New Precision Electroweak Tests in Supergravity Models

Abstract

We update the analysis of the precision electroweak tests in terms of 4 epsilon parameters, $ε_{1,2,3,b}$, to obtain more accurate experimental values of them by taking into account the new LEP data released at the 28th ICHEP (1996, Poland). We also compute $ε_1$ and $ε_b$ in the context of the no-scale $SU(5)\times U(1)$ supergravity model to obtain the updated constraints by imposing the correlated constraints in terms of the experimental ellipses in the $ε_1-ε_b$ plane and also by imposing the new bound on the lightest chargino mass, $m_{χ^\pm_1}\gtrsim 79$ $ GeV$. Upon imposing these new experimental results, we find that the situations in the no-scale model are much more favorable than those in the standard model, and if $m_t\gtrsim 170$ $ GeV$, then the allowed regions at the 95% C.~L. in the no-scale model are $\tanβ\gtrsim 4$ and $m_{χ^\pm_1}\lesssim 120 (82)$ $ GeV$ for $μ>0 (μ<0)$, which are in fact much more stringent than in our previous analysis. Therefore, assuming that $m_t\gtrsim 170$ $ GeV$, if the lightest chargino mass bound were to be pushed up only by a few GeV, the sign on the Higgs mixing term $μ$ in the no-scale model could well be determined from the $ε_1-ε_b$ constraint to be positive at the 95% C.~L. At any rate, better accuracy in the measured $m_t$ from the Tevatron in the near future combined with the LEP data is most likely to provide a decisive test of the no-scale $SU(5)\times U(1)$ supergravity model.