String no-scale supergravity

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. All soft-supersymmetry-breaking parameters involving untwisted fields are given explicitly and those involving twisted fields are conjectured. This class of models contain no free parameters, \ie, 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)\times U(1) model, and evidence that all such models contain extra (\r{10},\rb{10}) matter representations that allow gauge coupling unification at the string scale. Our candidate model possesses a novel assignment of supersymmetry breaking scalar masses which gives vanishing contribution to the extra (\r{10},\rb{10}) scalar masses, and may have interesting low-energy experimental consequences.