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

Using a data sample of $\left(448.1±2.9\right)×{10}^{6}\text{}\text{}\psi \left(3686\right)$ decays collected at an ${e}^{+}{e}^{-}$ center-of-mass energy of 3.686 GeV by the BESIII detector at Beijing Electron Positron Collider II, we report an observation of the hindered electromagnetic Dalitz decay $\psi \left(3686\right)\to {e}^{+}{e}^{-}{\eta }_{c}$ with a significance of $7.9\sigma $. The branching fraction is determined to be $B\left(\psi \left(3686\right)\to {e}^{+}{e}^{-}{\eta }_{c}\right)=\left(3.77±0.4{0}_{\mathrm{stat}}±0.1{8}_{\mathrm{syst}}\right)×{10}^{-5}$, agreeing well with the prediction of the vector meson dominance model. This is the first measurement of the electromagnetic Dalitz transition between the $\psi \left(3686\right)$ 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.