RARE DECAYS OF THE TOP QUARK IN THE STANDARD AND 2-HIGGS-DOUBLET MODELS

The decays t --> cV, where V = gamma, g, or Z, and t --> cH are calculated, keeping all masses, in the standard model (SM) for all possible values of the top-quark mass that are consistent with present data. We find that the branching fractions for these processes are small, the largest being B(t --> cH) approximately 10(-7) and B(t --> cg) approximately 10(-10). Our calculations are then extended to include the possible contributions of charged Higgs bosons to the transitions t --> cV in the context of general two-Higgs-doublet models. These new contributions can enhance the SM branching fractions by as much as 3-4 orders of magnitude, e.g., B(t --> cg) approximately 10(-7)-10(-6), for various values of the parameters. General expressions for flavor-changing neutral-current decays of heavy quarks including all masses and momenta, in the SM and in the two-Higgs-doublet models, are also given.