Upper Bounds on $K \to πν\bar ν$ and $K_L \to π^0 e^+ e^-$ from $ε^\prime/ε$ and $K_L \to μ^+ μ^-$

Abstract

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 $ε^\prime/ε$ provides at present the strongest constraint on $\Im Z_{ds}$. Assuming $0 \le ε^\prime/ε\le 2 \cdot 10^{-3}$ and Standard Model values for the CKM parameters we obtain the bounds ${\rm BR}(K_L \to π^0 ν\bar ν) \le 2.4 \cdot 10^{-10}$ and ${\rm BR}(K_L \to π^0 e^+ e^-) \le 3.6 \cdot 10^{-11}$ (which are substantially stronger than the bounds found recently by Colangelo and Isidori, using $ε$ instead of $ε^\prime/ε$). We illustrate how these bounds can be improved with the help of the forthcoming data on $ε^\prime/ε$. Using the bound on $\Re Z_{ds}$ from $K_L \to μ^+ μ^-$ we find ${\rm BR}(K^+ \to π^+ ν\bar ν) \le 2.3 \cdot 10^{-10}$. In this context we derive an analytic upper bound on ${\rm BR}(K^+ \to π^+ ν\bar ν)$ as a function of ${\rm BR}(K_L \to π^0 ν\bar ν)$ and the short distance contribution to ${\rm BR}(K_L \to μ^+ μ^-)$. 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.