Scrutinizing the Alignment Limit in Two-Higgs-Doublet Models. Part 1: $m_h = 125$ GeV

TL;DR

This paper investigates the alignment limit in Two-Higgs-Doublet Models where the 125 GeV Higgs mimics the Standard Model Higgs, analyzing both decoupling and non-decoupling scenarios with current experimental constraints.

Contribution

It provides a detailed analysis of the alignment limit in 2HDMs near 125 GeV, including phenomenological implications without decoupling and updated constraints from LHC data.

Findings

Alignment limit can be achieved without decoupling of heavy scalars.

Current LHC data constrains the parameter space of 2HDMs near the alignment limit.

Implications for future LHC runs include potential discovery of additional Higgs states.

Abstract

In the alignment limit of a multi-doublet Higgs sector, one of the Higgs mass eigenstates aligns with the direction of the scalar field vacuum expectation values, and its couplings approach those of the Standard Model (SM) Higgs boson. We consider CP-conserving Two-Higgs-Doublet Models (2HDMs) of Type I and Type II near the alignment limit in which the lighter of the two CP-even Higgs bosons, $h$, is the SM-like state observed at 125 GeV. In particular, we focus on the 2HDM parameter regime where the coupling of $h$ to gauge bosons approaches that of the SM. We review the theoretical structure and analyze the phenomenological implications of the regime of alignment limit without decoupling, in which the other Higgs scalar masses are not significantly larger than $m_h$ and thus do not decouple from the effective theory at the electroweak scale. For the numerical analysis, we perform scans of the 2HDM parameter space employing the software packages 2HDMC and Lilith, taking into account all relevant pre-LHC constraints, the latest constraints from the measurements of the 125 GeV Higgs signal at the LHC, as well as the most recent limits coming from searches for heavy Higgs-like states. We contrast these results with the alignment limit achieved via the decoupling of heavier scalar states, where $h$ is the only light Higgs scalar. Implications for Run 2 at the LHC, including expectations for observing the other scalar states, are also discussed.