Options for Orbifold-GUT Model Building from Five-Dimensional   Supergravity
  This is the first paper of a series that will examine the options for embedding supersymmetric orbifold-GUTs into five-dimensional N=2 Yang-Mills-Einstein supergravity theories (YMESGTs). In particular, we focus on the allowed couplings of charged hypermultiplets in the lowest dimensional reps of the gauge groups SU(5), SO(10) and E(6). Our results are within the classification of homogeneous quaternionic scalar manifolds. In the minimal coupling of a generation of bulk matter hypermultiplets, supergravity requires the field content of an SO(10) scenario. In the minimal coupling of $n$ bulk generations of matter and higgs hypermultiplets, supergravity requires the field content of an E(6) scenario.   We also discuss the coupling of tensors and non-compact gaugings in 5D YMESGTs, which can serve as alternative ways to obtain four-dimensional Higgs sectors. Charged tensor couplings seem to be difficult to work with phenomenologically since a U(1) gauge factor is always required when they are present, and it is not clear if tensors can be put in unified multiplets with other fields, if this is desired. This seems to imply that tensor coulpings in GUT scenarios may be better suited in higher dimensional settings. The non-compact gaugings discussed here are simple, and offer a novel unification scenario in which the supergravity and vector multiplets are connected by gauge transformations.   The main points are summarized in tables and the conclusion. Although the discussion is in the spirit of a "bottom-up" approach, M-theory is taken as a motivating background. 
Sean McReynolds