  To solve the doublet-triplet splitting problem in SU(5) grand unified
theories, we propose a four dimensional orbifold grand unified theory by acting
Z2 on the SU(5) gauge group. Without an adjoint Higgs, the orbifold procedure
breaks the SU(5) gauge symmetry down to the standard model gauge group, and
removes the triplet component of the fundamental SU(5) Higgs. In the
supersymmetric framework, we show that the orbifold procedure removes two
triplet superfields of the Higgs multiplets and leaves us with the minimal
supersymmetric standard model, which also solves the hierarchy problem and
realizes gauge coupling unification. We also discuss possible UV completions of
the orbifold theories.
