TASI Lectures on Electroweak Symmetry Breaking from Extra Dimensions

TL;DR

This work surveys how extra dimensions can illuminate electroweak symmetry breaking without a fundamental Higgs. It develops both flat and warped (AdS) realizations, emphasizing boundary-condition breaking, custodial SU(2) protection, and the AdS/CFT perspective to control unitarity and precision constraints. A central thread is the generation of realistic gauge-boson spectra and fermion masses via bulk fields, brane terms, and boundary conditions, while addressing oblique corrections (S,T,U) and the appearance of KK resonances. The framework provides a toolkit for constructing viable higgsless models and assessing their phenomenological viability, including localization of fermions and the impact on electroweak precision data and collider signatures.

Abstract

This is a pedagogical introduction into the possible uses and effects of extra dimensions in electroweak (TeV scale) physics, and in particular to models of electroweak symmetry breaking via boundary conditions ("higgsless models"). It is self contained: all the aspects of extra dimensional and electroweak physics used here are reviewed, before we apply these concepts to higgsless models. In the first lecture gauge theories in an extra dimension and on an interval are discussed. In the second lecture we describe the basic structure of higgsless models, while in the third lecture we discuss fermions in extra dimensions and the inclusion of fermions into higgsless models. The final lecture is devoted to the issue of electroweak precision observables in theories beyond the standard model and its applications to extra dimensional theories and in particular the higgsless models.

Figures (8)

• Figure 1: The action of the two $Z_2$ reflections in the extended circle picture. The fundamental domain of the $S^1/Z_2$ orbifold is just the interval between $0$ and $\pi R$, and the theory can be equivalently formulated on this line segment as well.
• Figure 2: Elastic scattering of longitudinal modes of KK gauge bosons, $n+n\to n+n$, with the gauge index structure $a+b\to c+d$.
• Figure 3: The tree-level diagrams contributing to the scattering of massive longitudinal gauge bosons in the SM without a Higgs.
• Figure 4: The four diagrams contributing at tree level to the elastic scattering amplitude of the nth KK mode.
• Figure 5: The symmetry breaking structure of the flat space higgsless toy model.