In light of the discovery of a standard model (SM)-like Higgs boson ($h$) at the LHC, we investigate the top quark to Higgs boson transition $t\to W^{*}bh$, which is the leading $t\to h$ decay mode in the SM. We find the decay branching fraction to be $1.80\times 10^{-9}$. In comparison, the two-body, loop-induced $t\to ch$ transition occurs at $\sim 10^{-14}$ in the SM. We consider the consequences of gauge-invariant dimension-six operators affecting the $t\overline{t}h$ interaction and find that the decay branching fraction may be increased by a factor of 2 within current constraints on the coupling parameters from collider experiments. We also extend the calculation to the $CP$-conserving Type I and Type II two-Higgs-doublet models, including both $CP$-even and $CP$-odd Higgs bosons. For neutral scalar masses at about 100 GeV, the decay rates can be several times larger than the SM result in the allowed range of model parameters. Observation prospects at present and future colliders are briefly addressed.

• Received 4 February 2014

DOI:https://doi.org/10.1103/PhysRevD.89.074045