Natural Standard Model Alignment in the Two Higgs Doublet Model

TL;DR

This paper introduces a symmetry-based approach to achieve natural Standard Model alignment in the Two Higgs Doublet Model without relying on decoupling or fine-tuning, and explores its phenomenological implications for collider experiments.

Contribution

It identifies three symmetry realizations that lead to natural alignment in 2HDMs with both doublets acquiring VEVs, providing a new theoretical framework.

Findings

Three symmetry realizations for natural alignment identified

Maximally-Symmetric 2HDM based on SO(5) symmetry discussed

New collider signals involving third-generation quarks proposed

Abstract

The current LHC Higgs data provide strong constraints on possible deviations of the couplings of the observed 125 GeV Higgs boson from the Standard Model (SM) expectations. Therefore, it now becomes compelling that any extended Higgs sector must comply with the so-called SM alignment limit. In the context of the Two Higgs Doublet Model (2HDM), this alignment is often associated with either decoupling of the heavy Higgs sector or accidental cancellations in the 2HDM potential. Here we present a new solution realizing natural alignment based on symmetries, without decoupling or fine-tuning. In particular, we show that in 2HDMs where both Higgs doublets acquire vacuum expectation values, there exist only three different symmetry realizations leading to natural alignment. We discuss some phenomenological implications of the Maximally-Symmetric 2HDM based on SO(5) symmetry group and analyze new collider signals for the heavy Higgs sector, involving third-generation quarks, which can be a useful observational tool during the Run-II phase of the LHC.