Higgs-Boson Decay to Four Fermions Including a Single Top Quark Below $t\bar{t}$ Threshold

Abstract

The rare decay modes Higgs $\to$ four light fermions, and Higgs $\to$ single top-quark + three light fermions for $m_t<M_H<2m_t$, 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 decay to single top, two tree-level graphs contribute in the standard model (SM); one couples the Higgs to $W^{+}{W}^{-}(\sim g{M}_W)$, and one to $t\bar{t}(\sim g_{topyukawa}=m_t/246\text{GeV})$. The large Yukawa coupling for $m_t>100\text{GeV}$ makes the second amplitude competitive or dominant for most $M_H,m_t$ values. Thus the Higgs decay rate to single top directly probes the SM universal mechanism generating both gauge boson and fermion masses, and offers a means to infer the Higgs-$t\bar{t}$ Yukawa coupling when $H\to t\bar{t}$ is kinematically disallowed. We find that the modes $pp\to Xt\bar{t}(H\to t\bar{b}{W}^{(\ast )})$ at the SSC, and $e^{+}{e}^{-}\to Zor\nu \bar{\nu }+(H\to t\bar{b}{W}^{(\ast )})$ at future high energy, high luminosity colliders, may be measureable if $2m_t$ is not too far above $M_H$. We classify non-standard Higgses as gaugeo-phobic, fermio-phobic or fermio-philic, and discuss the Higgs$\to$ single top rates for these classes.

Comments: 30 pages, 6 figures (figures available upon request); VAND-TH-93/4