Improved measurements of the Dalitz decays $\eta /{\eta }^{\prime }\to \gamma e^{+}{e}^{-}$

Abstract

Based on a data sample of 10 billion $J/\psi$ events collected with the BESIII detector, improved measurements of the Dalitz decays $\eta /{\eta }^{\prime }\to \gamma e^{+}{e}^{-}$ are performed, where the $\eta$ and ${\eta }^{\prime }$ are produced through the radiative decays $J/\psi \to \gamma \eta /{\eta }^{\prime }$. The branching fractions of $\eta \to \gamma e^{+}{e}^{-}$ and ${\eta }^{\prime }\to \gamma e^{+}{e}^{-}$ are measured to be $(7.07\pm 0.05\pm 0.23)\times 10^{-3}$ and $(4.83\pm 0.07\pm 0.14)\times 10^{-4}$, respectively. Within the single pole model, the parameter of electromagnetic transition form factor for $\eta \to \gamma e^{+}{e}^{-}$ is determined to be ${\Lambda }_{\eta }=(0.749\pm 0.027\pm 0.007)\mathrm{GeV}/c^2$. Within the multi-pole model, we extract the electromagnetic transition form factors for ${\eta }^{\prime }\to \gamma e^{+}{e}^{-}$ to be ${\Lambda }_{{\eta }^{\prime }}=(0.802\pm 0.007\pm 0.008)\mathrm{GeV}/c^2$ and ${\gamma }_{{\eta }^{\prime }}=(0.113\pm 0.010\pm 0.002)\mathrm{GeV}/c^2$. The results are consistent with both theoretical predictions and previous measurements. The characteristic sizes of the interaction regions for the $\eta$ and ${\eta }^{\prime }$ are calculated to be $(0.645\pm 0.023\pm 0.007)\mathrm{fm}$ and $(0.596\pm 0.005\pm 0.006)\mathrm{fm}$, respectively. In addition, we search for the dark photon in $\eta /{\eta }^{\prime }\to \gamma e^{+}{e}^{-}$, and the upper limits of the branching fractions as a function of the dark photon are given at 90\% confidence level.

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Cited by in corpus (2)

• Precision evaluation of the $\eta$- and ${\eta }^{\prime }$-pole contributions to hadronic light-by-light scattering in the anomalous magnetic moment of the muon
• Measurement of the $\eta$ transition form factor through ${\eta }^{\prime }\to {\pi }^{+}{\pi }^{-}\eta$ decay