Grand unified theories in higher dimensions: from the heterotic string to Randall-Sundrum.

We explore the phenomenology of effective field theories in two different cases. First, we look at a mirror universe model, using an orbifold compactification of the heterotic string, in the free fermionic approach. An expected non-chiral Pati-Salam mirror universe model is transformed into a chiral model with enhanced hidden sector gauge symmetry and reduced observable sector gauge symmetry: $[SU(4)C\times SU(2)L\times SU(2)R]O\times [SU(4)C\times SU(2)L\times SU(2)R]H$, is necessarily transformed into a chiral $[SU(4)C\times SU(2)L]O\times [SO(10)\times SU(2)R]H$.

Second, we look at a non-supersymmetric flipped SU(5) theory built within the Randall-Sundrum framework. We derive the low energy running of the couplings, and show how the effects of the numbers and types of representations used for model building can be constrained by looking at universal contributions from KK modes to the beta functions. We show that the Yukawa couplings are, in general, exponentially small, leading to an exponentially short proton lifetime.