Unification with enlarged Kaluza-Klein dimensions

In minimal theories with extra spatial dimensions at scales mu(0) much lowe r than the conventional GUT scale, unification can give too-large predictio ns for alpha(3)(M-Z) given alpha(1)(M-Z) and alpha(2)(M-Z) as empirical inp ut. We systematically study the effects of adding extra states above the co mpactification scale on running of the gauge couplings and find several sim ple examples that give unification where all alpha(i)(M-Z) are consistent w ith low-energy data. We study both the supersymmetric and nonsupersymmetric unification. (C) 1999 Published by Elsevier Science B.V. All rights reserv ed.