Two loop O(αsGFmt2) corrections to the fermionic decay rates of the standard-model Higgs boson

Low- and intermediate mass Higgs bosons decay preferably into fermion pairs. The one-loop electroweak corrections to the respective decay rates are dominated by a flavour-independent term of O(GFmt2). We calculate the two-loop gluon correction to this term. It turns out that this correction screens the leading high-mt behaviour of the one-loop result by roughly 10%. We also present the two-loop QCD correction to the contribution induced by a pair of fourth-generation quarks with arbitrary masses. As expected, the inclusion of the QCD correction considerably reduces the renormalization-scheme dependence of the prediction. (orig.)