Study of the electromagnetic Dalitz decay $ J/\psi \to {e}^{+}{e}^{-}{\pi }^{0}$

We report the first measurement of the dielectron invariant mass dependent transition form factor in the electromagnetic Dalitz decay $ J/\psi \to {e}^{+}{e}^{-}{\pi }^{0}$ using $ (10087\pm 44)\times {10}^{6}\text{}\text{}J/\psi $ events collected by the BESIII detector. A clear $ \rho -\omega $ 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 ($ {a}_{\mu }$) and refine the predictions of the vector meson dominance (VMD) model and hadronic light-by-light contribution to $ {a}_{\mu }$. By taking into account the contribution of this $ \rho -\omega $ interference structure, the branching fraction of $ J/\psi \to {e}^{+}{e}^{-}{\pi }^{0}$ in the full $ {e}^{+}{e}^{-}$ invariant mass range is also measured for the first time to be $ (8.06\pm 0.31(\mathrm{stat})\pm 0.38(\mathrm{syst}))\times {10}^{-7}$, approximately twice the nonresonant VMD prediction.