The 95 GeV Excess in Models with Two Higgs Doublets plus One Singlet: Model Distinction via Four-top Final States and Triple Higgs Couplings

Abstract

We investigate the prospects for experimentally distinguishing the Next-to-Two-Higgs-Doublet Model (N2HDM) and the Two-Higgs-Doublet Model with a Complex Singlet (2HDMS) in the Yukawa type II realization. Both models can successfully accommodate the reported Higgs-boson excesses around 95 GeV while satisfying current theoretical and experimental constraints, leading to very similar predictions for the observed Higgs spectrum and signal strengths. We analyze how they can nevertheless be discriminated through observables sensitive to the scalar potential and the CP-odd sector. Analytical expressions for the trilinear Higgs couplings are derived, exhibiting characteristic contributions induced by the additional cubic interactions of the 2HDMS. We study the corresponding phenomenology in two complementary collider environments: four-top production at the High-Luminosity LHC, probing the different pseudoscalar sectors. In a second step we analyze Higgs pair production at a future high-energy $e^+e^-$ collider, providing direct sensitivity to the trilinear Higgs couplings. We quantify the potential to distinguish the two models by introducing the minimum deviation from the N2HDM limit required for a $2σ$ difference between the two models. Although our numerical analysis focuses on benchmark scenarios describing the 95 GeV excesses, the proposed approach provides a general framework for discriminating between singlet-extended Higgs sectors at future colliders.