Effects of SM Kaluza-Klein excitations on electroweak observables
Manuel Masip, Alex Pomarol
TL;DR
The paper investigates the impact of a single TeV-scale extra dimension on electroweak observables by introducing a KK tower of SM gauge bosons that mix with the zero modes and mediate additional four-fermion interactions. It constructs a 5D framework on $S^1/Z_2$ with gauge fields in the bulk, fermions on 4D boundaries, and a Higgs sector that can be bulk, boundary, or a mixture (two Higgs doublets φ1 in bulk, φ2 on boundary), deriving leading corrections to W/Z masses and couplings. By confronting these corrections with precision data, it derives bounds on the compactification scale $M_c=1/R$—notably a model-independent lower bound around 2.5 TeV—and discusses constraints from leptonic Z width, LEP2 contact interactions, and Tevatron searches. The results indicate that any extra dimension must be sufficiently heavy to be consistent with current electroweak measurements, with multi-dimensional generalizations remaining highly model-dependent due to divergent KK sums.
Abstract
The presence of an extra dimension of size R \approx TeV^{-1} introduces a tower of Kaluza-Klein gauge boson excitations that affects the standard model (SM) relations between electroweak observables. The mixing of the W and Z bosons with their excitations changes their masses and couplings to fermions. This effect depends on the Higgs field, which may live in the bulk of the extra dimension, on its boundary, or may be a combination of both types of fields. We use high-precision electroweak data to constrain 1/R. We find limits from 1 to 3 TeV from different observables, with a model independent bound of 2.5 TeV.