Study of the electromagnetic Dalitz decay J=ψ → e+e−π0

We report the first measurement of the dielectron invariant mass dependent transition form factor in the electromagnetic Dalitz decay J=ψ → eþe−π0 using ð10087 ± 44Þ × 106 J=ψ events collected by the BESIII detector. A clear ρ − ω interference structure is observed, consistent with the pion form factor, which offers a novel approach to extract the hadronic vacuum polarization contribution to the anomalous muon magnetic moment (aμ) and refine the predictions of the vector meson dominance (VMD) model and hadronic light-by-light contribution to aμ. By taking into account the contribution of this ρ − ω interference structure, the branching fraction of J=ψ → eþe−π0 in the full eþe− invariant mass range is also measured for the first time to be ð8.06 ± 0.31ðstatÞ ± 0.38ðsystÞÞ × 10−7, approximately twice the nonresonant VMD prediction.