3-LOOP O(ALPHA(2)(S)G(F)M(T)(2)) CORRECTIONS TO HADRONIC HIGGS DECAYS

We calculate the top-quark-induced three-loop corrections of O(alpha(s )(2)G(F)M(t)(2)) to the Yukawa couplings of the first five quark flavo urs in the framework of the minimal standard model with an intermediat e-mass Higgs boson, with mass M(H) much less than 2M(t). The calculati on is performed using an effective-Lagrangian approach implemented wit h the hard-mass procedure. As an application, we derive the O(alpha(s) (2)G(F)M(t)(2)) corrections to the H --> <q(q)over bar> partial decay widths, including the case q = b. The couplings of the Higgs boson to pairs of leptons and intermediate bosons being known to O(alpha(s)(2)G (F)M(t)(2)), this completes the knowledge of such corrections in the H iggs sector. We express the results both in the <(MS)over bar> and on- shell schemes of mass renormalization. We recover the notion that the QCD perturbation expansions exhibit a worse convergence behaviour in t he on-shell scheme than they do in the <(MS)over bar> scheme. (C) 1997 Elsevier Science B.V.