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ISSN - 00319007, EISSN - 10797114, Physical Review Letters, 2012, vol. 109, p. -

Evidence for the direct two-photon transition from ψ(3686) to J/ψ

Ablikim M., Achasov M. N., Ambrose D. J., An F. F., An Q., An Z. H., Bai J. Z., Ferroli R. B., Ban Y., Becker J., Berger N., Bertani M. B., Bian J. M., Boger E., Bondarenko O., Boyko I., Briere R. A., Bytev V., Cai X., Calcaterra A. C., Cao G. F., Chang J. F., Chelkov G., Chen G., Chen H. S., Chen J. C., Chen M. L., Chen S. J., Chen Y., Chen Y. B., Cheng H. P., Chu Y. P., Cronin-Hennessy D., Dai H. L., Dai J. P., Dedovich D., Deng Z. Y., Denig A., Denysenko I., Destefanis M., Ding W. M., Ding Y., Dong L. Y., Dong M. Y., Du S. X., Fang J., Fang S. S., Fava L., Feldbauer F., Feng C. Q., Fu C. D., Fu J. L., Gao Y., 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. P., Han Y. L., Hao X. Q., Harris F. A., He K. L., He M., He Z. Y., Held T., Heng Y. K., Hou Z. L., Hu H. M., Hu J. F., Hu T., Huang B., Huang G. M., Huang J. S., Huang X. T., Huang Y. P., Hussain T., Ji C. S., Ji Q., Ji X. B., Ji X. L., Jia L. K., Jiang L. L., Jiang X. S., Jiao J. B., Jiao Z., Jin D. P., Jin S., Jing F. F., Kalantar-Nayestanaki N., Kavatsyuk M.

The two-photon transition ψ(3686)→γγJ/ψ is studied in a sample of 1.06×108 ψ(3686) decays collected by the BESIII detector. The branching fraction is measured to be (3.1±0.6(stat)-1.0+0. 8(syst))×10 -4 using J/ψ→e +e - and J/ψ→μ +μ - decays, and its upper limit is estimated to be 4.5×10 -4 at the 90% confidence level. This work represents the first measurement of a two-photon transition among charmonium states. The orientation of the ψ(3686) decay plane and the J/ψ polarization in this decay are also studied. In addition, the product branching fractions of sequential E1 transitions ψ(3686) →γχ cJ and χ cJ→γJ/ψ(J=0, 1,2) are reported. © 2012 American Physical Society.