The rare decay modes Higgs boson → four light fermions and Higgs boson → single top quark + three light fermions for $m_t<M_H<2\mathrm{}{m}_t$ are presented, and phenomenologically interpreted. The angular correlation between fermion planes is presented as a test of the spin and intrinsic parity of the Higgs particle. In Higgs-boson decay to a single top quark, two tree-level graphs contribute in the standard model (SM); one couples the Higgs boson to $W^{+}{W}^{-}\left(\sim g{M}_W\right)$, and one to $t\overline{t}(\sim g_{\mathrm{top}\mathrm{Yukawa}}=\frac{m_t}{246} \mathrm{GeV})$. The large Yukawa coupling for $m_t>100\mathrm{}$ GeV makes the second amplitude competitive or dominant for most $M_H$, $m_t$ values. Thus the Higgs-boson decay rate to a single top quark directly probes the SM universal mechanism generating both gauge-boson and fermion masses, and offers a means to infer the Higgs-boson-$t\overline{t}$ Yukawa coupling when $H\to t\overline{t}$ is kinematically disallowed. We find that the modes $\mathrm{pp}\to \mathrm{Xt}\overline{t}(H\to t\overline{b}{W}^{\mathrm{}(*)\mathrm{}})$ at the Superconducting Super Collider, and $e^{+}{e}^{-}\to Z$ or $\nu \overline{\nu }+(H\to t\overline{b}{W}^{\mathrm{}(*)\mathrm{}})$ at future high-energy, high-luminosity colliders, may be measurable if $2m_t$ is not too far above $M_H$. We classify nonstandard Higgs-bosons as gaugeophobic, fermiophobic, or fermiophilic, and discuss the Higgs boson → single top-quark rates for these classes.

• Received 4 November 1991

DOI:https://doi.org/10.1103/PhysRevD.47.3886