QCD CORRECTIONS TO THE (S)OVER-BAR-DZ VERTEX FOR ARBITRARY TOP-QUARK MASS

We calculate strong interaction O(alpha(s)) corrections to the effective one-loop induced neutral flavour changing vertex sdZBAR for an arbitrary top quark mass in 't Hooft-Feynman gauge for the W-field. This two-loop calculation is necessary for a meaningful use of a running top quark mass in rare K- and B-decays and consequently increases the precision of theoretical predictions. We find that the uncertainty in the strength of the effective vertex sdZBAR, due to the dependence of m(t) on the choice of the renormalization scale mu, is reduced from O(10%) to O(1%) by including the O(alpha(s)) corrections. For the choice mu = m(t) the corrections are less than 5% for 100 less-than-or-equal-to m(t) less-than-or-equal-to 200 GeV but they are substantially larger for mu = M(W). As a by-product we confirm the O(alpha(s)G(F)m(t)2) correction to the Z --> bbBAR partial decay width calculated recently in the large top quark mass limit.