Patterns of Custodial Isospin Violation from a Composite Top

TL;DR

This paper investigates how top quark compositeness influences electroweak parameters and Zbb coupling, identifying patterns of fermion mixing and symmetry breaking that impact phenomenology.

Contribution

It introduces effective field theory patterns for fermion mixing and symmetry breaking in composite top models, connecting them to extra-dimensional theories and phenomenological constraints.

Findings

One pattern of O(4) symmetry breaking is phenomenologically favored.

Models illustrate four ways to break extended O(4) symmetry.

Analysis links composite top models to five-dimensional gauge theories.

Abstract

In this paper we consider the effects of top quark compositeness on the electroweak parameters T-hat and S-hat and the Zbb coupling. We do so by using an effective field theory analysis to identify several promising patterns of mixing between standard model like and vector fermions, and then analyzing simple extensions of the Standard Model that realize those patterns. These models illustrate four ways in which an extended O(4) symmetry, which controls the size of radiative corrections to the observables discussed, may be broken. These models may also be viewed as highly-deconstructed versions of five-dimensional gauge theories dual to various strongly-interacting composite Higgs theories. We comment on how our results relate to extra-dimensional models previously considered, and we demonstrate that one pattern of O(4) breaking is phenomenologically favored.