R(B) AND R(C) IN THE 2-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-do ublet model with flavor-changing scalar currents (FCSC's). Implication s of the model for the rho parameter and for b-->s gamma are also cons idered. The experimental data on R(b) places stringent constraints on the model parameters that are difficult to reconcile with the constrai nts from b-->s gamma and rho. If we treat the couplings of the model a s 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 the se using constraints from b-->s gamma and the rho parameter only. We e mphasize 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 correc tions. Noting that in models with FCNC's the amplitude for Z-->c (c) o ver bar receives a contribution which grows with m(t)(2) the importanc e and uniqueness of precision Z-->c (c) over bar measurements for cons training flavor-changing t (c) over bar currents is underscored.