Warped Higgsless Models with IR--Brane Kinetic Terms

TL;DR

This work investigates warped Higgsless electroweak symmetry breaking in a 5D RS-like setup with IR-brane kinetic terms for U(1)_{B-L} and the SU(2)_D custodial sector. It finds that the U(1)_{B-L} brane term primarily affects neutral KK states and can improve alignment with tree-level SM relations for certain couplings, while the SU(2)_D term lowers custodial KK masses and can significantly delay perturbative unitarity violation in W_L^+W_L^- scattering to around 6–7 TeV for substantial parameter space. However, precision EW constraints and collider searches (Tevatron and LEP II) place nontrivial bounds on the brane-term sizes, leaving only limited viable regions, and highlighting tension between maintaining SM-like tree-level relations and delaying unitarity violation without new physics. Overall, multi-TeV perturbative unitarity remains challenging in this framework, though a constrained parameter space with κ≈1 and modest δ_D offers potential viability pending radiative corrections.

Abstract

We examine a warped Higgsless $SU(2)_L\times SU(2)_R\times U(1)_{B-L}$ model in 5--$d$ with IR(TeV)--brane kinetic terms. It is shown that adding a brane term for the $U(1)_{B-L}$ gauge field does not affect the scale ($\sim 2-3$ TeV) where perturbative unitarity in $W_L^+ W_L^- \to W_L^+ W_L^-$ is violated. This term could, however, enhance the agreement of the model with the precision electroweak data. In contrast, the inclusion of a kinetic term corresponding to the $SU(2)_D$ custodial symmetry of the theory delays the unitarity violation in $W_L^\pm$ scattering to energy scales of $\sim 6-7$ TeV for a significant fraction of the parameter space. This is about a factor of 4 improvement compared to the corresponding scale of unitarity violation in the Standard Model without a Higgs. We also show that null searches for extra gauge bosons at the Tevatron and for contact interactions at LEP II place non-trivial bounds on the size of the IR-brane terms.

Figures (8)

• Figure 1: $\sin^2 \theta$ in each of the three definitions as a function of $\delta_{B,D}$. The black horizontal solid and dashed curves correspond to the on-shell value $\pm 1\sigma$, the solid red (dashed blue) curve represents $\sin^2 \theta_{eff}$ for $\kappa=3 (1)$ while the dash-dotted green (dotted magenta) curve is for $\sin^2 \theta_{eg}$. The top (bottom) panel illustrates the effects of including the $U(1)_{B-L}$ ($SU(2)_D$) kinetic term. We take only one IR kinetic term to be non-vanishing at a time.
• Figure 2: Same as in the previous figure but now with both $\delta_{B,D}$ nonzero for the case $\kappa=1$. The solid magenta curve is the value of $\sin^2 \theta_{eff}$ while the dash-dotted curves are all for $\sin^2 \theta_{eg}$ for, from left to right, $\delta_B=0,10,12,15,20$ and 30, respectively.
• Figure 3: $\delta \rho_{eff}^Z$ as a function of $\delta_{B,D}$ for $\kappa=1$ and 3. We take only one IR kinetic term to be non-vanishing at a time.
• Figure 4: Values of the pseudo-oblique parameters $S^*$ (solid red, dash dotted green) and $U^*$ (blue dashed, dotted magenta) for of $\kappa=(3,1)$ as labeled as functions of $\delta_{B_D}$. We take only one IR kinetic term to be non-vanishing at a time.
• Figure 5: Behavior of the neutral KK mass spectrum as a function of $\delta_{B,D}$. From bottom to top on the left the curves correspond to the states $Z_{2,3,..}$. $\kappa=1$ has been assumed. We take only one IR kinetic term to be non-vanishing at a time.