Observation of the hindered electromagnetic Dalitz decay $\psi (3686)\to e^{+}{e}^{-}{\eta }_c$

Using a data sample of $(448.1\pm 2.9)\times {10}^6\psi (3686)$ decays collected at an $e^{+}{e}^{-}$ center-of-mass energy of $3.686\mathrm{G}\mathrm{e}\mathrm{V}$ by the BESIII detector at BEPCII, we report an observation of the hindered electromagnetic Dalitz decay $\psi (3686)\to e^{+}{e}^{-}{\eta }_c$ with a significance of $7.9\sigma$. The branching fraction is determined to be $\mathcal{B}(\psi (3686)\to e^{+}{e}^{-}{\eta }_c)=(3.77\pm {0.40}_{\mathrm{s}\mathrm{t}\mathrm{a}\mathrm{t}\mathrm{.}}\pm {0.18}_{\mathrm{s}\mathrm{y}\mathrm{s}\mathrm{t}\mathrm{.}})\times {10}^{-5}$, agreeing well with the prediction of the vector meson dominance model. This is the first measurement of the electromagnetic Dalitz transition between the $\psi (3686)$ and the ${\eta }_c$, which provides new insight into the electromagnetic properties of this decay, and offers new opportunities to measure the absolute branching fractions of ${\eta }_c$ decays.