Holographic Theories of Electroweak Symmetry Breaking without a Higgs Boson

TL;DR

This work develops holographic, Higgsless models of electroweak symmetry breaking by embedding the dynamics in a warped 5D AdS space where EW breaking occurs via IR boundary conditions, dual to a 4D strongly coupled gauge sector G with coupling $\tilde{g}$ and size $N$. It demonstrates that, in the minimal warped model, the oblique S parameter is too large unless the 5D theory is strongly coupled or an additional negative-S sector is introduced, outlining two viable paths to viability: extend the model to cancel S or accept a strongly coupled regime with limited calculability but potentially acceptable precision electroweak corrections. The framework yields SM-like fermions and masses below ~2 TeV, with distinctive string-type resonances and a possible light radion at a nearby scale; the top-quark sector and flavor physics provide further phenomenological handles, including moderate deviations in $t_RZ$ couplings and potential FCNC signals. Overall, the paper offers a calculable, dual perspective on dynamical EW symmetry breaking without a Higgs, predicting testable TeV-scale signatures and guiding directions for UV completion.

Abstract

Recently, realistic theories of electroweak symmetry breaking have been constructed in which the electroweak symmetry is broken by boundary conditions imposed at a boundary of higher dimensional spacetime. These theories have equivalent 4D dual descriptions, in which the electroweak symmetry is dynamically broken by non-trivial infrared dynamics of some gauge interaction, whose coupling g and size N satisfy g^2N > 16pi^2. Such theories allow one to calculate electroweak radiative corrections, including the oblique parameters S, T and U, as long as g^2N/16pi^2 and N are large. We study how the duality between the 4D and 5D theories manifests itself in the computation of various physical quantities. In particular, we study a warped 5D theory where the electroweak symmetry is broken by boundary conditions at the infrared brane. We show that S exceeds the experimental bound if the minimal theory is in a weakly coupled regime. This requires either an extension of the model or departure from weak coupling. An interesting scenario is obtained if the gauge couplings in the 5D theory take the largest possible values -- the value suggested by naive dimensional analysis. We argue that such a theory can provide a potentially consistent picture for dynamical electroweak symmetry breaking: corrections to the electroweak observables are sufficiently small while realistic fermion masses are obtained without conflicting with bounds from flavor violation. The theory contains only the standard model quarks, leptons and gauge bosons below \sim 2 TeV, except for a possible light radion. At \sim 2 TeV increasingly broad string resonances appear. An analysis of top-quark phenomenology and flavor violation is also presented, which is applicable to both the weakly-coupled and strongly-coupled cases.

Figures (11)

• Figure 1: Schematic description for the evolution of the coupling parameter $\kappa \equiv \tilde{g}^2 N/16\pi^2$ in various theories of dynamical electroweak symmetry breaking. The behaviors of (a), (b), (c), and (d) represent those of 5D flat space, 5D warped space, walking technicolor and technicolor theories, respectively.
• Figure 2: The diagram contributing to the $W$ boson propagators. Similar diagrams exist with one or two external $W_\mu$'s replaced by $B_\mu$.
• Figure 3: The diagrams represented by the gray disk. Here, $\psi_G$ and $A^G_\mu$ represent matter and gauge fields of the strongly-coupled $G$ sector. The size of each diagram is also shown. The contribution from this set of diagrams will be of order $N/16\pi^2$.
• Figure 4: The diagram representing the mixing between the elementary $W$ boson and the composite $W$ states arising from the dynamics of $G$. Cutting the diagrams at $A$ gives the states which have the same quantum numbers and spin as $W$. Similar diagrams also exist for $W_\mu$ replaced by the gauge boson of $U(1)_Y$, $B_\mu$.
• Figure 5: The diagrams with an elementary loop (a), with the $G$ effect at the next-to-leading order in $1/N$ (b), and with an additional loop of elementary fields on top of the leading $G$ effect (c,d). For (a) and (c), similar diagrams using gauge 4-point vertices exist. For (b) and (d), there are also similar diagrams with $B_\mu$ on some of external lines.