Flavor Changing Neutral Currents

A study of Rb and Rc is presented in the context of a Two Higgs Doublet Model (2HDM) with flavor changing scalar currents (FCSC). 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. The configuration of the model needed to account for Rb is found to be irreconcilable with constraints from b → sγ and ρ. In particular, if R b > R SM b persists then this version of 2HDM will be ruled out or require significant modifications. Noting that aspects of the experimental analysis for Rb and Rc may be of some concern, we also disregard R b and R exp c and give predictions for these using constraints from b → sγ and ρ parameter only. We emphasize the theoretical and experimental advantages of the observable Rb+c ≡ Γ(Z → bb̄ or cc̄)/Γ(Z → hadrons). We also stress the role of Rl ≡ Γ(Z → hadrons)/Γ(Z → l+l−) in testing the Standard Model (SM) despite its dependence on QCD corrections. Noting that in models with FCNC the amplitude for Z → cc̄ receives a contribution which grows with mt , the importance and uniqueness of precision Z → cc̄ measurements for constraining flavor changing tc̄ currents is underscored.