εb constraints on the minimal SU(5) and SU(5)xU(1) supergravity models

We have performed a systematic analysis to compute the one-loop electroweak corrections to the Z→b bar b vertex in terms of εb and Rb in the context of the minimal SU(5) and no-scale SU(5)xU(1) supergravity models. With the measured top mass, mt=174±10-12+13 GeV, recently announced by CDF, we use the latest CERN LEP data on εb and Rb [≡Γ(Z→b bar b)/Γ(Z→hadrons)] in order to constrain further the two models. We find that the present LEP data on εb and Rb constrain the two models rather severely. Especially the low-tanβ region is constrained more severely. tanβ approx-lt 2.5 (4.0) is excluded by εb at 90% C.L. for mt approx-gt 170 [180] GeV in the minimal SU(5) [no-scale SU(5)xU(1)] supergravity. An even more stringent constraint comes from Rb. It excludes tanβ approx-lt 4.0 at 90% C.L. for mt approx-gt 160 [170] GeV in the minimal SU(5) [no-scale SU(5)xU(1)] supergravity. We also find that the sign on μ in the two models can be determined from εb and Rb at 90% C.L