Evidence for the Direct Two-Photon Transition from $\psi (3686)$ to $J/\psi$

BESIII Collaboration, M. Ablikim, M. N. Achasov +354

The two-photon transition $\psi (3686)\to \gamma \gamma J/\psi$ is studied in a sample of 106 million $\psi (3686)$ decays collected by the BESIII detector. The branching fraction is measured to be $(3.1\…