Classification of the chiral Z2XZ2 fermionic models in the heterotic   superstring
  The first particle physics observable whose origin may be sought in string theory is the triple replication of the matter generations. The class of Z2XZ2 orbifolds of six dimensional compactified tori, that have been most widely studied in the free fermionic formulation, correlate the family triplication with the existence of three twisted sectors in this class. In this work we seek an improved understanding of the geometrical origin of the three generation free fermionic models. Using fermionic and orbifold techniques we classify the Z2XZ2 orbifold with symmetric shifts on six dimensional compactified internal manifolds. We show that perturbative three generation models are not obtained in the case of Z2XZ2 orbifolds with symmetric shifts on complex tori, and that the perturbative three generation models in this class necessarily employ an asymmetric shift. We present a class of three generation models in which the SO(10) gauge symmetry cannot be broken perturbatively, while preserving the Standard Model matter content. We discuss the potential implications of the asymmetric shift for strong-weak coupling duality and moduli stabilization. We show that the freedom in the modular invariant phases in the N=1 vacua that control the chiral content, can be interpreted as vacuum expectation values of background fields of the underlying N=4 theory, whose dynamical components are projected out by the Z2-fermionic projections. In this class of vacua the chiral content of the models is determined by the underlying N=4 mother theory. 
A.E. Faraggi, C. Kounnas, S.E.M. Nooij, J. Rizos