Gauge Coupling Unification and Phenomenology of Selected Orbifold 5D N=1 SUSY Models

We study gauge coupling unification and various phenomenological issues, such as baryon number conservation, the $\mu $ problem and neutrino anomalies, within SUSY 5D orbifold models. The 5D MSSM on an $S^{(1)}/Z_2$ orbifold with 'minimal' field content does not lead to low scale unification, while some of its extensions can give unification near the multi TeV scale. Within the orbifold SU(5) GUT, low scale unification can not be realized due to full SU(5) multiplets participating in the renormalization above the compactification scale. As alternative examples, we construct 5D N=1 SUSY Pati-Salam $SU(4)_c\tm SU(2)_L\tm SU(2)_R\equiv G_{422}$ and flipped $SU(5)\tm U(1)\equiv G_{51}$ GUTs [both maximal subgroups of SO(10)] on an $S^{(1)}/Z_2\tm Z_2'$ orbifold. New examples of low scale unifications within $G_{422}$ are presented. For $G_{51}$ the unification scale is shown to be necessarily close to $\sim 10^{16}$ GeV. The possible influence of brane couplings on the gauge coupling unification is also outlined. For the resolution of the various phenomenological problems extensions with a discrete ${\cal Z}$ symmetry turn out to be very effective.