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

Abstract

We report the first measurement of the di-electron invariant mass dependent transition form factor in the electromagnetic Dalitz decay $J/\psi \to e^+e^- \pi^0$ using $(10087 \pm 44) \times 10^6$ $J/\psi$ 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 $(8.06 \pm 0.31 (\rm{stat}) \pm 0.38 (\rm{syst}))\times 10^{-7}$, approximately twice the non-resonant VMD prediction.