The numbers of ψ(3686) events accumulated by the BESIII detector for the data taken during 2009 and 2012 are determined to be (107.0±0.8)×10 6 and (341.1±2.1)×10 6 , respectively, by counting inclusive hadronic events, where the uncertainties are systematic and the statistical uncertainties are negligible. The number of events for the sample taken in 2009 is consistent with that of the previous measurement. The total number of ψ(3686) events for the two data taking periods is (448.1±2.9)×10 6 .

Observation of e + e − → π 0 π 0 h c and a neutral charmoniumlike structure Z c (4020)

We report the first observation of the Dalitz decay η 0 → γe þ e − , based on a data sample of 1.31 billion J=ψ events collected with the BESIII detector. The η 0 mesons are produced via the J=ψ → γη 0 decay process. The ratio Γðη 0 → γe þ e − Þ=Γðη 0 → γγÞ is measured to be ð2.13 AE 0.09ðstatÞ AE 0.07ðsysÞÞ × 10 −2 . This corresponds to a branching fraction Bðη 0 → γe þ e − Þ ¼ ð4.69 AE 0.20ðstatÞ AE 0.23ðsysÞÞ × 10 −4 . The transition form factor is extracted and different expressions are compared to the measured dependence on the e þ e − invariant mass. The results are consistent with the prediction of the vector meson dominance model.

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