ar X iv : h ep - p h / 01 07 19 0 v 3 2 5 Ju l 2 00 1 1 Fermion Mass Hierarchy in the Grand Unified Theory on S

We suggest a simple grand unified theory where the fifth dimensional coordinate is compactified on an S1/(Z2 × Z ′ 2) orbifold. This model is based on the supersymmetric flipped SU(5) × U(1) grand unified theory, which can realize not only the triplet-doublet splitting but also the natural fermion mass hierarchies. The triplet-doublet splitting is realized by S1/(Z2 × Z ′ 2) orbifolding, which also reduces the gauge group as SU(5) × U(1) → SU(3)c × SU(2)L × U(1)Z × U(1)X . The suitable fermion mass hierarchies are generated by integrating out extra three sets of vector-like heavy fields which can propagate in five dimensions. The radiative corrections to the large Yukawa coupling of right-handed neutrinos can reduce the gauge group to SU(3)c × SU(2)L × U(1)Y .