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

We consider minimal 5-dimensional extensions of the standard model compactified on an S1/Z2 orbifold, in which the SU(2)L and U(1)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 Rξgauge 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 ∼4 TeV on a universal compactification scale may be considerably relaxed to ∼3 TeV in a minimal scenario, in which the SU(2)L gauge boson is the only field that feels the presence of the fifth dimension.