Power-Like Threshold Corrections to Gauge Unification in Extra   Dimensions
  One of the much-debated novel features of theories with extra dimensions is the presence of power-like loop corrections to gauge coupling unification, which have the potential of allowing a significant reduction of the unification scale. A recognized problem of such scenarios is the UV sensitivity of the above power corrections. We consider situations where the grand unified group is broken by the vacuum expectation value of a bulk field and find that, because of the softness of this extra-dimensional symmetry breaking mechanism, power-like threshold corrections are calculable and generic in many of the most relevant settings. While the precision is limited by the presence of higher-dimension bulk operators, the most dangerous of these operators are naturally forbidden by symmetries of the bulk theory. Particularly interesting and constrained scenarios arise in the context of higher-dimensional supersymmetry. Our phenomenological exploration of SU(5) models in 5d, linked in particular with more recently discussed orbifold GUT models, shows promising results. 
A. Hebecker, A. Westphal