Dispersive treatment of K_S → γγ and K_S → γl⁺l^-

We analyse the rare kaon decays K_S → γγ and K_S → γl⁺l^- (l = e or μ) in a dispersive framework in which the weak Hamiltonian carries momentum. Our analysis extends predictions from lowest order SU(3)_L x SU(3)_R chiral perturbation theory (χPT₃) to fully account for effects from final-state interactions, and is free from ambiguities associated with extrapolating the kaon off-shell. Given input from K_S → ππ and γγ^(*) → ππ, we solve the once-subtracted dispersion relations numerically to predict the rates for K_S → γγ and K_S → γl⁺l^-. In the leptonic modes, we find sizeable corrections to the χPT₃ predictions for the integrated rates. (orig.)