Using 106 × 10 6 ψ ′ events collected with the BESIII detector at the BEPCII storage ring, the higher-order multipole amplitudes in the radiative transition ψ ′ → γχ c2 → γπ + π − /γK + K − are measured. A fit to the χ c2 production and decay angular distributions yields M 2 = 0.046 ± 0.010 ± 0.013 and E3 = 0.015±0.008±0.018, where the first errors are statistical and the second systematic.Here M 2 denotes the normalized magnetic quadrupole amplitude and E3 the normalized electric octupole amplitude. This measurement shows evidence for the existence of the M 2 signal with 4.4σ statistical significance and is consistent with the charm quark having no anomalous magnetic moment.

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