Measurement of the η transition form factor through the η' → π+ π− η decay

Based on a sample of (1.0087 ± 0.0044) × 1010 J/ψ events collected at BESIII, the transition form factor of the η meson is extracted by analyzing J/ψ → γη',η' → π+π−η and η → γl+l- (l = e, μ) events. The measured slope of the transition form factor is Λ−2 = 1.668 ± 0.093stat ± 0.024sys(GeV=/c2)−2 for the di-electron channel and Λ−2 = 1.645 ± 0.343stat ± 0.017sys(GeV/c2)−2 for the di-muon channel. The branching fractions for η → γe+e− and η → γμ+μ− are measured to be B(η → γe+e−) = (6.79 ± 0.05stat ± 0.36sys) × 10−3 and B(η → γμ+μ−) = (2.97 ± 0.12stat ± 0.07sys) × 10−4. By combining with the results based on the J/ψ → γη and η → γe+e− events from the previous BESIII measurement, we determine Λ−2 = 1.707 ± 0.076stat ± 0.029sys(GeV/c2)−2 and B(η → γe+e−) =(6.93 ± 0.28tot) × 10−3. In addition, we search for the dark photon (A') using the combined events. No significant signal is observed, and the upper limits on B(η → γA',A' → e+e−) are set at the 90% confidence level for different A' mass hypotheses.