Probing mis-aligned type-I two-Higgs-doublet model in e + e -→ ZH at future lepton colliders

Abstract

A detailed collider study of Higgs production in the process $e^+e^- \to ZH$ is presented within the framework of the Two-Higgs-Doublet Model (2HDM), with particular emphasis on mis-aligned scenarios in the Type-I realization. The analysis is performed at a future $e^+e^-$ collider with $\sqrt{s}=500$~GeV and an integrated luminosity of $10~\mathrm{ab}^{-1}$, considering heavy Higgs masses in the range $160 \leq m_H \leq 400$~GeV. The analysis covers heavy CP-even Higgs states with non-fermionic decay modes such as $H\to WW$ and $H\to hh$, and incorporates final states arising from leptonic and invisible $Z$ boson decays. Fast parametrized detector-level simulations are employed to evaluate selection efficiencies, signal--background discrimination, and exclusion sensitivities. It is shown that values of $|c_{\beta-\alpha}|>0.1$ can be excluded at the $95\%$ confidence level over almost the entire range $1\leq\tan\beta\leq10$ in the $H\to WW$ scenario, while the $H\to hh$ channel provides lower sensitivity.