R(b) and R(c) in the two-Higgs-doublet model with flavor-changing neutral currents

A study of R(b) and R(c) is presented in the context of a two-Higgs-doublet model with flavor-changing scalar currents (FCSC's). Implications of the model for the rho parameter and for b-->s gamma are also considered. The experimental data on R(b) places stringent constraints on the model parameters that are difficult to reconcile with the constraints from b-->s gamma and rho. If we treat the couplings of the model as purely phenomenological, then the model can still survive albeit in a rather narrow region of the parameter space. Noting that aspects of the experimental analysis for R(b) and R(c) may be of some concern, we also disregard R(b)(expt) and R(c)(expt) and give predictions for these using constraints from b-->s gamma and the rho parameter only. We emphasize the theoretical and experimental advantages of the observable R(b+c)=Gamma(Z-->b (b) over bar or c (c) over bar/Gamma(Z--> hadrons). We also stress the role of R(l)=Gamma(Z--> hadrons)/Gamma(Z-->l(+)l(-)) in testing the standard model despite its dependence on QCD corrections. Noting that in models with FCNC's the amplitude for Z-->c (c) over bar receives a contribution which grows with m(t)(2) the importance and uniqueness of precision Z-->c (c) over bar measurements for constraining flavor-changing t (c) over bar currents is underscored.