Flat directions in three-generation free-fermionic string models

In quasi-realistic string models that contain an anomalous U(1) the non-zer o Fayet-Iliopoulos term triggers the shifting of the original vacuum to a n ew one along some flat direction, so that SUSY is preserved but the gauge g roup is partially broken. The phenomenological study of these models thus r equires as a first step the mapping of the space of flat directions. We inv estigate F- and D-flat directions in several three-generation SU(3)c x SU(2 )(L) x U(1)(Y) free-fermionic string models and discuss the typical scenari os that generically arise. When they exist, we systematically construct the flat directions that preserve hypercharge, only break abelian group factor s, and can be proven to remain F-flat to all orders in the non-renormalizab le superpotential. (C) 1999 Elsevier Science B.V.