Decoupling of heavy Kaluza-Klein modes in models with five-dimensional scalar fields

We investigate the decoupling of heavy Kaluza-Klein modes in phi(4) theory and scalar QED with spacetime topology R-3,R-1 x S-1. We calculate the one-loop low-energy effective action by integrating out heavy KK modes. We construct generalized RGE's for the couplings with respect to the compactification scale M. With the solutions to the RG-like equations we find the M-scale dependence of the effective theory due to higher dimensional quantum effects. We find that the heavy modes decouple in phi(4) theory, but do not in scalar QED. All the non-decoupling effects are due to the zero mode of the fifth component of the 5D gauge field A(5). Because A(5) is a scalar under 4D Lorentz transformations, there is no gauge symmetry protecting it from getting mass and A(5)(4) interaction terms after loop corrections. In addition, operators are induced that violate charge universality. In light of these unpleasant features, we explore S-1/Z(2) compactifications, which eliminate A5, allowing for the heavy modes to decouple at low energies. A set of RG-like equations with respect to the compactification scale is derived for the couplings in scalar QED on the orbifold. It is found that both couplings become weaker as the compactification scale increases. We also explore the possibility of restoring decoupling by including higher dimensional operators. It is found that this is possible, but a high degree of fine tuning is required.