QCD analysis of electromagnetic Dalitz decays $J/\psi \to {\eta }^{(\prime )}{\ell }^{+}{\ell }^{-}$

Abstract

The electromagnetic Dalitz decays $J/\psi \to {\eta }^{(\prime )}{e}^{+}{e}^{-}$ with large recoil momentum are studied in the framework of perturbative QCD. Meanwhile, the soft contributions from the small recoil momentum region are described by the overlap of soft wave functions, and the resonance contributions are estimated by the vector meson dominance model. Based on this dynamical picture, the transition form factors $f_{\psi {\eta }^{(\prime )}}(q^2)$ in full kinematic region are calculated for the first time, and we find that the transition form factors are insensitive to the shapes of ${\eta }^{(\prime )}$ distribution amplitudes. Our prediction of the normalized transition form factor $F_{\psi \eta }(q^2)\equiv f_{\psi \eta }(q^2)/f_{\psi \eta }(0)$ agrees well with its experimental data. In addition, we also find that the branching ratios $\mathcal{B}(J/\psi \to {\eta }^{(\prime )}{e}^{+}{e}^{-})$ are dominated by the contributions of perturbative QCD, and the resonance contributions are negligibly small as well as the soft contributions due to the suppression of the kinematic factor. With all these contributions, our results of the branching ratios $\mathcal{B}(J/\psi \to {\eta }^{(\prime )}{e}^{+}{e}^{-})$ and the ratio $R_{J/\psi }^e=\mathcal{B}(J/\psi \to \eta e^{+}{e}^{-})/\mathcal{B}(J/\psi \to {\eta }^{\prime }{e}^{+}{e}^{-})$ are in good agreement with their experimental data. Using the obtained $F_{\psi {\eta }^{(\prime )}}(q^2)$, we give the predictions of the branching ratios $\mathcal{B}(J/\psi \to {\eta }^{(\prime )}{\mu }^{+}{\mu }^{-})$ and their ratio $R_{J/\psi }^{\mu }$.

arXiv comment28 pages, 6 figures and 2 tables

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