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Measurement of B (J /ψ →η′e+e-) and search for a dark photon

Ablikim M., Achasov M. N., Ahmed S., Albrecht M., Alekseev M., Amoroso A., An F. F., An Q., Bai J. Z., Bai Y., Bakina O., Baldini Ferroli R., Ban Y., Begzsuren K., Bennett D. W., Bennett J. V., Berger N., Bertani M., Bettoni D., Bianchi F., Boger E., Boyko I., Briere R. A., Cai H., Cai X., Cakir O., Calcaterra A., Cao G. F., Cetin S. A., Chai J., Chang J. F., Chelkov G., Chen G., Chen H. S., Chen J. C., Chen M. L., Chen P. L., Chen S. J., Chen X. R., Chen Y. B., Chu X. K., Cibinetto G., Cossio F., 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., Dou Z. L., Du S. X., Duan P. F., Fang J., Fang S. S., Fang Y., Farinelli R., Fava L., Fegan S., Feldbauer F., Felici G., Feng C. Q., Fioravanti E., Fritsch M., Fu C. D., Gao Q.

© 2019 authors. Published by the American Physical Society. Published by the American Physical Society under the terms of the »https://creativecommons.org/licenses/by/4.0/» Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article's title, journal citation, and DOI. Funded by SCOAP 3 . Using a data sample of (1310.6±7.0)×106 J/ψ decay events collected with the BESIII detector at BEPCII, we study the electromagnetic Dalitz decay J/ψ→η′e+e- with two dominant η′ decay modes, η′→γπ+π- and η′→π+π-η. The branching fraction is determined to be B(J/ψ→η′e+e-)=(6.59±0.07±0.17)×10-5, which improves in precision by a factor of 2 over the previous BESIII measurement. A search for the dark photon (γ′) is performed via J/ψ→η′γ′, γ′→e+e-. Excluding the ω and φ mass regions, no significant signal is observed in the mass range from 0.1 to 2.1 GeV/c2. We set upper limits at the 90% confidence level on B(J/ψ→η′γ′)×B(γ′→e+e-), B(J/ψ→η′γ′) and the mixing strength as a function of dark photon mass. This is among the first searches for dark photons in charmonium decays.