Observation of ψ(3686)→η′e+e−

Using a data sample of 448.1×106ψ(3686) events collected with the BESIII detector at the BEPCII collider, we report the first observation of the electromagnetic Dalitz decay ψ(3686)→η′e+e−, with significances of 7.0σ and 6.3σ when reconstructing the η′ meson via its decay modes η′→γπ+π− and η′→π+π−η (η→γγ), respectively. The weighted average branching fraction is determined to be B(ψ(3686)→η′e+e−)=(1.90±0.25±0.11)×10−6, where the first uncertainty is statistical and the second systematic.