ISSN - 15507998, Physical Review D - Particles, Fields, Gravitation and Cosmology, 2015, vol. 92, p. -
Observation of the Dalitz decay η' →γ<sup>e+</sup><sup>e-</sup>
© 2015 American Physical Society. © 2015 American Physical Society. We report the first observation of the Dalitz decay η′→γ<sup>e+e-</sup>, based on a data sample of 1.31 billion J/ψ events collected with the BESIII detector. The η′ mesons are produced via the J/ψ→γη′ decay process. The ratio Γ(η′→γ<sup>e+e-</sup>)/Γ(η′→γγ) is measured to be (2.13±0.09(stat)±0.07(sys))×10-2. This corresponds to a branching fraction B(η′→γ<sup>e+e-</sup>)=(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 <sup>e+e-</sup> invariant mass. The results are consistent with the prediction of the vector meson dominance model.