Bulk standard model in the Randall-Sundrum background

We discuss issues in an attempt to put the standard model (SM) in five-dimensional anti–de Sitter spacetime compactified on $S^1{/Z}_2.$ The recently proposed approach to the gauge hierarchy problem by using this background geometry, with the SM confined on a boundary, is extended to a situation where (some of) the SM particles reside in the five-dimensional bulk. In particular, we find a localization of zero modes of bulk fermions near the boundary with a negative tension. Unlike the compactification with the flat metric, these fermion zero modes couple to Kaluza-Klein (KK) excitations of the SM gauge bosons. Interestingly, only low-lying modes of such KK gauge bosons have non-negligible couplings. Current electroweak precision data give a constraint that the first KK mode be heavier than $9$ TeV. We also argue that at least the Higgs field should be confined on the brane to utilize the Randall-Sundrum background as a solution to the gauge hierarchy.