$ε_b$ Constraints on the Minimal $SU(5)$ and $SU(5)\times U(1)$ Supergravity Models

Abstract

We have performed a systematic analysis to compute the one-loop electroweak corrections to the Z->b b-bar vertex in terms of $ε_b$ and $R_b$ in the context of the minimal $SU(5)$ and no-scale $SU(5)\times U(1)$ supergravity models. With the measured top mass, $m_t=174\pm 10^{+13}_{-12}$ $\GeV$ , recently announced by CDF, we use the latest LEP data on $ε_b$ and $R_b$ ($\equiv{Γ(Z->b b-bar)/{Γ(Z->hadrons)}}$) in order to constrain further the two models. We find that the present LEP data on $ε_b$ and $R_b$ constrain the two models rather severely. Especially, the low-$\tanβ$ region is constrained more severely. $\tanβ\lsim 2.5$ $(4.0)$ is excluded by $ε_b$ at $90\%$ C.~L. for $m_t\gsim 170$ $(180)$ $\GeV$ in the minimal $SU(5)$ (no-scale $SU(5)\times U(1)$) supergravity. Even more stringent constraint comes from $R_b$. It excludes $\tanβ\lsim 4.0$ at $90\%$ C.~L. for $m_t\gsim 160$ $(170)$ $\GeV$ in the minimal $SU(5)$ (no-scale $SU(5)\times U(1)$) supergravity. We also find that the sign on $μ$ in the two models can be determined from $ε_b$ and $R_b$ at $90\%$ C.~L.