String derived MSSM and M-theory unification

The recent conjecture of possible equivalence between the string scale Ms a nd the minimal supersymmetric standard model unification scale M-U approxim ate to 2.5 x 10(16) GeV is considered in the context of string models. This conjecture suggests that the observable gauge group just below the string scale should be SU(3)(C) x SU(2)(L) x U(1)(Y) and that the SU(3)(C) x SU(2) (L) x U(1)(Y)-charged spectrum of the observable sector should consist sole ly of the MSSM spectrum. We demonstrate that string models can actually be constructed that possess these observable features. Two aspects generic to many classes of three family SU(3)(C) x SU(2)(L) x U(1)(Y) string models ar e both an extra local anomalous U(1)(A) and numerous (often fractionally ch arged) exotic particles beyond the MSSM. Thus, for these classes, the key t o obtaining an M-S = M-U approximate to 2.5 x 10(16) GeV string model is th e existence of F- and D-flat directions that near the string scale can simu ltaneously break the anomalous U(1) and give mass to all exotic SM-charged observable particles, decoupling them from the low energy spectrum. In this letter we show, in the context of free fermionic strings, that string mode ls with flat directions possessing these features do exist. We present one such string derived model in which all such exotic observable states beyond the MSSM receive mass at the scale generated by the Fayet-Iliopoulos term. The associated F- and D-flat direction is proven flat to all orders of the superpotential. (C) 1999 Elsevier Science B.V, All rights reserved.