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We propose a direct method to calculate the decay width of a hadron decayingto two photons, which is conventionally obtained by an extrapolation forvarious off-shell form factors. The new method provides an simple way toexamine the finite-volune effects. As an example, we apply the method to$\eta_c\rightarrow 2\gamma$. Using three $N_f=2$ twisted-mass gauge ensembleswith different lattice spacings, we obtain the final decay width$\Gamma_{\eta_c\gamma\gamma}=6.57(15)_{\textrm{stat}}(8)_{\textrm{syst}}$ keV,where the systematic error accounts for the fine-tuning of the charm quarkmass. This method can also be applied for other processes which involve theleptonic or radiative particles in the final states.

A new method for a lattice calculation of $\eta_c \rightarrow 2\gamma$