Kaluza-Klein thresholds and regularization (in)dependence

We present a method to control the regularization scheme dependence in the running of couplings in Kaluza-Klein theories. Specifically we consider the scalar theory in five dimensions, assuming that one dimension is compactif ied, and we study various regularization schemes in order to analyze concre tely the regularization scheme dependence of the Kaluza-Klein threshold eff ects. We find that in one-loop order, although the beta-functions are diffe rent for the different schemes, the net difference in the running of the co upling among the different schemes is very small for the entire range of en ergies. Our results have been extended to include more than one radius, and the gauge coupling unification is re-examined. Strings are: also used as a regulator. We obtain a particular regularization scheme of the effective f ield theory which can accurately describe the string Kaluza-Klein threshold effects. (C) 2000 Elsevier Science B.V. All rights reserved.