Flat directions, string compactification and three generation models

We show how identification of absolutely flat directions allows the constru ction of a new class of compactified string theories with reduced gauge sym metry that may or may not be continuously connected to the original theory. We use this technique to construct a class of three generation models with just the Standard Model gauge group after compactification. We discuss the low-energy symmetries necessary for a phenomenologically viable low-energy model and construct an example in which these symmetries are identified wi th string symmetries which remain unbroken down to the supersymmetry breaki ng scale. Remarkably the same symmetry responsible for stabilising the nucl eon is also responsible for ensuring one and only one pair of Higgs doublet s is kept light. We show how the string symmetries also lead to textures in the quark and lepton mass matrices which can explain the hierarchy of ferm ion masses and mixing angles, (C) 1999 Elsevier Science B.V. All rights res erved.