Observation of the Dalitz Decay $eta to gamma e^+e^-$

We report the first observation of the Dalitz decay $eta to gamma e^+e^-$, based on a data sample of 1.31 billion $J/psi$ events collected with the BESIII detector. The $eta$ mesons are produced via the $J/psi to gamma eta$ decay process. The ratio $Gamma(eta to gamma e^+ e^-)/Gamma(etatogammagamma)$ is measured to be $(2.13pm0.09(text{stat.})pm0.07(text{sys.}))times10^{-2}$. This corresponds to a branching fraction ${cal B}(eta to gamma e^+e^-)= (4.69 pm0.20(text{stat.})pm0.23(text{sys.}))times10^{-4}$. 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.

Observation of Electromagnetic Dalitz decays J/psi to P e^+e^-

Based on a sample of (225.3pm2.8)times 10^{6} J/psi events collected with the BESIII detector, the electromagnetic Dalitz decays of J/psi to P e^+e^-(P=eta/eta/pi^0) are studied. By reconstructing the pseudoscalar mesons in various decay modes, the decays J/psi to eta e^+e^-, J/psi to eta e^+e^- and J/psi to pi^0 e^+e^- are observed for the first time. The branching fractions are determined to be mathcal{B}(J/psito eta e^+e^-) = (5.81pm0.16pm0.31)times10^{-5}, mathcal{B}(J/psito eta e^+e^-) = (1.16pm0.07pm0.06)times10^{-5}, and mathcal{B}(J/psito pi^0 e^+e^-)=(7.56pm1.32pm0.50)times10^{-7}, where the first errors are statistical and the second ones systematic.