Spinor-vector duality in fermionic Z2 x Z2 heterotic orbifold models

We continue the classification of the fermionic Z(2) x Z(2) heterotic string vacua with symmetric internal shifts. The space of models is spanned by working with a fixed set of boundary condition basis vectors and by varying the sets of independent Generalized GSO (GGSO) projection coefficients (discrete torsion). This includes the Calabi-Yau like compactifications with (2, 2) world-sheet superconformal symmetry, as well as more general vacua with only (2, 0) superconformal symmetry. In contrast to our earlier classification that utilized a Monte Carlo technique to generate random sets of GGSO phases, in this paper we present the results of a complete classification of the subclass of the models in which the four-dimensional gauge group arises solely from the null sector. In line with the results of the statistical classification we find a bell shaped distribution that peaks at vanishing net number of generations and with similar to 15% of the models having three net chiral families. The complete classification reveals a novel spinor-vector duality symmetry over the entire space of vacua. The S-t <-> V duality interchanges the spinor plus anti-spinor representations with vector representations. We present the data that demonstrates the spinor-vector duality. We illustrate the existence of a duality map in a concrete example. We provide a general algebraic proof for the existence of the S-t <-> V duality map. We discuss the case of self-dual solutions with an equal number of vectors and spinors, in the presence and absence of E-6 gauge symmetry, and presents a couple of concrete examples of self-dual models without E-6 symmetry. (c) 2007 Elsevier B.V. All rights reserved.