The two-photon transition ψ(3686)→γγJ/ψ is studied in a sample of 106 million ψ(3686) decays collected by the BESIII detector. The branching fraction is measured to be (3.1±0.6(\unitstat)+0.8−1.0(\unitsyst))×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% conference 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)→γχcJ,χcJ→γJ/ψ(J=0,1,2) are reported.
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