Grand unification at intermediate mass scales through extra dimensions

One of the drawbacks of conventional grand unification scenarios has been t hat the unification scale is too high to permit direct exploration. In this paper, we show that the unification scale can be significantly lowered (pe rhaps even to the TeV scale) through the appearance of extra space-time dim ensions, Such extra dimensions are a natural consequence of string theories with large-radius compactifications. We show that extra space-time dimensi ons naturally lead to gauge coupling unification at intermediate mass scale s, and moreover may provide a natural mechanism for explaining the fermion mass hierarchy by permitting the fermion masses to evolve with a power-law dependence on the mass scale, We also show that proton-decay constraints ma y be satisfied in our scenario due to the higher-dimensional cancellation o f proton-decay amplitudes to all orders in perturbation theory. Finally, we extend these results by considering theories without supersymmetry; experi mental collider signatures; and embeddings into string theory. The latter a lso enables us to develop several novel methods of explaining the fermion m ass hierarchy via D-branes. Our results therefore suggest a new approach to wards understanding the physics of grand unification as well as the phenome nology of large-radius string compactifications. (C) 1999 Elsevier Science B.V.