Implications of High Precision Experiments and the CDF Top Quark Candidates

Abstract

We discuss the consequences of recent experimental results from CDF, SLC, LEP and elsewhere for the Standard Model and for new physics. A global fit to all indirect precision data yields $m_t = 175 \pm 11^{+17}_{-19}$~GeV, $\sin^2\theta_{MS} = 0.2317(3)(2)$, and $\alpha_s = 0.127(5)(2)$, where the central values are for $M_H = 300$ GeV and the second uncertainties are for $M_H \rightarrow 1000$ GeV (+) and 60 GeV (-). The $m_t$ value is in remarkable agreement with the value $m_t=174 \pm 16$ GeV suggested by the CDF candidate events. There is a slight preference for a light Higgs with $M_H < 730$ (880) GeV at 95\% c.l. if the CDF $m_t$ value is (not) included. The sensitivity is, however, due almost entirely to the anomalously large observed values for the $Z \rightarrow b\bar{b}$ width and left-right asymmetry. The value of alpha_s (from the lineshape) is clean theoretically assuming the Standard Model, but is sensitive to the presence of new physics contributions to the $Z \rightarrow b\bar{b}$ vertex. Allowing a vertex correction $\delta_{b\bar{b}}^{\rm new}$ one obtains the significantly lower value alpha_s = 0.111 \pm 0.009, in better agreement with low energy determinations, and $\delta_{b\bar{b}}^{\rm new} = 0.023 \pm 0.011$. There is now enough data to perform more general fits to parameters describing new physics effects and to separate these from $m_t$ and $M_H$. Allowing the parameter $\rho_0$, which describes sources of $SU(2)$ breaking beyond the Standard Model, to be free one finds $\rho_0=1.0012 \pm 0.0017 \pm 0.0017$, remarkably close to unity. One can also separate the new physics contributions to the oblique parameters $S_{\rm new}$, $T_{\rm new}$ and $U_{\rm new}$, which all take values consistent with zero. The effects of supersymmetry on the determination of the SM parameters are discussed.