HIGGS-BOSON DECAY TO 4 FERMIONS INCLUDING A SINGLE TOP-QUARK BELOW T(T)OVER-BAR THRESHOLD

The rare decay modes Higgs boson --> four light fermions and Higgs bos on --> single top quark + three light fermions for m(t) < M(H) < 2m(t) are presented, and phenomenologically interpreted. The angular correl ation between fermion planes is presented as a test of the spin and in trinsic 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- (approximately gM(W)), and one to ttBAR (approximately g(top Yukawa) = m(t)/246 GeV). The large Yukaw a coupling for m(t) > 100 GeV makes the second amplitude competitive o r 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 gene rating both gauge-boson and fermion masses, and offers a means to infe r the Higgs-boson-ttBAR Yukawa coupling when H-->ttBAR is kinematicall y disallowed. We find that the modes pp-->XttBAR(H-->tbW()BAR) at the Superconducting Super Collider, and e+e--->Z or nunuBAR+(H-->tbW()BA R) at future high-energy, high-luminosity colliders, may be measurable if 2m(t) is not too far above M(H). We classify nonstandard Higgs-bos ons as gaugeophobic, fermiophobic, or fermiophilic, and discuss the Hi ggs boson --> single top-quark rates for these classes.