Gauge unification in 5D SU(5) model with orbifold breaking of GUT symmetry

We consider a five-dimensional SU(5) model wherein the symmetry is broken to the four-dimensional standard model by compactification of the fifth dimension on an S¹/(Z₂ × Z'₂) orbifold. SU(5) symmetry is recovered above the compactification scale and the strong, weak and electromagnetic gauge couplings are unified. For all SU(5) representations up to 75, we identify those members which have zero modes under this compactification. Assuming a single intermediate scale, we examine how these light scalars affect gauge coupling evolution and find that only a few cases can meet the requirement of unification of the couplings. The 5D compactification scale is determined via this renormalization group analysis. When SO(10) is considered as the GUT group there are only two acceptable cases, as long as a few low dimensional scalar multiplets up to 126 are included.