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Abstract

Maximally supersymmetric SO ( 10 ) and SU ( 6 ) unified theories are constructed on the orbifold T 2 / ( Z 2 × Z 2 ′ ) , with one length scale R 5 taken much larger than the other, R 6 . The effective theory below 1 / R 6 is found to be the highly successful SU ( 5 ) theory in 5D with natural doublet–triplet splitting, no proton decay from operators of dimension four or five, unified mass relations for heavier generations only, and a precise prediction for gauge coupling unification. A more unified gauge symmetry, and the possibility of Higgs doublets being components of the higher-dimensional gauge multiplet, are therefore compatible with a large energy interval where physics is described by SU ( 5 ) gauge symmetry in 5D. This leads to the distinctive branching ratios for proton decay from SU ( 5 ) gauge boson exchange, p → l + π 0 , l + K 0 , ν ¯ π + , ν ¯ K + ( l = e , μ ), for well-motivated locations for matter. Several phenomenological features of the higher unified gauge symmetry are discussed, including the role of an extra U ( 1 ) gauge symmetry, which survives compactification, in the generation of neutrino masses.