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 Lehmann-Symanzik-Zimmermann reduction to relate a photon to a sum of hadronic vector eigenstates, we compute form factors in both the spacelike and timelike 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.