String scale unification in an SU(6)xSU(2) GUT

We construct and analyze an SU(6) X SU(2) GUT. The model is k=1 string embedable in the sense that we employ only chiral representations allowed at the k = 1 level of the associated Kac-Moody Algebra. Both cases SU(6) X SU(2), and SU(6) X SU(2), are realized. The model is characterized by the SU(6) X SU(2) --> SU(4) X SU(2) X SU(2) breaking scale M-x, and the SU(4) X SU(2) X SU(2) --> SU(3)(C) X SU(2)(L) X U(1)(Y) breaking scale M-R. The spectrum bellow M-R includes an extra pair of charge-1/3 colour-triplets of mass M-I less than or equal to M-R that does not couple to matter fields and, possibly, an extra pair of isodoublets. Above M-X the SU(6) and SU(2) gauge couplings always unify at a scale which can be taken to be the string unification scale M-s similar to 5 X 10(17) GeV. The model has Yukawa coupling unification since quarks and leptons obtain their masses from a single Yukawa coupling. Neutrinos obtain acceptably small masses through a see-saw mechanism. Coloured triplets that couple to matter fields are naturally split from the coexisting isodoublets without the need of any numerical fine tuning. (C) 1997 Elsevier Science B.V.