Gauge Coupling Unification from Unified Theories in Higher Dimensions

A two-stage framework for unification is proposed involving grand unification, supersymmetry and extra dimensions, leading to a prediction for the strong gauge coupling, αKK s = 0.118±0.004±0.003, which is more successful and precise than that obtained from conventional supersymmetric grand unification, αSGUT s = 0.130 ± 0.004 ± ∆SGUT. Orbifold breaking of the unified gauge group leads to a non-universal, logarithmic running of the standard model gauge couplings over a large energy interval from the compactification scale to the string scale, where the theory is strongly coupled. Within this framework, only a single theory gives precise agreement with the experimental result αexp s = 0.117±0.002, and has SU(5) gauge interactions and two Higgs hypermultiplets propagating in a single extra dimension. The theory necessarily has a strongly coupled top quark located on a fixed point and part of the lightest generation propagating in the bulk. The string and compactification scales are determined to be around 1017 GeV and 1015 GeV, respectively. Implications for string theory are mentioned. For the case of strongly coupled heterotic string theory, the eleventh “gravity-only” dimension has a size of approximately 1016 GeV.