Dispersive treatment of K_Stoγγ and K_Stoγell^+ell^-

We analyse the rare kaon decays $K_S \to \gamma\gamma$ and $K_S \to \gamma\ell^+\ell^-$ $(\ell = e \mbox{ or } \mu)$ in a dispersive framework in which the weak Hamiltonian carries momentum. Our analysis extends predictions from lowest order $SU(3)_L\times SU(3)_R$ chiral perturbation theory ($\chi$PT$_3$) to fully account for effects from final-state interactions, and is free from ambiguities associated with extrapolating the kaon off-shell. Given input from $K_S \to \pi\pi$ and $\gamma\gamma^{(*)}\to\pi\pi$, we solve the once-subtracted dispersion relations numerically to predict the rates for $K_S \to \gamma\gamma$ and $K_S \to \gamma\ell^+\ell^-$. In the leptonic modes, we find sizeable corrections to the $\chi$PT$_3$ predictions for the integrated rates.