Rb in supersymmetric models.

We compute the supersymmetric contribution to Rb ≡ Γ(Z → bb̄)/Γ(Z → hadrons) in a variety of supersymmetric models. In the context of supergravity models with universal soft-supersymmetry-breaking and radiative electroweak breaking we find R b <∼ 0.0004, which does not shift significantly the Standard Model prediction (R b = 0.2157 for mt = 175GeV; R exp b = 0.2204 ± 0.0020). We also compute Rb in the minimal supersymmetric standard model (MSSM), and delineate the region of parameter space which yields interestingly large values of Rb. This region entails light charginos and top-squarks, but is strongly restricted by the combined constraints from B(b → sγ) and a not-too-large invisible top-quark branching ratio: only a few percent of the points with R b > 0.0020 (1σ) are allowed. CTP-TAMU-25/95 ACT-10/95 June 1995