The Simplest, String-Derivable, Supergravity Model and its Experimental Predictions

Abstract

We present the simplest, string-derivable, supergravity model and discuss its experimental consequences. This model is a new string-inspired flipped $SU(5)$ which unifies at the string scale $M_U=10^{18}\text{GeV}$ due to the introduction of an additional pair of \r{10},\rb{10} flipped $SU(5)$ representations which contain new intermediate scale `gap' particles. We study various model-building issues which should be addressed in string-derived incarnations of this model. We focus our study on the no-scale supergravity mechanism and explore thoroughly the three-dimensional parameter space of the model ($m_{\tilde{g}},m_t,\tan \beta$), thus obtaining several simple relationships among the particle masses, such as: $m_{\tilde{q}}\approx m_{\tilde{g}}$, $m_{{\tilde{e}}_L}\approx m_{\tilde{\nu }}\approx 0.30m_{\tilde{g}}$, $m_{{\tilde{e}}_R}\approx 0.18m_{\tilde{g}}$, and $m_{{\chi }_2^0}\approx 2m_{{\chi }_1^0}\approx m_{{\chi }_1^{\pm }}$. In a strict interpretation of the no-scale supergravity scenario we solve for $\tan \beta$ as a function of $m_t$ and $m_{\tilde{g}}$, and show that $m_t$ determines not only the sign of the Higgs mixing parameter $\mu$ but also whether the lightest Higgs boson mass is above or below $100\text{GeV}$. We also find that throughout the parameter space the neutralino relic abundance is within observational bounds (${\Omega }_{\chi }{h}_0^2\text{lsim}0.25$) and may account for a significant portion of the dark matter in the Universe.

Comments: 29 pages plus nine figures (not included). CTP-TAMU-68/92, CERN-TH.6667/92