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

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

1994-08-03

arxiv.org/abs/hep-ph/9408218v1

Phys.Rev.D50:6686-6690,1994

Physics

High Energy Physics

High Energy Physics - Phenomenology

RevTeX 3.0, 15 Pages+2 figures(not included but available as a uuencoded file from gtpark@phyb.snu.ac.kr), SNUTP-94-66

Scientific paper

10.1103/PhysRevD.50.R6686

We have performed a systematic analysis to compute the one-loop electroweak corrections to the Z->b b-bar vertex in terms of $\epsilon_b$ 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 $\epsilon_b$ and $R_b$ ($\equiv{\Gamma(Z->b b-bar)/{\Gamma(Z->hadrons)}}$) in order to constrain further the two models. We find that the present LEP data on $\epsilon_b$ and $R_b$ constrain the two models rather severely. Especially, the low-$\tan\beta$ region is constrained more severely. $\tan\beta\lsim 2.5$ $(4.0)$ is excluded by $\epsilon_b$ 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 $\tan\beta\lsim 4.0$ 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 $\mu$ in the two models can be determined from $\epsilon_b$ and $R_b$ at $90\%$ C.~L.

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