  We construct two SU(5) models on the space-time $M^4 \times T^2/(Z_2 \times
Z_2^{\prime})$ where the gauge and Higgs fields are in the bulk and the
Standard Model fermions are on the brane at the fixed point or line. For the
zero modes, the SU(5) gauge symmetry is broken down to $SU(3) \times SU(2)
\times U(1) $ due to non-trivil orbifold projection. In particular, if we put
the Standard Model fermions on the 3-brane at the fixed point in Model II, we
only have the zero modes and KK modes of the Standard Model gauge fields and
two Higgs doublets on the observable 3-brane.
  So, we can have the low energy unification, and solve the triplet-doublet
splitting problem, the gauge hierarchy problem, and the proton decay problem.

High Energy Physics - Theory;High Energy Physics - Phenomenology