Physics at LEP and Yukawa Coupling Unification

Abstract

We discuss the impact of the recent precision measurements at LEP on the minimal supersymmetric standard model. We show that the values of $\tan\beta$ necessary to induce large positive corrections to $R_b$ are the same as the ones preferred by the unification of the bottom and $\tau$ Yukawa couplings in the MSSM for the current measured value of $M_t$. We discuss the relevance of the preferred parameter space for the Higgs and sparticle searches at LEP2. Remarkably, it follows that the LEP measurements can provide information about the structure of soft supersymmetry breaking parameters at $M_{GUT}$. Finally, we briefly discuss the properties of a supersymmetric model with four generations, for which the fit to $R_b$ naturally improves with respect to the one in the Standard Model.