Implications of a W⁺W^-(ZZ)-Higgs boson- tc̅ interaction for e⁺e^-→tc̅ ν_eν̅ _e, tc̅ e⁺e^-, tc̅ Z and for t→cW⁺W^-, cZZ in a two Higgs doublet model

The standard model with one extra Higgs doublet may give rise to an enhanced tree-level flavor-changing-scalar coupling of a neutral Higgs boson to a pair of top-charm quarks. This coupling may drive a large tree-level effective W⁺W^-(ZZ)-Higgs boson-tc̅ interaction. As a result we find that the reactions e⁺e^-→tc̅ ν_eν̅ _e, tc̅ e⁺e^-, tc̅ Z and the two rare top decays t→cW⁺W^-, t→cZZ become very sensitive probes of such an effective interaction. The most promising ones, e⁺e^-→tc̅ ν_eν̅ _e, tc̅ e⁺e^-, may yield several hundreds and up to thousands of such events at the Next Linear Collider with a center of mass energy of sqrt[s]=0.5–2 TeV if the mass of the lightest neutral Higgs boson is a few hundred GeV. The rare decays t→cW⁺W^- and t→cZZ may be accessible at the CERN LHC if the mass of the lightest neutral Higgs boson lies in the narrow window 150 GeV≲m_h≲200 GeV.