arXiv subjects

Q. S. Yan

Publications and source records attributed to Q. S. Yan.

11 recordsLinked to original sources

arXiv2026

VLQBounds: Confronting Vector-Like Quark Models with LHC Searches

We present VLQBounds, a public, data-driven Python framework for testing Vector-Like Quark (VLQ) scenarios against Large Hadron Collider (LHC) exclusion limits from ATLAS and CMS. The framework incorporates public results on both pair and single VLQ production and supports the main parameterisations used in experimental interpretations, including mass-mixing, mass-coupling, and mass-width representations. For each parameter point, the predicted cross-section or effective coupling is compared channel by channel to the corresponding observed and expected experimental limits through interpolation over machine-readable grids. The most sensitive analysis is automatically identified and a 95\% Confidence-Level exclusion verdict is returned, together with the observed and expected sensitivity ratios and the metadata needed for reproducible reinterpretation. The modular structure of VLQBounds makes it suitable for fast phenomenological scans, validation of public limits, and future extensions to new collider searches and non-minimal VLQ decay patterns.

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arXiv2026

Analysis of the $ q\bar q\to Z^* \to hA \to4τ$ process within the lepton-specific 2HDM at the LHC

We analyse light Higgs scalar and pseudoscalar associated hadro-production in the 2-Higgs Doublet Model (2HDM) Type-X (or lepton-specific) within the parameter space allowed by theoretical self-consistency requirements as well as the latest experimental constraints from the Large Hadron Collider (LHC), precision data and $B$ physics. Over the viable regions of such a scenario, the Standard Model-like Higgs boson discovered at the LHC in 2012 is the heavier CP-even state $H$. Furthermore, in the Type-X scenario, due to large $\tanβ$, the lighter Higgs scalar $h$ and the pseudoscalar $A$ mainly decay into two $τ$ leptons. Therefore, we concentrate on analysing the signal process $pp\to Z^{*} \to hA\to τ^{+}τ^{-}τ^{+}τ^{-}\to \ell ν_\ell \ell ν_\ell τ_h τ_h$ (where $\ell= e, μ$ whereas $τ_h$ represents the hadronic decay of the $τ$) and explore the feasibility of conducting such a search at the LHC with a centre-of-mass energy of $\sqrt{s}~=$ 14 TeV and a luminosity of $L~=~300~fb^{-1}$. To suppress the huge SM background, we confine ourselves to consider the fraction of signal events with two same-sign $τ$ leptons further decaying into same-sign leptons while the other two $τ$ leptons decay hadronically. We find that a combination of kinematical selection and machine learning (ML) analysis will yields significant sensitivity to this process at the end of the LHC Run 3.

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arXiv2025

Confront a dilaton model with the LHC measurements

The origin of the Higgs boson ($H_{125}$), discovered in 2012, remains a mystery. In the metric affine theory (MAT) framework, we study the scalar potential and investigate a couple of scenarios for the symmetry breaking mechanisms with a dilaton model which is derived from the geometry. The LHC constraints for the couplings of Yukawa couplings, Higgs-weak vector bosons and Higgs self-couplings, in this model are examined, which identify the parameter space where the discovered Higgs boson $m_h=125$ GeV can be dilaton-dominant and the features of Higgs self-couplings are explored. It is found that via the measurements of Higgs pair production, the High Luminosity LHC (HL-LHC) running can either confirm or rule out the dilaton dominance.

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arXiv2024

Probing a 2HDM Type-I light Higgs state via $H_{\rm SM} \to hh \to b\bar bγγ$ at the LHC

We study the discovery potential for a light Higgs boson via $gg \to H_{\text{SM-like}} \to hh \to b\bar{b}γγ$ process at the Large Hadron Collider (LHC). Focusing on the 2-Higgs Doublet Model (2HDM) Type-I, which can accommodate light neutral Higgs states, of $\mathcal{O}(100)$ GeV or less in mass, while agreeing with theoretical and up-to-date experimental constraints, we explore the feasibility of a light CP-even Higgs state $h$ via the largely unexplored final state $b\bar{b}γγ$ at Run-3 of the LHC. We further propose a few Benchmark Points (BPs) for future searches.

