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Higher-order multipole amplitude measurement in $ψ(2S)\toγχ_{c2}$

Using $106\times10^6$ $ψ(2S)$ events collected with the BESIII detector at the BEPCII storage ring, the higher-order multipole amplitudes in the radiative transition $ψ(2S)\toγχ_{c2}\toγππ/γKK$ are measured. A fit to the $χ_{c2}$ production and decay angular distributions yields $M2=0.046\pm0.010\pm0.013$ and $E3=0.015\pm0.008\pm0.018$, 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 $4.4σ$ statistical significance and is consistent with the charm quark having no anomalous magnetic moment.