2-LOOP O(G(F)(2)M(H)(4)) CORRECTIONS TO THE FERMIONIC DECAY-RATES OF THE HIGGS-BOSON

We calculate the dominant O(GF2MH4) two-loop electroweak corrections to the fermionic decay widths of a heavy Higgs boson in the standard model. Use of the Goldstone-boson equivalence theorem reduces the problem to one involving only the physical Higgs boson H and the Goldstone bosons w± and z of the unbroken theory. The two-loop corrections are opposite in sign to the one-loop electroweak corrections, exceed the one-loop corrections in magnitude for MH>1114 GeV, and increase in relative magnitude as MH2 for larger values of MH. We conclude that the perturbation expansion in powers of GFMH2 breaks down for MH1100 GeV. We discuss briefly the QCD and the complete one-loop electroweak corrections to H→bb̄,tt̄, and comment on the validity of the equivalence theorem. Finally, we note how a very heavy Higgs boson could be described in a phenomenological manner. © 1995 The American Physical Society.