ISSN - 00319007, EISSN - 10797114, Physical Review Letters, 2012, vol. 109, p. -
Evidence for the direct two-photon transition from ψ(3686) to J/ψ
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.