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

We construct and analyze an $SU(6)\times SU(2)$ 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 Ka\v{c}-Moody Algebra. Both cases $SU(6)\times SU(2){L}$and$SU(6)\times SU(2){R}$ are realized. The model is characterized by the $SU(6)\times SU(2)\to SU(4)\times SU(2)\times SU(2)$ breaking scale $M_X$, and the $SU(4)\times SU(2)\times SU(2) \to SU(3){C}\times SU(2){L}\times U(1){Y}$breakingscale$M{R}$\mathrm{.}Thespectrumbellow$M_R$includesanextrapairofcharge-1\mathrm{/}3colour-tripletsofmass$M_{I}\leq M_{R}$ 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 $M_{s}\sim 5\times 10^{17} GeV$. 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.