Z2 orbifold compactification of heterotic string and 6D SO(16) and E7×SU(2) flavor unification models

A Z2 orbifold compactification of the heterotic string is considered. The resulting 6D GUT groups can be SO(16) or E7×SU(2) plus some hidden sector groups. The N=4 supersymmetry is reduced to N=2. In particular, the SO(16) 6D model with one spinor representation 128 can reduce to the previous 5D SO(16) or SO(14) family unification models after compactifying the sixth dimension. To obtain one spinor, we have to take into account the left-over center of SO(16). We also comment on the E7×SU(2) model.