  When computing radiative corrections in models with compactified extra dimen-
sions, one has to sum over the entire tower of Kaluza-Klein excitations inside
the loops. The loop corrections generate a difference between the coupling
strength of a zero-mode gauge boson and the coupling strength of its
Kaluza-Klein excitation, although both originate from the same
higher-dimensional gauge interaction. Furthermore, this dis- crepancy will in
general depend on the cutoff scale and assumptions about the UV completion of
the extra-dimensional theory. In this article, these effects are studied in
detail within the context of the minimal universal extra dimension model
(MUED). The broad features of the cutoff scale dependence can be captured
through the so- lution of the functional flow equation in five-dimensional
space. However, an explicit diagrammatic calculation reveals some modifications
due to the compactification of the extra dimension. Nevertheless, when imposing
a physical renormalization condition, one finds that the UV sensitivity of the
effective Kaluza-Klein gauge-boson vertex is relatively small and not very
important for most phenomenological purposes. Similar conclusions should hold
in a larger class of extra-dimensional models besides MUED.
