Kaluza-Klein decomposition and gauge coupling unification in orbifold GUTs

We discuss Kaluza-Klein (KK) decomposition in 5-dimensional (5D) field theories with orbifold compactification. Kinetic terms localized at orbifold fixed points, which are inevitably present in any realistic model, modify the standard KK mass spectrum and interactions of KK modes. This, in turn, can significantly affect phenomenology of the orbifold models. As an example, we discuss gauge coupling unification in N=1 supersymmetric 5D orbifold SU(5) model. We have found that uncertainties in the predictions of the model related to modification of the KK masses are large and essentially uncontrollable.