Onset of strong interactions in the Higgs sector of the standard model:H→ff¯ at two loops

We present the dominant two-loop O(${\mathit{G}}_{\mathit{F}}^{2}$${\mathit{M}}_{\mathit{H}}^{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 ${\mathit{M}}_{\mathit{H}}$\ensuremath{\gtrsim}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.