Observation of the electromagnetic Dalitz transition $J/ψ\to e^+ e^- η_c$

Using $(10.087\pm0.044)\times10^9$ $J/ψ$ events collected with the BESIII detector at the $e^+e^-$ BEPCII collider, we present the first observation of the electromagnetic Dalitz decay $J/ψ\to e^+ e^- η_c$. The relative branching fraction $R \equiv \frac{Γ(J/ψ\to e^+e^-η_c)}{Γ(J/ψ\to γη_c)}$ is dete