Higher-order multipole amplitude measurement in $\psi (2S)\to \gamma {\chi }_{c2}$

Using $106\times {10}^6$ $\psi (2S)$ events collected with the BESIII detector at the BEPCII storage ring, the higher-order multipole amplitudes in the radiative transition $\psi (2S)\to \gamma {\chi }_{c2}\to \gamma \pi \pi \mathrm{/}\gamma KK$ are measured. A fit to the ${\chi }_{c2}$ production and decay angular distributions yields $M2=0.046\pm 0.010\pm 0.013$ and $E3=0.015\pm 0.008\pm 0.018$, where the first errors are statistical and the second systematic. Here $M2$ denotes the normalized magnetic quadrupole amplitude and $E3$ the normalized electric octupole amplitude. This measurement shows evidence for the existence of the $M2$ signal with $4.4\sigma$ statistical significance and is consistent with the charm quark having no anomalous magnetic moment.