Constraints on the electroweak universal parameters and the top and Higgs masses from updated LEP/SLC data

Abstract

A global analysis is performed using the latest data from LEP and SLC. Constraints on the electroweak universal parameters ($S,T,U$) and on the masses of the top quark and Higgs boson within the Standard Model (SM) are investigated. The uncertainties due to the QCD and QED effective couplings, $\alpha_s(\mz)$ and $\bar{\alpha}(\mmz)$, are examined in detail. Even though the mean value of $S$ is increased to be consistent with zero, the naive Technicolor models are still disfavored due to its reduced error. Within the SM, we find the 90\,\%CL constraints; $133\gev<m_t<190\gev$ and $10\gev<\mh<440\gev$ for $\alpha_s(\mz)=0.116$ and $1/\bar{\alpha}(\mmz) =128.72$. The experimental constraints on the $\zbb$ vertex form-factor, $\delb(\mmz)$, play an important role in disfavoring the region of large $m_t(m_t \sim 200\gev)$ and large $\mh(\mh \sim 1000\gev)$. If $m_t$ is precisely known, the present electroweak data give a rather strict upper bound on the Higgs mass, $\mh<140\,(300)\gev\,$ at 95\%~CL, for $m_t=160\, (175)\gev$ and for the above $\alpha_s(\mz)$ and $\bar{\alpha}(\mmz)$.