Minimal higher-dimensional extensions of the standard model and electroweak observables

We consider minimal 5-dimensional extensions of the standard model compactified on an $S^1{/Z}_2$ orbifold, in which the $\mathrm{SU}{\mathrm{}\left(2\right)\mathrm{}}_L$ and $\mathrm{U}{\mathrm{}\left(1\right)\mathrm{}}_Y$ gauge fields and Higgs bosons may or may not all propagate in the fifth dimension while the observable matter is always assumed to be confined to a 4-dimensional subspace. We pay particular attention to consistently quantize the higher-dimensional models in the generalized $R_{\xi }$ gauge and derive analytic expressions for the mass spectrum of the resulting Kaluza-Klein states and their couplings to matter. Based on recent data from electroweak precision tests, we improve previous limits obtained in the 5-dimensional standard model with a common compactification radius and extend our analysis to other possible 5-dimensional standard-model constructions. We find that the usually derived lower bound of $\sim 4\hspace{0.5em}\mathrm{TeV}$ on a universal compactification scale may be considerably relaxed to $\sim 3\hspace{0.5em}\mathrm{TeV}$ in a minimal scenario, in which the $\mathrm{SU}{\mathrm{}\left(2\right)\mathrm{}}_L$ gauge boson is the only field that feels the presence of the fifth dimension.