First observation of the electromagnetic Dalitz decay $\psi (3686)\to {\mu }^{+}{\mu }^{-}{\eta }^{\prime }$

Abstract

Utilizing $(2712.4\pm 14.3)\times 10^6\psi (3686)$ events collected by the BESIII detector at the symmetric $e^{+}{e}^{-}$ collider BEPCII, we report the first observation of the electromagnetic Dalitz decay $\psi (3686)\to {\mu }^{+}{\mu }^{-}{\eta }^{\prime }$ with a statistical significance of 6.1$\sigma$. The branching fraction is determined to be $\mathcal{B}(\psi (3686)\to {\mu }^{+}{\mu }^{-}{\eta }^{\prime })=(4.1\pm 1.0_{stat.}\pm 0.4_{syst.})\times 10^{-7}$. The ratio to the branching fraction of the radiative decay $\psi (3686)\to \gamma {\eta }^{\prime }$ is estimated to be $(3.3\pm 0.9)\times 10^{-3}$, which is consistent with the prediction of the vector meson dominance model within $1\sigma$. Furthermore, using the branching fraction of $\psi (3686)\to e^{+}{e}^{-}{\eta }^{\prime }$ previously measured by the BESIII experiment, the ratio between the muon and the electron channels is evaluated to be $0.22\pm 0.07$, which is consistent with the calculation of the vector meson dominance model within $1\sigma$, and no significant violation of lepton flavor universality is found.

11 page, 2 figures