Wilson lines corrections to gauge couplings from a field theory approach

Using an effective field theory approach, we address the effects on the gauge couplings of one and two additional compact dimensions in the presence of a constant background (gauge) field. Such background fields are a generic presence in models with extra dimensions and can be employed for gauge symmetry breaking mechanisms in the context of 4D N=1 supersymmetric models. The structure of the ultraviolet (UV) and infrared (IR) divergences that the gauge couplings develop in the presence of Wilson line vev's is investigated. One-loop radiative corrections to the gauge couplings due to overlapping effects of the compact dimensions and Wilson line vev's are computed for generic 4D N=1 models. Values of Wilson lines vev's corresponding to points (in the ``moduli'' space) of enhanced gauge symmetry cannot be smoothly reached {\it perturbatively} from those corresponding to the broken phase. The one-loop corrections are compared to their (heterotic) string counterpart in the ``field theory'' limit alpha'->0 to show remarkably similar results when no massless states are present in a Kaluza-Klein tower. An additional correction to the gauge coupling exists in the effective field theory approach when for specific Wilson lines vev's massless Kaluza-Klein states are present. This correction is not recoverable by the limit alpha'->0 of the (infrared regularised) string because the infrared regularisation limit and the limit alpha'->0 of the string result do not commute.