2026-07-072026-07-07http://salesiana.dossiersoluciones.com/handle/123456789/189143We 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 $η_c \to γ^* γ^*$ and $χ_{c0} \to γ^* γ^*$. At the on-shell point we find approximate agreement with experimental world-average values.Replaced with version to be published in PRL. Expanded discussion of possible systematic errorsHigh Energy Physics - PhenomenologyHigh Energy Physics - Latticetext