Improved measurements of the Dalitz decays η/η′ →γe+e-

Based on a data sample of 10 billion J/ψ events collected with the BESIII detector, improved measurements of the Dalitz decays η/η′→γe+e- are performed, where the η and η′ are produced through the radiative decays J/ψ→γη/η′. The branching fractions of η→γe+e- and η′→γe+e- are measured to be (7.07±0.05±0.23)×10-3 and (4.83±0.07±0.14)×10-4, respectively. Within the single-pole model, the parameter of electromagnetic transition form factor for η→γe+e- is determined to be Λη=(0.749±0.027±0.007) GeV/c2. Within the multipole model, we extract the electromagnetic transition form factors for η′→γe+e- to be Λη′=(0.802±0.007±0.008) GeV/c2 and γη′=(0.113±0.010±0.002) GeV/c2. The results are consistent with both theoretical predictions and previous measurements. The characteristic sizes of the interaction regions for the η and η′ are calculated to be (0.645±0.023±0.007) fm and (0.596±0.005±0.006) fm, respectively. In addition, we search for the dark photon in η/η′→γe+e-, and the upper limits of the branching fractions as a function of the dark photon are given at 90% confidence level.