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)+0.8−1.0(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 E1transitions ψ(3686)→γχcJ and χcJ→γJ/ψ(J=0,1,2) are reported.