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) ≃ <InlineEquation ID="IEq1"> <EquationSource Format="MATHML"><math display="inline"> <mfrac> <mn>1</mn> <mrow> <mo>tan</mo> <mi>β</mi> </mrow> </mfrac> </math></EquationSource> <EquationSource Format="TEX">$ \frac{1}{\tan \beta } $</EquationSource> </InlineEquation>, light A with cos(β − α) ≃ <InlineEquation ID="IEq2"> <EquationSource Format="MATHML"><math display="inline"> <mfrac> <mn>1</mn> <mrow> <mo>tan</mo> <mi>β</mi> </mrow> </mfrac> <mfrac> <mrow> <mn>2</mn> <msubsup> <mi>m</mi> <mi>H</mi> <mn>2</mn> </msubsup> <mo>−</mo> <msubsup> <mi>m</mi> <mi>h</mi> <mn>2</mn> </msubsup> </mrow> <mrow> <msubsup> <mi>m</mi> <mi>H</mi> <mn>2</mn> </msubsup> <mo>−</mo> <msubsup> <mi>m</mi> <mi>h</mi> <mn>2</mn> </msubsup> </mrow> </mfrac> </math></EquationSource> <EquationSource Format="TEX">$ \frac{1}{\tan \beta}\frac{2{m}_H^2-{m}_h^2}{m_H^2-{m}_h^2} $</EquationSource> </InlineEquation>, 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.

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

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2026年3月27日

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Xueying Qi,
Huayang Song,
Wei Su

摘要

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) ≃ $\frac{1}{tan β}$ $ \frac{1}{\tan \beta } $ , light A with cos(β − α) ≃ $\frac{1}{tan β} \frac{2 m_{H}^{2} - m_{h}^{2}}{m_{H}^{2} - m_{h}^{2}}$ $ \frac{1}{\tan \beta}\frac{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.