Study of the electromagnetic Dalitz decay $J/\ddot{I}ˆ\to e^{+}{e}^{-}\ddot{I}{€}^0$

Abstract

We report the first measurement of the di-electron invariant mass dependent transition form factor in the electromagnetic Dalitz decay $J/\ddot{I}ˆ\to e^{+}{e}^{-}\ddot{I}{€}^0$ using $(10087\pm 44)\times 10^6$ $J/\ddot{I}ˆ$ events collected by the BESIII detector. A clear $\ddot{I}-\ddot{I}‰$ 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 ($a_{\hat{I}}¼$) and refine the predictions of the Vector Meson Dominance (VMD) model and hadronic light-by-light contribution to $a_{\hat{I}}¼$. By taking into account the contribution of this $\ddot{I}-\ddot{I}‰$ interference structure, the branching fraction of $J/\ddot{I}ˆ\to e^{+}{e}^{-}\ddot{I}{€}^0$ 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 non-resonant VMD prediction.

9 pages, 4 figures