Grand Uni cation at Intermediate Mass Scales through Extra Dimensions

One of the drawbacks of conventional grand uni cation scenarios has been that the uni cation scale is too high to permit direct exploration. In this paper, we show that the uni cation scale can be signi cantly lowered (perhaps even to the TeV scale) through the appearance of extra spacetime dimensions. Such extra dimensions are a natural consequence of string theories with largeradius compacti cations. We show that extra spacetime dimensions naturally lead to gauge coupling uni cation at intermediate mass scales, 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 may be satis ed in our scenario due to the higher-dimensional cancellation of proton-decay amplitudes to all orders in perturbation theory. Finally, we extend these results by considering theories without supersymmetry; experimental collider signatures; and embeddings into string theory. The latter also enables us to develop several novel methods of explaining the fermion mass hierarchy via D-branes. Our results therefore suggest a new approach towards understanding the physics of grand uni cation as well as the phenomenology of large-radius string compacti cations. E-mail address: [email protected] yE-mail address: [email protected] zE-mail address: [email protected]