Observation of the dalitz decay η’→γe⁺e⁻

We report the first observation of the Dalitz decay η’→γe⁺e⁻, based on a data sample of 1.31 billion J/ψ events collected with the BESIII detector. The η’ mesons are produced via the J/ψ→γη’ decay process. The ratio Γ(η’→γe⁺e⁻)/Γ(η’→γγ) is measured to be (2.13±0.09(stat)±0.07(sys))×10⁻². This corresponds to a branching fraction B(η’→γe⁺e⁻)=(4.69±0.20(stat)±0.23(sys))×10⁻⁴. The transition form factor is extracted and different expressions are compared to the measured dependence on the e⁺e⁻ invariant mass. The results are consistent with the prediction of the vector meson dominance model.