Rbin supersymmetric models

We compute the supersymmetric contribution to ${\mathit{R}}_{\mathit{b}}$≡Γ(Z→bb¯)/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 ${\mathit{R}}_{\mathit{b}}^{\mathrm{SUSY}}$≲0.0004, which does not shift significantly the standard model prediction (${\mathit{R}}_{\mathit{b}}^{\mathrm{SM}}$=0.2157 for ${\mathit{m}}_{\mathit{t}}$=175 GeV; ${\mathit{R}}_{\mathit{b}}^{\mathrm{expt}}$=0.2204±0.0020). We also compute ${\mathit{R}}_{\mathit{b}}$ in the minimal supersymmetric standard model (MSSM), and delineate the region of parameter space which yields interestingly large values of ${\mathit{R}}_{\mathit{b}}$. 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 ${\mathit{R}}_{\mathit{b}}^{\mathrm{SUSY}}$>0.0020 (1σ) are allowed.