Measurement of higher-order multipole amplitudes in $\psi (3686)\to \gamma {\chi }_{c1,2}$ with ${\chi }_{c1,2}\to \gamma J/\psi$ and search for the transition ${\eta }_c(2S)\to \gamma J/\psi$

M. Ablikim, M. N. Achasov, X. C. Ai +424

Using 106~million $\psi (3686)$ events collected with the BESIII detector, we measure multipole amplitudes for the decay $\psi (3686)\to \gamma {\chi }_{c1,2}\to \gamma \gamma J/\psi$ beyond the dominant elect…