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arXiv2024

Probing a light Higgs in $ H \to hh \to b\bar{b}ττ$ in Type-I 2HDM at the LHC

In this study, we explore the potential to probe the process $gg \to H_{\text{SM-like}} \to hh \to b\bar{b}ττ$ at the Large Hadron Collider (LHC), within the framework of the Two Higgs Doublet Model (2HDM) Type-I. After performing a detailed Monte Carlo (MC) simulation, we focus on isolating the signal from the Standard Model (SM) backgrounds. Our analysis employs a dedicated trigger choice and optimised kinematic selection to improve the signal sensitivity. We demonstrate some sensitivity to this decay channel at Run 3, while the High-Luminosity LHC (HL-LHC) can provide discovery evidence.

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arXiv2024

Analysis of the $gg\to H\to hh\to4τ$ process in the 2HDM lepton specific model at the LHC

We analyse the signature of a light Higgs boson pair in the 2-Higgs Doublet Model(2HDM) Type-X (or lepton specific) over the parameter spaces allowed by theoretical self-consistency requirements as well as the latest experimental constraints from the Large Hadron Collider (LHC), precision test data and $B$ physics. Over the viable regions of the latter, wherein the Standard Model (SM)-like Higgs boson discovered at the LHC in 2012 is the heavier CP-even state of the 2HDM, $H$, it is found that the SM-like Higgs boson can decay into a pair of the lighter CP-even Higgs boson, $h$, via the process $H\to hh$ with a Branching Ratio (BR) of $5\%-10\%$ or so, (with $2 m_h < m_H =125$ GeV). Furthermore, in the Type-X scenario, the lighter Higgs bosons $h$ can dominantly decay into two $τ$'s due to a large $\tanβ$. Therefore, the pair of lighter Higgs bosons can altogether decay into a 4 $τ$ final state. In order to suppress the huge SM background events, we confine ourself to consider the fraction of signal events with two Same-Sign (SS) $τ$'s further decaying into same sign leptons while the other two $τ$'s decay hadronically. By using Monte Carlo (MC) and Machine Learning (ML) tools, we thus focus on the analysis of the signal process $pp\to H\to hh\to τ^{+}τ^{-}τ^{+}τ^{-}\to \ell v_\ell \ell v_\ell τ_h τ_h$ (where $\ell= e, μ$ and $τ_h$ means a hadronic decay of the $τ$) and explore the feasibility of such a search at the LHC for a collision energy $\sqrt{s}=~\text{14 TeV}$ and a luminosity $\text{300}~\text{fb}^{-1}$

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arXiv2023

Probing a light charged Higgs boson at the LHC Run 3

We study the Large Hadron Collider (LHC) discovery prospects of a light charged Higgs boson decaying into a $W$ boson and a non-Standard Model (SM)-like Higgs within the 2-Higgs Doublet Model type-I. In the analysis, we consider the associated production of a charged Higgs boson with a light neutral one, $pp \rightarrow H^{\pm} h$, with the subsequent $H^\pm \rightarrow W^{\pm*} h$. We then investigate the emerging $W^{\pm*} + 4b$ final state and provide several benchmark points for signal-to-background analysis. We therefore show that this signal could be an excellent avenue for identifying $H^\pm$ at the LHC.

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arXiv2023

Searching for $H \to hh \to b\bar bττ$ in the 2HDM Type-I at the LHC

Unlike other realisations of the 2-Higgs Doublet Model (2HDM), the so-called Type-I allows for a very light Higgs boson spectrum. Specifically, herein, the heaviest of the two CP-even neutral Higgs states, $H$, can be the one discovered at the Large Hadron Collider (LHC) in 2012, with a mass of $\approx 125$ GeV and couplings consistent with those predicted by the Standard Model (SM). In such a condition of the model, referred to as `inverted mass hierarchy', the decay of the SM-like Higgs state into pairs of the lightest CP-even neutral Higgs boson, $h$, is possible, for masses of the latter ranging from $M_H/2\approx 65$ GeV down to 15 GeV or so, all compatible with experimental constraints. In this paper, we investigate the scope of the LHC in accessing the process $gg\to H \to hh\to b\bar bττ$ by performing a Monte Carlo (MC) analysis aimed at extracting this signal from the SM backgrounds, in presence of a dedicated trigger choice and kinematic selection. We prove that some sensitivity to such a channel exists already at Run 3 of the LHC while the High-Luminosity LHC (HL-LHC) will be able to either confirm or disprove this theoretical scenario over sizable regions of its parameter space.

