Analysis of electroweak precision data and prospects for future improvements

Abstract.

We update our previous work on an analysis of the electroweak data by including new and partly preliminary data available up to the 1996 summer conferences. The new results on the $Z$ partial decay widths into $b$ and $c$ hadrons now offer a consistent interpretation of all data in the minimal standard model. The value extracted for the strong interaction coupling constant $\alpha_s(m_Z)$ agrees well with determinations in other areas. New constraints on the universal parameters $S$, $T$ and $U$ are obtained from the updated measurements. No signal of new physics is found in the $S$, $T$, $U$ analysis once the SM contributions with $m_t \sim 175$GeV and those of not a too heavy Higgs boson are accounted for. The naive QCD-like technicolor model is now ruled out at the 99% CL even for the minimal model with ${\rm SU(2)_{TC}}$. In the absence of a significant new physics effect in the electroweak observables, constraints on masses of the top quark, $m_t$, and Higgs boson, $m_H$, are derived as a function of $\alpha_s$ and the QED effective coupling $\bar{\alpha}(m_Z^2)$. The preferred range of $m_H$ depends rather strongly on the actual value of $m_t$: $m_H<360 {\rm GeV}$ for $m_t=170{\rm GeV}$, while $m_H>130{\rm GeV}$ for $m_t=180{\rm GeV}$ at 95% CL. Prospects due to forthcoming improved measurements of asymmetries, the mass of the weak boson $W$ $m_W$, $m_t$ and $\bar{\alpha}(m_Z^2)$ are discussed. Anticipating uncertainties of 0.00020 for $\bar s^2(m^2_Z )$, 20 MeV for $m_W$, and 2 GeV for $m_t$, the new physics contributions to the $S$, $T$, $U$ parameters will be constrained more severely, and, within the SM, the logarithm of the Higgs mass can be constrained to about $\pm 0.35$. The better constraints on $S$, $T$, $U$ and on $m_H$ within the minimal SM should be accompanied with matching precision in $\bar{\alpha}(m_Z^2)$.