Rb and Rc in the two-Higgs-doublet model with flavor-changing neutral currents

A study of Rb and Rc is presented in the context of a two-Higgs-doublet model with flavor-changing scalar currents (FCSC close-quote s). Implications of the model for the ρ parameter and for b→sγ are also considered. The experimental data on Rb places stringent constraints on the model parameters that are difficult to reconcile with the constraints from b→sγ and ρ. 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 Rb and Rc may be of some concern, we also disregard Rbexpt and Rcexpt and give predictions for these using constraints from b→sγ and the ρ parameter only. We emphasize the theoretical and experimental advantages of the observable Rb+c≡Γ(Z→b bar borc bar c)/Γ(Z→hadrons). We also stress the role of Rl≡Γ(Z→hadrons)/Γ(Z→l+l-) in testing the standard model despite its dependence on QCD corrections. Noting that in models with FCNC close-quote s the amplitude for Z→c bar c receives a contribution which grows with m2t the importance and uniqueness of precision Z→c bar c measurements for constraining flavor-changing t bar c currents is underscored. copyright 1996 The American Physical Society