Observation of the dalitz decay ?’??e+e?

Abstract

We report the first observation of the Dalitz decay ?’??e+e?, based on a data sample of 1.31 billion J/? events collected with the BESIII detector. The ?’ mesons are produced via the J/????’ decay process. The ratio ?(?’??e+e?)/?(?’???) is measured to be (2.13±0.09(stat)±0.07(sys))×10?2. This corresponds to a branching fraction B(?’??e+e?)=(4.69±0.20(stat)±0.23(sys))×10?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. © 2015 American Physical Society.