Observation of $psi(3686) to eta e^+ e^- $

Using a data sample of $448.1 times 10^6$ $psi(3686)$ events collected with the BESIII detector at the BEPCII collider, we report the first observation of the electromagnetic Dalitz decay $psi(3686) to eta e^+ e^-$, with significances of 7.0$sigma$ and 6.3$sigma$ when reconstructing the $eta$ meson via its decay modes $etatogamma pi^+ pi^-$ and $etatopi^+pi^-eta$ ($eta to gammagamma$), respectively. The weighted average branching fraction is determined to be $mathcal{B}(psi(3686) to eta e^+ e^-)= (1.90 pm 0.25 pm 0.11) times 10^{-6}$, where the first uncertainty is statistical and the second systematic.