Flavor Changing Transitions and the Top Quark in a Two Higgs Doublets Model; Application to e^+e^- --> t bar-c nu_e bar-nu_e, t bar -c e^+ e^-, t bar-c Z, t --> cW^+W^- and t --> cZZ

Abstract

Extension of the SM with one extra Higgs doublet may give rise to flavor-changing-scalar coupling of a neutral Higgs to a pair of top-charm quarks. This coupling gives rise to a large tree-level effective W^+W^-;ZZ - Higgs - t bar-c interaction. We find that the reactions e^+e^- --> t bar-c nu_e bar-nu_e, t bar-c e^+e^-, t bar-c Z and the two rare top decays t --> cW^+W^-, t --> cZZ are very sensitive probes of such an effective interaction. The most promising ones, e^+e^- --> t bar-c nu_e bar-nu_e, t bar-c 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 light neutral Higgs is a few hundred GeV. We also find that the rare decays t --> cW^+W^- and t --> cZZ may be accessible in the LHC if the mass of the light neutral Higgs lies in the narrow window 150 GeV < m_h < 200 GeV.