Two Photon Decays of Charmonia from Lattice QCD

We make the first calculation in lattice QCD of two-photon decays of mesons. Working in the charmonium sector, using the LSZ reduction to relate a photon to a sum of hadronic vector eigenstates, we compute form-factors in both the space-like and time-like domains for the transitions $\eta_c \to \gamma^* \gamma^*$ and $\chi_{c0} \to \gamma^* \gamma^*$. At the on-shell point we find approximate agreement with experimental world-average values.