Two-photon decays of from lattice QCD

We present an exploratory lattice study for the two-photon decay of using twisted mass lattice QCD gauge configurations generated by the European Twisted Mass Collaboration. Two different lattice spacings of fm and fm are used in the study, both of which are of physical size of 2 fm. The decay widths are found to be 1.025(5) KeV for the coarser lattice and 1.062(5) KeV for the finer lattice, respectively, where the errors are purely statistical. A naive extrapolation toward the continuum limit yields KeV, which is smaller than the previous quenched result and most of the current experimental results. Possible reasons are discussed.