CONSTRAINTS ON AN ENHANCED s̄dZ VERTEX FROM ε′/ε1

We analyze rare kaon decays in models in which the dominant new effect is an enhanced s̄dZ vertex Zds. We point out that in spite of large theoretical uncertainties the CP-violating ratio ε /ε provides at present the strongest constraint on ImZds. Assuming 1.5 · 10−3 ≤ ε/ε ≤ 3 · 10−3 and Standard Model values for the CKM parameters we obtain the bounds BR(KL → π0νν̄) ≤ 3.9 · 10−10 and BR(KL → πee)dir ≤ 7.9 · 10−11. Using the bound on ReZds from KL → μ+μ− we find BR(K+ → π+νν̄) ≤ 2.6 · 10−10. We also discuss new physics scenarios in which in addition to an enhanced s̄dZ vertex also neutral meson mixing receives important new contributions. In this case our most conservative bounds are BR(KL → π0νν̄) ≤ 1.8 · 10−9, BR(KL → πee)dir ≤ 2.8 · 10−10 and BR(K+ → π+νν̄) ≤ 6.1 · 10−10. The dependence of the upper bound for BR(KL → π0νν̄) on the CKM parameters is also analyzed. Talk given at the XXXIVnd Rencontres de Moriond “ELECTROWEAK INTERACTIONS AND UNIFIED THEORIES”, Les Arcs, France, 13-20 March 1999.