John March-Russell

Abstract:

We show that the recently constructed 5-dimensional supersymmetric $S^1/(Z_2\times Z_2')$ orbifold GUT models allow an appealing explanation of the observed hierarchical structure of the quark and lepton masses and mixing angles. Flavor hierarchies arise from the geometrical suppression of some couplings when fields propagate in different numbers of dimensions, or on different fixed branes. Restrictions arising from locality in the extra dimension allow interesting texture zeroes to be easily generated. In addition the detailed nature of the SU(5)-breaking orbifold projections lead to simple theories where $b-\tau$ unification is maintained but similar disfavored SU(5) relations for the lighter generations are naturally avoided. We find that simple 5d models based on $S^1/(Z_2\times Z_2')$ are strikingly successful in explaining many features of the masses and mixing angles of the 2nd and 3rd generation. Successful three generation models of flavor including neutrinos are constructed by generalizing the $S^1/(Z_2\times Z'_2)$ model to six dimensions. Large angle neutrino mixing is elegantly accommodated. Novel features of these models include a simple $m_u=0$ configuration leading to a solution of the strong CP problem.

Abstract:

We investigate supersymmetric SU(5) grand-unified theories (GUTs) realized in 5 space-time dimensions and broken down to the MSSM by SU(5)-violating boundary conditions on a S^1/(Z_2 x Z_2') orbifold with two 3-branes. The doublet-triplet splitting problem is entirely avoided by locating the MSSM Higgs doublets on the brane on which SU(5) is not a good symmetry. An extremely simple model is then described in which the MSSM matter is also located on this SU(5)-violating brane. Although this model does not unify the MSSM matter within SU(5) multiplets, it explains gauge coupling unification. A second model with MSSM matter in the SU(5)-symmetric bulk preserves both the SU(5) explanation of fermion quantum numbers as well as gauge-coupling unification. Both models naturally avoid problematic SU(5) predictions for the Yukawa couplings of the first two generations and are consistent with proton decay constraints. We analyse the running of gauge couplings above the compactification scale in terms of a 5d effective action and derive the implications for the values of compactification scale, unification scale and of the scale at which the bulk gauge theory becomes strongly coupled.