A $U_{e}(1)$-covariant $R_{\xi}$ gauge for the two-Higgs doublet model based on BRST (Becchi–Rouet–Stora–Tyutin) symmetry is introduced. This gauge allows one to remove a significant number of nonphysical vertices appearing in conventional linear gauges, which greatly simplifies the loop calculations, since the resultant theory satisfies QED-like Ward identities. The presence of four ghost interactions in these types of gauges and their connection with the BRST symmetry are stressed. The Feynman rules for those new vertices that arise in this gauge, as well as for those couplings already present in the linear $R_{\xi}$ gauge but that are modified by this gauge-fixing procedure, are presented.

Effects of universal extra dimensions on Standard Model observables first arise at the one-loop level. The quantization of this class of theories is therefore essential in order to perform predictions. A comprehensive study of the $SU_{C}(3) \times SU_{L}(2) \times U_{Y}(1)$ Standard Model defined in a space-time manifold with one universal extra dimension, compactified on the oribifold $S^{1}/Z_{2}$, is presented. The fact that the four-dimensional Kaluza–Klein theory is subjected to two types of gauge transformations is stressed and its quantization under the basis of the BRST symmetry discussed. $A SU_{C}(3) \times SU_{L}(2) \times U_{Y}(1)$-covariant gauge-fixing procedure for the Kaluza–Klein excitations is introduced. The connection between gauge and mass eigenstate fields is established in an exact way. An exhaustive list of the explicit expressions for all physical couplings induced by the Yang–Mills, Currents, Higgs, and Yukawa sectors is presented. The one-loop renormalizability of the standard Green’s functions, which implies that the Standard Model observables do not depend on a cut-off scale, is stressed.