A bstract Recently, the study of long-lived particles (LLPs) has attracted increasing attention. In this work, we analyze the full parameter space of the Type-I Two-Higgs-Doublet Model (2HDM) that allows for light long-lived scalar (H) and pseudoscalar (A) particles. When involving a light beyongd Standard Model (BSM) Higgs, the ∆ S could be the main contribution during the global fit of the oblique parameters, which is different to ∆ T being the main factor for heavy BSM Higgs cases. By imposing theoretical constraints such as vacuum stability and perturbative unitarity, together with current experimental bounds, we summarize a complete region for a potential light H with cos (β − α) ≃ 1 1 tan β, light A with cos (β − α) ≃ 1 2mH²-{mₕ²}mH²-{mₕ²} 1 tan β 2 m H 2 − m h 2 m H 2 − m h 2, and point out the invisible Higgs decay is the most important constraint. We further identify viable regions for LLPs and propose four benchmark regions that simultaneously accommodate a light long-lived particle and the CDF-II measured W boson mass. For these benchmarks, we present the reaches of FASER and FASER 2, where FASER 2 improves the sensitivity by approximately two orders of magnitude compared to FASER.
Qi et al. (2026) studied this question.