Next-to-leading order unitarity fits in Two-Higgs-Doublet models with soft $\mathbb{Z}_2$ breaking

TL;DR

This paper applies next-to-leading order unitarity constraints to the Two-Higgs-Doublet model with a softly broken _2 symmetry, refining bounds on couplings and Higgs mass degeneracies using comprehensive data and a simplified implementation.

Contribution

It introduces a simplified method for implementing NLO unitarity conditions in 2HDMs and combines these with experimental data for improved parameter bounds.

Findings

Upper bounds of 4.2 on quartic couplings

Mass degeneracy limits of 235 GeV (Type I) and 100 GeV (Type II)

Exclusion of unbroken _2 symmetry in Type II at 95% confidence

Abstract

We fit the next-to-leading order unitarity conditions to the Two-Higgs-Doublet model with a softly broken $\mathbb{Z}_2$ symmetry. In doing so, we alleviate the existing uncertainty on how to treat higher order corrections to quartic couplings of its Higgs potential. A simplified approach to implementing the next-to-leading order unitarity conditions is presented. These new bounds are then combined with all other relevant constraints, including the complete set of LHC Run I data. The upper $95\%$ bounds we find are $4.2$ on the absolute values of the quartic couplings, and $235$ GeV ($100$ GeV) for the mass degeneracies between the heavy Higgs particles in the type I (type II) scenario. In type II, we exclude an unbroken $\mathbb{Z}_2$ symmetry with a probability of $95\%$. All fits are performed using the open-source code HEPfit.