Upper Bounds on $K \to \pi \nu \bar \nu$ and $K_L \to \pi^0 e^+ e^-$   from $\epsilon^\prime/\epsilon$ and $K_L \to \mu^+ \mu^-$
  We analyze rare kaon decays in models in which the dominant new effect is an enhanced $\bar s d Z$ vertex $Z_{ds}$. We point out that in spite of large theoretical uncertainties the CP-violating ratio $\epsilon^\prime/\epsilon$ provides at present the strongest constraint on $\Im Z_{ds}$. Assuming $0 \le \epsilon^\prime/\epsilon \le 2 \cdot 10^{-3}$ and Standard Model values for the CKM parameters we obtain the bounds ${\rm BR}(K_L \to \pi^0 \nu \bar \nu) \le 2.4 \cdot 10^{-10}$ and ${\rm BR}(K_L \to \pi^0 e^+ e^-) \le 3.6 \cdot 10^{-11}$ (which are substantially stronger than the bounds found recently by Colangelo and Isidori, using $\epsilon$ instead of $\epsilon^\prime/\epsilon$). We illustrate how these bounds can be improved with the help of the forthcoming data on $\epsilon^\prime/\epsilon$. Using the bound on $\Re Z_{ds}$ from $K_L \to \mu^+ \mu^-$ we find ${\rm BR}(K^+ \to \pi^+ \nu \bar \nu) \le 2.3 \cdot 10^{-10}$. In this context we derive an analytic upper bound on ${\rm BR}(K^+ \to \pi^+ \nu \bar \nu)$ as a function of ${\rm BR}(K_L \to \pi^0 \nu \bar \nu)$ and the short distance contribution to ${\rm BR}(K_L \to \mu^+ \mu^-)$. We also discuss new physics scenarios in which in addition to an enhanced $\bar s d Z$ vertex also neutral meson mixing receives important new contributions. In this case larger values of the branching ratios in question cannot be excluded. 
Andrzej J. Buras and Luca Silvestrini