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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>

Ablikim M., Achasov M. N., Ai X. C., Albayrak O., Albrecht M., Ambrose D. J., Amoroso A., An F. F., An Q., Bai J. Z., Ferroli R. Baldini, Ban Y., Bennett D. W., Bennett J. V., Bertani M., Bettoni D., Bian J. M., Bianchi F., Boger E., Bondarenko O., Boyko I., Briere R. A., Cai H., Cai X., Cakir O., Calcaterra A., Cao G. F., Cetin S. A., Chang J. F., Chelkov G., Chen G., Chen H. S., Chen H. Y., Chen J. C., Chen M. L., Chen S. J., Chen X., Chen X. R., Chen Y. B., Cheng H. P., Chu X. K., Cibinetto G., Cronin-Hennessy D., Dai H. L., Dai J. P., Dbeyssi A., Dedovich D., Deng Z. Y., Denig A., Denysenko I., Destefanis M., De Mori F., Ding Y., Dong C., Dong J., Dong L. Y., Dong M. Y., Du S. X., Duan P. F., Fan J. Z., Fang J., Fang S. S., Fang X., Fang Y., Fava L., Feldbauer F., Felici G., Feng C. Q., Fioravanti E., Fritsch M., Fu C. D., Gao Q., Gao X. Y., Gao Y., Gao Z., Garzia I., Geng C., Goetzen K., Gong W. X., Gradl W., Greco M., Gu M. H., Gu Y. T., Guan Y. H., Guo A. Q., Guo L. B., Guo Y., Guo Y. P., Haddadi Z., Hafner A., Han S., Han Y. L., Hao X. Q.

© 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.