Observation of the Charmonium Decay ηc→γγ in J/ψ→γηc

Using (2712.4±14.3)×106 ψ(3686) events collected with the BESIII detector at the BEPCII collider, the decay ηc→γγ in J/ψ→γηc is observed. We determine the product branching fraction B(J/ψ→γηc)×B(ηc→γγ)=(5.23±0.26stat±0.30syst)×10-6. This result is consistent with the lattice QCD calculation (5.34±0.16)×10-6 from HPQCD in 2023. By using the world-average values of B(J/ψ→γηc) and the total decay width of ηc, the partial decay width Γ(ηc→γγ) is determined to be (11.30±0.56stat±0.66syst±1.14ref) keV, which deviates from the corresponding world-average value by 3.4σ.