Missing doublet multiplet as the origin of the doublet-triplet splitting in SUSY SU(6)

In a SU(6) gauge theory we found the irreducible representation (175-plet) which does not contain the Higgs doublet. Using this representation we construct two SUSY SU(6) models in which the doublet-triplet splitting occurs naturally, without fine tuning. The crucial role is played by the “custodial” global SU(2)H in combination with discrete or continuous R symmetries.