CP Violation in $t\to W^+ b$ Decays in Two-Higgs Doublet Model

Due to the large mass of top quark, CP violation in the top-quark decay is sensitive to the interactions mediated by Higgs bosons. We consider CP violation in $t\rightarrow W^+ b$ decay by calculating consistently in unitary gauge the CP-violating up-down asymmetry of the leptons from $W$ boson decays in $t\rightarrow W^+ b$, defined by Grz\c{a}dkowski and Gunion, in the two-Higgs doublet model with CP-violating neutral sector. The asymmetry is shown to be at most of the order of $(1-3)\times 10^{-4}$ for $\tan\beta=1.0$, where $\tan\beta$ is the ratio of vacuum expectation values for the two neutral Higgs bosons.