Three-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 flavours in the framework of the minimal standard model with an intermediate-mass Higgs boson, with mass M(H) much less than 2M(t). The calculation is performed using an effective-Lagrangian approach implemented with 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 Higgs 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 the on-shell scheme than they do in the <(MS)over bar> scheme. (C) 1997 Elsevier Science B.V.