Onset of strong interactions in the Higgs sector of the standard model: [ital H][r arrow][ital f[bar f]] at two loops

We present the dominant two-loop [ital O]([ital G][sub [ital F]][sup 2][ital M][sub [ital H]][sup 4]) electroweak corrections to the fermionic decay widths of a high-mass Higgs boson in the standard model. The corrections are negative and quite significant, and are larger in magnitude than the one-loop electroweak corrections for [ital M][sub [ital H]][approx gt]400 GeV. This indicates the onset of a breakdown of perturbation theory in the Higgs sector of the standard model at a surprisingly low value of the Higgs-boson mass.