Modular Invariance, Soft Breaking, $\mu$ and $\tan \beta$ in Superstring Models

Abstract

We go beyond parameterizations of soft terms in superstring models and investigate the dynamical assumptions that lead to the relative strength of the dilaton {\it vs} the moduli contributions in the soft breaking. Specifically, we discuss in some simple heterotic orbifold models sufficient conditions to achieve dilaton dominance. Assuming self-dual points to be minima we find multiple solutions to the trilinear and bilinear soft parameters $A_0$ and $B_0$. We discuss the constraints on $\mu$ and $\tan \beta$ in superstring models in the context of radiative breaking of the electroweak symmetry. We show that string models prefer a small to a moderate value of $\tan \beta$, {\it i.e}. $\tan \beta \le 10$, and a value much larger than this requires a high degree of fine tuning. Further, we show that for large $\tan \beta$ the radiative electroweak symmetry breaking constraint leads to a value ${\alpha }_{string}=g_{string}^2/4\pi$ which is typically an order of magnitude smaller than implied by the LEP data and the heterotic superstring relation $g_{string}=k_i{g}_i$, where $g_i$ is the gauge coupling constant for the gauge group $G_i$ and $k_i$ is the corresponding Kac-Moody level in the class of models considered. This situation can be overcome by another fine tuned cancellation between the dilaton and the moduli contributions in the soft parameters.

Comments: 15 pages, Latex, including 3 figures. Revised version. Accepted for publication in Physics Letters B