Oblique Corrections from Higgsless Models in Warped Space

TL;DR

The paper investigates tree-level oblique corrections in a Higgsless model of electroweak symmetry breaking realized in a warped extra dimension with gauge group $\text{SU(2)}_L\times\text{SU(2)}_R\times\text{U(1)}_{B-L}$. Using an AdS$_5$ background and boundary conditions that implement custodial symmetry, the authors compute the oblique parameters $S$, $T$, and $U$, finding $S\approx1.15$ and $T,U\approx0$ in the minimal setup, analogous to technicolor-like theories. They then explore deformations via asymmetric bulk couplings and brane-localized kinetic terms: Planck-brane terms and unequal left-right couplings can lower $S$ but tend to push KK modes and threaten tree-level unitarity, while TeV-brane terms for $\text{SU}(2)_D$ raise $S$, whereas a TeV-brane $U(1)_{B-L}$ term can drive $S$ toward zero and predict a light $Z'$ near $300$ GeV. The authors caution that experimental interpretations of $S$, $T$, and $U$ must account for the absence of a Higgs and the need to incorporate the full KK tower at loop level, and they highlight potential non-oblique corrections to third-generation fermions that require further study.

Abstract

We calculate the tree-level oblique corrections to electroweak precision observables generated in higgless models of electroweak symmetry breaking with a 5D SU(2)_L x SU(2)_R x U(1)_{B-L} gauge group on a warped background. In the absence of brane induced kinetic terms (and equal left and right gauge couplings) we find the S parameter to be ~1.15, while T,U~0, as in technicolor theories. Planck brane induced kinetic terms and unequal left-right couplings can lower S, however for sufficiently low values of S tree-level unitarity will be lost. A kinetic term localized on the TeV brane for SU(2)_D will generically increase S, however an induced kinetic term for U(1)_{B-L} on the TeV brane will lower S. With an appropriate choice of the value of this induced kinetic term S~0 can be achieved. In this case the mass of the lowest Z' mode will be lowered to about ~300 GeV.

Figures (2)

• Figure 1: Plot of the oblique parameters as functions of the $B-L$ kinetic term on the TeV brane, normalized as $\eta' = \tau' /(R \log R'/R)$. The grid lines point to the region where $S$ vanishes. These results include higher order corrections in the log and $\tau'$ expansion, and we also checked them numerically.
• Figure 2: The spectrum of the first three $Z$ resonances as a function of the $B-L$ kinetic term, normalized as $\eta' = \tau' /(R \log R'/R)$.