Complete Light Long-Lived Particles searches in Type-I 2HDM

TL;DR

This paper explores the parameter space of the Type-I 2HDM for light long-lived particles, identifying viable regions and proposing benchmarks that could explain anomalies and be tested by FASER detectors.

Contribution

It provides a comprehensive analysis of light LLPs in Type-I 2HDM, including theoretical constraints, viable parameter regions, and experimental prospects for detection.

Findings

Identified regions where light LLPs are consistent with constraints.

Proposed four benchmark scenarios for future collider searches.

FASER~2 can significantly improve sensitivity to these LLPs.

Abstract

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 $\Delta S$ could be the main contribution during the global fit of the oblique parameters, which is different to $\Delta T$ 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 $\cos(\beta - \alpha) \simeq \frac{1}{\tan \beta}$, light $A$ with $\cos(\beta - \alpha) \simeq \frac{1}{\tan\beta} \frac{2m_H^2 - m_h^2}{m_H^2 - m_h^2}$, 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 explain the $W$ boson mass anomaly. 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.