Two-loop electroweak heavy-fermion corrections to Higgs-boson production and decay

The dominant electroweak corrections to the production cross sections and partial decay widths of a light Standard Model Higgs boson, with mass M-H << 2m(r), are due to top-quark loops. By means of a low-energy theorem, we study at the two-loop level the leading shifts in the Higgs-boson couplings to pairs of light fermions and gauge bosons induced by a sequential isodoublet of high-mass fermions. For tree-level and loop-induced Higgs-boson couplings, these corrections are of relative order O(G(mu)(2)m(F)(4)) and O(G(mu)m(F)(2)), respectively, where m(F) represents a generic heavy-fermion mass, with m(F) >> M-W, M-H. Except for the Hb (b) over bar coupling, all results carry over to the case of the top-quark-induced corrections. We discuss possible phenomenological consequences of our results. (C) 1998 Elsevier Science B.V.