The strongest experimental constraints on SU(5)xU(1) supergravity models

We consider a class of well motivated string-inspired flipped $SU(5)$ supergravity models which include four supersymmetry breaking scenarios: no-scale, strict no-scale, dilaton, and special dilaton, such that only three parameters are needed to describe all new phenomena $(m_t,\tan\beta,m_{\tilde g})$. We show that the LEP precise measurements of the electroweak parameters in the form of the ${ϵ}_1$ variable, and the CLEOII allowed range for $\text{bsg}$ are at present the most important experimental constraints on this class of models. For $m_t\text{gsim}155(165)\text{GeV}$, the ${ϵ}_1$ constraint (at 90(95)%CL) requires the presence of light charginos ($m_{{\chi }_1^{\pm }}\text{lsim}50-100\text{GeV}$ depending on $m_t$). Since all sparticle masses are proportional to $m_{\tilde{g}}$, $m_{{\chi }_1^{\pm }}\text{lsim}100\text{GeV}$ implies: $m_{{\chi }_1^0}\text{lsim}55\text{GeV}$, $m_{{\chi }_2^0}\text{lsim}100\text{GeV}$, $m_{\tilde g}\lsim360\GeV$,$m_{\tilde q}\lsim350,(365)\GeV$,$m_{\tilde e_R}\lsim80,(125)\GeV$,$m_{\tilde e_L}\lsim120,(155)\GeV$,and$m_{\tilde\nu}\lsim100,(140)\GeV$intheno-scale(dilaton)flipped$SU(5)$supergravitymodel\mathrm{.}The$\bsg$ constraint excludes a significant fraction of the otherwise allowed region in the $(m_{{\chi }_1^{\pm }},\tan \beta )$ plane (irrespective of the magnitude of the chargino mass), while future experimental improvements will result in decisive tests of these models. In light of the ${ϵ}_1$ constraint, we conclude that the outlook for chargino and selectron detection at LEPII and at HERA is quite favorable in this class of models.