Supersymmetry, supergravity and Rb revisited in the light of LEP 2

A previous study of supersymmetric models has indicated that they are unlikely to make a large contribution to Rb ≡ Γ(Z0 → bb)Γ(Z0 → hadrons). We revisit this analysis, taking into account the improved lower limits on sparticle masses provided recently by LEP 2 and the Tevatron, finding that a generic supersymmetric model cannot contribute more than about one-and-a-half current experimental standard deviations to Rb. We then specialize this analysis to minimal supergravity models with universal high-energy boundary conditions, and find a much more stringent upper bound Rbsusy < 0.0003. We discuss in detail why such models can only attain values of Rbsusy that are considerably smaller than those obtainable in more general supersymmetric models.