Based on a sample of ${\eta }^{\prime }$ mesons produced in the radiative decay $J/\psi \to \gamma {\eta }^{\prime }$ in $1.31\times {10}^{9}J/\psi$ events collected with the BESIII detector, the decay ${\eta }^{\prime }\to \omega e^{+}{e}^{-}$ is observed for the first time, with a statistical significance of $8\sigma$. The branching fraction is measured to be $\mathcal{B}({\eta }^{\prime }\to \omega e^{+}{e}^{-})=(1.97\pm 0.34(\text{stat})\pm 0.17(\text{syst}))\times {10}^{-4}$, which is in agreement with theoretical predictions. The branching fraction of ${\eta }^{\prime }\to \omega \gamma$ is also measured to be $(2.55\pm 0.03(\text{stat})\pm 0.16(\text{syst}))\times {10}^{-2}$, which is the most precise measurement to date, and the relative branching fraction $\frac{\mathcal{B}({\eta }^{\prime }\to \omega e^{+}{e}^{-})}{\mathcal{B}({\eta }^{\prime }\to \omega \gamma )}$ is determined to be $(7.71\pm 1.34(\text{stat})\pm 0.54(\text{syst}))\times {10}^{-3}$.

• Received 26 July 2015

DOI:https://doi.org/10.1103/PhysRevD.92.051101