Descendants of NAHE – based free fermionic and Type I Z n 2 models

The NAHE–set, that underlies the realistic free fermionic models, corresponds to Z2 × Z2 orbifold at an enhanced symmetry point, with (h11, h21) = (27, 3). Alternatively, a manifold with the same data is obtained by starting with a Z2 × Z2 orbifold at a generic point on the lattice and adding a freely acting Z2 involution. In this paper we study type I orientifolds on the manifolds that underly the NAHE–based models by incorporating such freely acting shifts. We present new models in the Type I vacuum which are modulated by Z n 2 for n = 2, 3. In the case of n = 2, the Z2 × Z2 structure is a composite orbifold Kaluza Klein shift arrangement. The partition function provides a simpler spectrum with chiral matter. For n = 3, the case discussed is a Z2 modulation of the T /Z2 × Z2 spectrum. The additional projection shows an enhanced closed and open sector with chiral matter. The brane stacks are correspondingly altered from those which are present in the Z2 × Z2 orbifold. In addition, we discuss the models arising from the open sector with and without discrete torsion. [email protected] [email protected]