SU(5)xU(1): a string paradigm of a TOE and its experimental consequences

We present a string-inspired/derived supergravity model based on the flipped $SU(5)\times U(1)$ structure supplemented by a minimal set of additional matter representations such that unification occurs at the string scale ($\sim10^{18}\GeV$). This model is complemented by two string supersymmetry breaking scenaria: the $SU(N,1)$ no-scale supergravity model and a dilaton-induced supersymmetry breaking scenario. Both imply universal soft supersymmetry breaking parameters: $m_0=0, A=0$ and $m_0=\coeff{1}{\sqrt{3}}m_{1/2}, A=-m_{1/2}$ respectively. In either case the models depend on only three parameters: $m_t$, $\tan\beta$, and $m_{\tilde g}$. We present a comparative study of the sparticle and Higgs spectra of both models and conclude that even though both can be partially probed at the Tevatron, LEPII, and HERA, a larger fraction of the parameter space of the no-scale model is actually accessible. In both cases there is a more constrained version which allows to determine $\tan\beta$ in terms of $m_t,m_{\tilde g}$. In the strict no-scale case we find that the value of $m_t$ determines the sign of $\mu$ ($\mu>0:\,m_t\lsim135\GeV$, $\mu<0:\,m_t\gsim140\GeV$) and whether the lightest Higgs boson mass is above or below $100\GeV$. In the more constrained version of the dilaton scenario, $\tan\beta\approx1.4-1.6$ and $m_t\lsim155\GeV$, $61\GeV\lsim m_h\lsim91\GeV$ follow. Thus, continuing Tevatron top-quark searches and LEPI,II Higgs searches could probe this restricted scenario completely.