Loop induced flavor changing neutral decays of the top quark in a general two-Higgs-doublet model

Decays of the top quark induced by flavor changing neutral currents (FCNC) are known to be extremely rare events within the Standard Model. This is so not only for the decay modes into gauge bosons, but most notably in the case of the Higgs channels, e.g., t --> H-SM + c, with a branching fraction of 10(-13) at most, Therefore, detection of FCNC top quark decays in a future high-energy and high-luminosity, machine like the LHC or the LC would be an indisputable signal of new physics. In this paper we show that within the simplest extension of the SM, namely the general two-Higgs-doublet model, the FCNC top quark decays into Higgs bosons, t --> (h(0), H-0, A(0)) + c, can be the most favored FCNC modes - comparable or even more efficient than the gluon channel t --> g + c. In both cases the optimal results are obtained for Type II models. However, only the Higgs channels can have rates reaching the detectable level (10(-5)), with a maximum of order 10(-4) which is compatible with the charged Higgs bounds from radiative B-meson decays. We compare with the previous results obtained in the Higgs sector of the MSSM. (C) 2001 Elsevier Science B.V. All rights reserved.