THE RARE DECAY K+-]PI+NU-NU-BAR AS A STANDARD MODEL STANDARD

A priori one would expect that extensions of the Standard Model can significantly enhance (or suppress) BR(K+ --> pi+ nu-nu-BAR). We have analyzed many different classes of such extensions: models with a non-minimal Higgs sector or with right-handed currents; fourth family extensions; SUSY in both the minimal and non-minimal version, and even with broken R-parity. We find that -apart from a few somewhat exotic exceptions - these extensions have little direct impact on K+ --> pi+ nu-nu-BAR due to constraints that are inferred from B(d)-B(d)BAR, and K0-K0BAR mixing and upper bounds on B --> K*gamma. Accordingly K+ --> pi+ nu-nu-BAR probes very cleanly the top mass and the KM parameter \V(td)V(ts)\, two fundamental parameters in the Standard Model.