$R_b$ in supergravity models

We compute the supersymmetric contribution to $R_b\equiv \mathrm{\Gamma }(Z\to b\bar{b})\mathrm{/}\mathrm{\Gamma }(Z\to \mathrm{h}\mathrm{a}\mathrm{d}\mathrm{r}\mathrm{o}\mathrm{n}\mathrm{s})$ in a variety of supergravity models. We find $R_b^{\mathrm{s}\mathrm{u}\mathrm{s}\mathrm{y}}\text{lsim}0.0004$, which does not shift significantly the Standard Model prediction ($R_b^{\mathrm{S}\mathrm{M}}=0.2162$ for $m_t=160\text{GeV}$). An improvement in experimental precision by a factor of four would be required to be sensitive to such an effect.