Gauge Unification in Higher Dimensions

A complete 5-dimensional SU(5) unified theory is constructed which, on compactification on the orbifold S/(Z2 ×Z ′ 2), yields the minimal supersymmetric standard model. The orbifold accomplishes SU(5) gauge symmetry breaking, doublet-triplet splitting, and a vanishing of proton decay from operators of dimension 5. Until 4d supersymmetry is broken, all proton decay from dimension 4 and dimension 5 operators is forced to vanish by an exact, anomaly free U(1)R symmetry. Quarks and leptons and their Yukawa interactions are located at the Z2 orbifold fixed points, where SU(5) is unbroken. A new mechanism for introducing SU(5) breaking into the quark and lepton masses is introduced, which originates from the SU(5) violation in the zero-mode structure of bulk multiplets. Even though SU(5) is absent at the Z ′ 2 orbifold fixed point, the brane threshold corrections to gauge coupling unification are argued to be negligibly small, while the logarithmic corrections are small and in a direction which improves the agreement with the experimental measurements of the gauge couplings. Furthermore, the X gauge boson mass is lowered, so that p → e+π0 is expected with a rate within about one order of magnitude of the current limit. Supersymmetry breaking occurs on the Z ′ 2 orbifold fixed point, and is felt directly by the gauge and Higgs sectors, while squarks and sleptons acquire mass via gaugino mediation, solving the supersymmetric flavor problem.