Evidence for the Direct Two-Photon Transition from $\ddot{I}ˆ(3686)$ to $J/\ddot{I}ˆ$

Abstract

The two-photon transition $\ddot{I}ˆ(3686)\to {\hat{I}}^3{\hat{I}}^{3}J/\ddot{I}ˆ$ is studied in a sample of 106 million $\ddot{I}ˆ(3686)$ decays collected by the BESIII detector. The branching fraction is measured to be $(3.1\pm0.6(\unit{stat})^{+0.8}_{-1.0}(\unit{syst})) \times10^{-4}$ using $J/\ddot{I}ˆ\to e^{+}{e}^{-}$ and $J/\ddot{I}ˆ\to \hat{I}{¼}^{+}\hat{I}{¼}^{-}$ decays, and its upper limit is estimated to be $4.5\times 10^{-4}$ at the 90% conference level. This work represents the first measurement of a two-photon transition among charmonium states. The orientation of the $\ddot{I}ˆ(3686)$ decay plane and the $J/\ddot{I}ˆ$ polarization in this decay are also studied. In addition, the product branching fractions of sequential $E1$ transitions $\ddot{I}ˆ(3686)\to {\hat{I}}^3\ddot{I}{‡}_{cJ},\ddot{I}{‡}_{cJ}\to {\hat{I}}^{3}J/\ddot{I}ˆ(J=0,1,2)$ are reported.

6 pages, 3 figures, 2 tables