\Gamma}}({\ensuremath{\eta}}^{\ensuremath{'}}\ensuremath{\rightarrow}\ensuremath{\gamma}{e}^{+}{e}^{\ensuremath{-}})/\mathrm{\ensuremath{\Gamma}}({\ensuremath{\eta}}^{\ensuremath{'}}\ensuremath{\rightarrow}\ensuremath{\gamma}\ensuremath{\gamma})$ is measured to be $(2.13\ifmmode\pm\else\textpm\fi{}0.09(\text{stat})\ifmmode\pm\else\textpm\fi{}0.07(\text{sys}))\ifmmode\times\else\texttimes\fi{}1{0}^{\ensuremath{-}2}$. This corresponds to a branching fraction $\mathcal{B}({\ensuremath{\eta}}^{\ensuremath{'}}\ensuremath{\rightarrow}\ensuremath{\gamma}{e}^{+}{e}^{\ensuremath{-}})=(4.69\ifmmode\pm\else\textpm\fi{}0.20(\text{stat})\ifmmode\pm\else\textpm\fi{}0.23(\text{sys}))\ifmmode\times\else\texttimes\fi{}1{0}^{\ensuremath{-}4}$. The transition form factor is extracted and different expressions are compared to the measured dependence on the ${e}^{+}{e}^{\ensuremath{-}}$ invariant mass. The results are consistent with the prediction of the vector meson dominance model.

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