We explore the postulates of string no-scale supergravity in the context of free-fermionic string models. The requirements of vanishing vacuum energy, flat directions of the scalar potential, and stable no-scale mechanism impose strong restrictions on possible string no-scale models, which must possess only two or three moduli, and a constrained massless spectrum. The soft-supersymmetry-breaking parameters involving all twisted and untwisted fields are given explicitly. Our calculations take into account the possible existence of an anomalous U A(1) factor in the gauge group, and reveal some novel effects concerning the stability of the no-scale mechanism in the presence of U A(1). This class of models contains no free parameters, i.e. in principle all supersymmetric particle masses and interactions are completely determined. A computerized search for free-fermionic models with the desired properties yields a candidate SU (5)×U(1) model containing extra [Formula: see text] matter representations that allow gauge coupling unification at the string scale. Our candidate model possesses a benign nonuniversal assignment of supersymmetry-breaking scalar masses, which may have interesting low-energy experimental consequences.
Supersymmetric particle and Higgs boson masses from the landscape: Dynamical versus spontaneous supersymmetry breaking
