Maximally Symmetric Two Higgs Doublet Model with Natural Standard Model Alignment

TL;DR

This paper explores a maximally symmetric Two Higgs Doublet Model that naturally aligns with the Standard Model Higgs, analyzing its spectrum, stability under quantum corrections, and proposing collider tests to probe its heavy Higgs sector.

Contribution

It introduces a SO(5)-based maximally symmetric 2HDM that achieves natural SM alignment independently of certain parameters and suggests new collider signals for testing the model.

Findings

The MS-2HDM naturally aligns with the SM Higgs without fine-tuning.

Renormalization effects can break the alignment, affecting model predictions.

Proposes a four-top-quark signal to test the heavy Higgs sector at the LHC.

Abstract

We study the Higgs mass spectrum as predicted by a Maximally Symmetric Two Higgs Doublet Model (MS-2HDM) potential based on the SO(5) group, softly broken by bilinear Higgs mass terms. We show that the lightest Higgs sector resulting from this MS-2HDM becomes naturally aligned with that of the Standard Model (SM), independently of the charged Higgs boson mass and $\tan \beta$. In the context of Type-II 2HDM, SO(5) is the simplest of the three possible symmetry realizations of the scalar potential that can naturally lead to the SM alignment. Nevertheless, renormalization group effects due to the hypercharge gauge coupling $g'$ and third-generation Yukawa couplings may break sizeably this SM alignment, along with the custodial symmetry inherited by the SO(5) group. Using the current Higgs signal strength data from the LHC, which disfavour large deviations from the SM alignment limit, we derive lower mass bounds on the heavy Higgs sector as a function of $\tan\beta$, which can be stronger than the existing limits for a wide range of parameters. In particular, we propose a new collider signal based on the observation of four top quarks to directly probe the heavy Higgs sector of the MS-2HDM during the run-II phase of the LHC.