Final State Interactions in Kaon Decays

We quantify the important effect of strong final state interactions in the weak $K\to 2\pi$ amplitudes, using the measured $\pi$-$\pi$ phase shifts with J=0 and $I=0,2$. The main results of this analysis, with their implications for $\epsilon'/\epsilon$ and the $\Delta I =1/2$ rule, have been already presented in a previous paper [1]. Here we provide a detailed formal derivation of those results and further discuss the Standard Model prediction of $\epsilon'/\epsilon$.