String Derived Exophobic SU(6)xSU(2) GUTs
  With the apparent discovery of the Higgs boson, the Standard Model has been confirmed as the theory accounting for all sub-atomic phenomena. This observation lends further credence to the perturbative unification in Grand Unified Theories (GUTs) and string theories. The free fermionic formalism yielded fertile ground for the construction of quasi--realistic heterotic--string models, which correspond to toroidal Z2xZ2 orbifold compactifications. In this paper we study a new class of heterotic-string models in which the GUT group is SU(6)xSU(2) at the string level. We use our recently developed fishing algorithm to extract an example of a three generation SU(6)xSU(2) GUT model. We explore the phenomenology of the model and show that it contains the required symmetry breaking Higgs representations. We show that the model admits flat directions that produce a Yukawa coupling for a single family. The novel feature of the SU(6)xSU(2) string GUT models is that they produce an additional family universal anomaly free U(1)symmetry that may remain unbroken below the string scale. The massless spectrum of the model is free of exotic states. 
Laura Bernard, Alon E Faraggi, Ivan Glasser, John Rizos and Hasan   Sonmez
hep-th hep-ph