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 (2HDM) with avor changing scalar currents (FCSC). Implications of the model for the -parameter and for b ! s are also considered. The experimental data on Rb places stringent constraints on the model parameters. The con guration of the model needed to account for Rb is found to be irreconcilable with constraints from b ! s and . In particular, if Rexp b > R SM b persists then this version of 2HDM will be ruled out or require signi cant modi cations. Noting that aspects of the experimental analysis for Rb and Rc may be of some concern, we also disregard Rexp b and R exp c and give predictions for these using constraints from b ! s and parameter only. We emphasize the theoretical and experimental advantages of the observable Rb+c (Z ! b b or c c)= (Z ! hadrons). We also stress the role of R` (Z ! hadrons)= (Z ! `` ) in testing the Standard Model (SM) despite its dependence on QCD corrections. Noting that in models with FCNC the amplitude for Z ! c c receives a contribution which grows with m2t , the importance and uniqueness of precision Z ! c c measurements for constraining avor changing t c currents is underscored.