The Other Natural Two Higgs Doublet Model

TL;DR

This paper explores a natural two Higgs doublet model where electroweak symmetry breaking is driven by pseudo-Nambu-Goldstone bosons, offering an alternative to supersymmetry with specific symmetry assumptions and distinctive scalar spectrum predictions.

Contribution

It constructs a low-energy effective Lagrangian for a two Higgs doublet model based on strong sector symmetries, detailing its scalar potential and symmetry properties, and analyzes its phenomenological implications.

Findings

Scalar potential determined by few parameters related to the top sector

Presence of a discrete symmetry controlling the T-parameter

Predictions for scalar spectrum and coupling deviations from standard models

Abstract

We characterize models where electroweak symmetry breaking is driven by two light Higgs doublets arising as pseudo-Nambu-Goldstone bosons of new dynamics above the weak scale. They represent the simplest natural two Higgs doublet alternative to supersymmetry. We construct their low-energy effective Lagrangian making only few specific assumptions about the strong sector. These concern their global symmetries, their patterns of spontaneous breaking and the sources of explicit breaking. In particular we assume that all the explicit breaking is associated with the couplings of the strong sector to the Standard Model fields, that is gauge and (proto)-Yukawa interactions. Under those assumptions the scalar potential is determined at lowest order by very few free parameters associated to the top sector. Another crucial property of our scenarios is the presence of a discrete symmetry, in addition to custodial SO(4), that controls the $T$-parameter. That can either be simple CP or a $Z_2$ that distinguishes the two Higgs doublets. Among various possibilities we study in detail models based on SO(6)/SO(4)$\times$ SO(2), focussing on their predictions for the structure of the scalar spectrum and the deviations of their couplings from those of a generic renormalizable two Higgs doublet model.