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arXiv2023

Discovering a light charged Higgs boson via $W^{\pm *}$ + 4$b$ final states at the LHC

Most of the current experimental searches for charged Higgs bosons at the Large Hadron Collider (LHC) concentrate upon the $tb$ and $τν$ decay channels. In the present study, we analyze instead the feasibility of the bosonic decay channel $W^{\pm *} h$, with the charged gauge boson being off-shell and $h$ being a neutral light Higgs boson, which decays predominantly into $b\bar{b}$. We perform a Monte Carlo (MC) analysis for the associate production of a charged Higgs with such a light neutral one, $pp\to H^\pm h$, at the LHC followed by the aforementioned charged Higgs boson decay, which leads to a $W^{\pm *} +4b$ final state. The analysis is performed within the 2-Higgs Doublet Model (2HDM) with Yukawa texture of Type-I. We take into account all available experimental constraints from LEP, Tevatron and the LHC as well as the theoretical requirements of self-consistency of this scenario. In order to study the full process $pp \rightarrow H^{\pm} h \rightarrow W^{\pm *} h h \rightarrow \ell^\pm ν+ 4b$ ($\ell=e,μ$), we provide several Benchmark Points (BPs) amenable to further analysis, with $M_{H^\pm}+M_{b} < M_{t}$, for which we prove that there is a strong possibility that this spectacular signal could be found at the LHC with center of mass energy 14 TeV and luminosity 300 $\rm{fb}^{-1}$.

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arXiv2017

$R_{K^{(*)}}$ anomaly in type-III 2HDM

Recent experimental results provided by the CMS and LHCb, Belle and BaBar collaborations are showing a tension with the SM predictions in $R_{K^{(*)}}$, which might call for an explanation from new physics. In this work, we examine this tension in the type-III two-Higgs doublet models. We focus on the contributions of charged Higgs boson to the observable(s) $R_{K^{(*)}}$ and other rare processes $ΔM_q$ ($q=s,d$), $B \to X_s γ$ $B_s \to μ^+ μ^{-}$ and $B_q \to X_s μ^+ μ^{-}$, which are governed by the same effective Hamiltonian. It is found that regions of large $\tanβ$ and light charged Higgs mass $m_{H^\pm}$ can explain the measured value of $R_{K^{(*)}}$ and accommodate other B physics data as well. In contrast, the type-II two-Higgs doublet model can not.

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arXiv2003

The renormalization of the effective gauge theory with spontaneous symmetry breaking: the $SU(2)\times U(1)$ case

We formulate the electroweak chiral Lagrangian in its mass eigenstates, and study the its one-loop renormalization and provide its renormalization group equations to the same order, so as to complete it as the low energy effective theory of the standard model below a few TeV. In order to make our computation consistent, we have provided a modified power counting rule to estimate the contributions of higher loop and higher operators. As one of the application of its renormalization group equations, we analyze the solution to the effects of the Higgs scalar. We find that similar to the SU(2) case, that the triple anomalous couplings are sensitive to the quartic couplings (here $\al_5$). While the quadratic anomalous couplings are not sensitive, due to the large leading contributions and the accidental cancellation. The differences in the triple anomalous couplings between the direct method and renormalization group equation method are well within the detection power of the LHC and LC, if the Higgs scalar is not too heavy (say, 300 or 400 GeV). We also suggest a new mechanism to generate the negative $S$ parameter through the radiative corrections of the anomalous couplings. Comparison of the renormalization group equation method and direct methods is provided in the full theory, the standard model, to reveal the basic differences of them. The problem of the unitarity violation is also addressed for our assumption in the modified power counting rule.

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