Complete light long-lived particles searches in Type-I 2HDM

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/tan β 1 tan β , light A with cos( β − α ) ≃ 1/tan β2mH²-mₕ²mH²-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.