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The decays t +cV ( V = y,g, or Z ) induced through loop processes are calculated in the minimal supersymmetric model. We find that these new contributions can enhance the standard model branching fractions by as much as 3-4 orders of magnitude, i.e., ~( t + c~) -l~-~, B (~+ c z ) -~o -~, andB ( t -c y ) -lop8, for favorable values of the parameters.

We analyze charm meson semileptonic decays using the measured ratios Γ L /Γ T and Γ + /Γ − from D →K * and the branching ratios for D →K * and D →K. First we introduce the light vector mesons in a model which combines the heavy quark effective Lagrangian and the chiral perturbation approach. We propose a method which predicts the behavior of the form factors. Using the available experimental data we determine the values of some model parameters and reproduce the observed branching ratios for D s → Φ, D s → (η + η ′ ) and D → π. We make predictions for the yet unmeasured branching ratios and polarization observables.

A new formula for the heavy quark-antiquark spin dependent potential is given by using the techniques developed in the heavy quark effective theory. The leading logarithmic quark mass terms emerging from the loop contributions are explicitly extracted and summed up. There is no renormalization scale ambiguity in this new formula. The spin-dependent potential in the new formula is expressed in terms of three independent color-electric and color-magnetic field correlation functions, and it includes both the Eichten-Feinberg's formula and the one-loop QCD result as special cases.