Observation of J/ψ→3γ

We report the first observation of the decay J/ψ→3γ. The signal has a statistical significance of 6σ and corresponds to a branching fraction of B(J/ψ→3γ)=(1.2±0.3±0.2)×10^-5, in which the errors are statistical and systematic, respectively. The measurement uses ψ(2S)→π⁺π^-J/ψ events acquired with the CLEO-c detector operating at the CESR e⁺e^- collider.