The decays of the negative G-parity meson G-parity into even numbers of pions violate G-parity. Such decays, as well as other G-parity decays into hadrons, can be parametrized in terms of three main intermediate virtual states: one photon, one photon plus two gluons, and three gluons. Since the electromagnetic interaction does not conserve G-parity, G-parity decays into positive G-parity final states should be dominantly electromagnetic. Nevertheless, the one-photon contribution to J/ψ→π+π-, that can be estimated by exploiting the cross section σ(e+e-→π+π-), differs from the observed decay probability for more than 4.5 standard deviations. We present a computation of the ggγ amplitude based on a phenomenological description of the decay mechanism in terms of dominant intermediate states ηγ, η′γ, and f1(1285)γ. The obtained value is of the order of the electromagnetic contribution.
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