PENGUIN-BOX EXPANSION - FLAVOR-CHANGING NEUTRAL-CURRENT PROCESSES AND A HEAVY TOP QUARK

We propose a Penguin-Box Expansion (PBE) for flavour-changing neutral-current (FCNC) processes. The basic elements of this expansion are universal m(t)-dependent functions resulting from various box and penguin diagrams common to many FCNC processes. The m(t)-independent coefficients of these functions can be calculated by using renormalization group techniques. Of particular interest is a manifestly gauge independent formulation of PBE. We illustrate these ideas by discussing the semi-leptonic decays K --> pi-nu-nu-BAR, K(L) --> mu-mu-BAR, K(L) --> pi-0 e+ e- and the ratio epsilon'/epsilon. We find simple analytic formulae for all these decays which exhibit the dependence on m(t) and other parameters of the standard model. We point out that due to the decrease of sin-delta with increasing m(t), the branching ratios for CP-violating semi-leptonic decays exhibit, in contrast to CP-conserving decays, a much weaker dependence on m(t) than expected on the basis of the box and penguin diagram functions alone. New insight in the suppression of epsilon'/epsilon for large m(t) is obtained. Analyzing the relevant hadronic matrix elements beyond the 1/N approach, we find that for 100 GeV less-than-or-equal-to m(t) less-than-equal-to 300 GeV, the values -2.5 X 10(-3) less-than-or-equal-to epsilon'/epsilon less-than-or-equal-to 3.2 X 10(-3) are possible. The structure of PBE beyond the leading logarithmic approximation is presented.