arXiv subjects

Michihisa Takeuchi

Publications and source records attributed to Michihisa Takeuchi.

At least 19 recordsLinked to original sources

arXiv2026

Quark masses and mixing in the D_5 model under spontaneous CP violation

It is known that CP violation occurs in flavor physics, and the CKM matrix is complex. We investigate the Yukawa sector of a four-Higgs model based on D_5 symmetry. To understand all possible sources of CP violation in the Yukawa sector, we systematically analyze the impact of all possible representation assignments of the left-handed and right-handed quark fields under the D_5 group. After imposing the conditions of non-block-diagonal CKM matrix and the absence of massless quarks, we find that the only viable representation consists of one D_5 doublet and one singlet. In this case, two distinct types of quark mass models emerge in the quark sector. For each model, we introduce the most general vacuum that allows spontaneous CP violation (SCPV) into the mass matrices. Due to the specific structure of the D_5 symmetry, the vacuum expectation values (vevs) satisfy particular relations such that, for both quark models, the up- and down-quark mass squared matrices (i.e., M_d M_d^\dagger and M_u M_u^\dagger) can be simultaneously block-diagonalized by the same unitary transformation, forcing the CKM matrix to be block-diagonal and thus yielding vanishing mixing angles and CP violation. This result rules out the possibility of explaining experimental observations within the D_5 4HDM under the SCPV framework.

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arXiv2026

Baryon Asymmetry Transfer to Majorana Dark Matter from a Colored Partner

We propose a mechanism in which a primordial baryon asymmetry determines the relic abundance of self-conjugate dark matter through a heavier partner sector. In a simplified model with a Majorana fermion $\chi$ and a colored scalar partner $\tilde q$, QCD annihilations deplete the symmetric partner population while a residual asymmetric component survives as a temporary reservoir. For feeble portal couplings, this asymmetry is transferred to $\chi$ at late times, after dark matter freeze-out. The final relic abundance is controlled by the amount of surviving asymmetry, the timing of its transfer, and the subsequent dark matter annihilation. Solving the coupled Boltzmann equations with the relevant chemical-potential effects, we find that an intermediate range of portal couplings can reproduce the observed relic abundance without requiring a compressed mass spectrum. The qualitative mechanism remains robust against variations in the mass hierarchy, dark matter annihilation mode, and lepton asymmetry. The small portal couplings also make the colored partner potentially long lived, allowing LHC searches for heavy long-lived particles to constrain part of the viable parameter space.

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arXiv2026

A D_5 Model of Quarks with Explicit CP Violation

We investigate the quark sector of a four-Higgs-doublet model with an exact D_5 symmetry. In the framework of explicit CP violation, we considered the most general real vacuum, derived the quark mass matrices, and performed a numerical fit to the quark masses and the CKM mixing matrix at the scale mu = 1 TeV using a differential evolution algorithm, based on the 2024 PDG experimental data.The results show that the model simultaneously fits the quark mass spectrum and the CKM mixing angles with an excellent overall chi^2 = 6.97, accurately reproducing the six-quark mass hierarchy and perfectly matching the nine squared moduli of the CKM matrix. The resulting Jarlskog invariant J_CP = 3.144 \times 10^{-5} agrees with the experimental value 3.12 \times 10^{-5} within 0.19\sigma, indicating that the new source of CP violation from the complex phases in the quark Yukawa couplings alone is sufficient to generate the observed CP violation. Moreover, the hierarchical structures of the vacuum expectation values and the quark Yukawa couplings naturally explain the observed quark mass hierarchy through their interplay.

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arXiv2026

Spontaneous CP violation in the D_5-symmetric four-Higgs-doublet models

We constructed a four-Higgs-doublet model (4HDM) invariant under D_5 symmetry and investigated its complete neutral vacuum structure in detail. Assuming explicit CP conservation in the scalar potential, we examined whether CP symmetry can be spontaneously broken. We provided a complete list of all possible real and complex vacua, along with the constraints on the potential parameters required for each vacuum solution to exist. We also discussed the positive-definiteness conditions that the Hessian must satisfy for each vacuum to be a local minimum of the potential. The results show that, after spontaneous symmetry breaking, some complex vacua lead to spontaneous CP violation in the potential, whereas the remaining complex vacua still preserve CP conservation. Among these CP-violating complex vacua, one can be regarded as the most general form. Furthermore, we discussed the relationship between real and complex vacua.

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arXiv2025

Probing Scalar-Mediator Quark Couplings via CLFV Lepton-Nucleon Scattering

We investigate charged lepton flavor violating (CLFV) deep-inelastic scattering, focusing on the gluon-initiated subprocess $\ell_i g \to \ell_j g$ via the gluon effective operator $\phi\, G_{\mu \nu}^a G_a^{\mu \nu}$, and demonstrate how to probe the nature of the CLFV mediator $\phi$, specifically its mass and interaction with quarks. We consider two benchmark scenarios for the mediator-quark coupling: (i) $h$-like scenario, in which the mediator couples to heavy quarks in proportion to their masses, and (i\hspace{-1pt}i) $b$-only scenario, where the coupling is restricted to bottom quark only. We demonstrate that these scenarios can be discriminated by examining the dependence of the differential cross section on the momentum transfer. Furthermore, we show that the peak position of the differential cross section exhibits a pronounced sensitivity to both the mass of the mediator and the coupling strengths with quarks.

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arXiv2025

Neutralino dark matter in gauge mediation

We explore the potential of neutralino dark matter within the framework of gauge-mediated supersymmetry (SUSY) breaking. Gauge mediation offers an advantage over gravity mediation, as SUSY CP/flavor problems are avoided more easily, leading to significantly milder fine-tuning for electroweak symmetry-breaking compared to gravity mediation. In our models, the lightest neutralino, as the lightest SUSY particle (LSP), is a viable dark matter candidate, assuming a gravitino mass of $\mathcal{O}(100)~\mathrm{TeV}$. The models are formulated in five-dimensional space-time, where the SUSY breaking field and the matter fields are placed on separate branes to avoid issues related to flavor and CP violation. Four distinct neutralino dark matter scenarios are studied: bino-wino coannihilation, higgsino dark matter, wino dark matter, and entropy-diluted bino dark matter. For each case, we determine the allowed parameter spaces and evaluate their consistency with existing experimental limits. Additionally, we examine the potential for testing these models through future investigations at the High-Luminosity Large Hadron Collider (HL-LHC) and through dark matter direct detection experiments.

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arXiv2024

Probing right-handed neutrinos via tri-lepton signals at the HL-LHC

Neutrino oscillation experiments have provided direct evidence for the existence of neutrino masses. The seesaw mechanism explains the smallness of these masses through the introduction of heavy right-handed neutrino (RHN) states. The RHN states can aslo generate Dirac neutrino masses at tree or loop level. These heavy states can exist at the electroweak scale, approximately in the $\mathcal{O}(\mathrm{GeV})$ range, and can be investigated through current and future collider experiments. This scenario, where other new physics interactions occur at scales much higher than the RHN scale, can be described using an effective field theory (EFT) framework known as $N_R$-EFT. This study focuses on constraining the Wilson coefficients of $N_R$-EFT operators, which primarily contribute to tri-lepton production and missing energy signals at the LHC. We examine both the scenarios where the RHN mass $M_N$ is less than and greater than the $W$ boson mass $M_W$, and provide predictions for the High-Luminosity run of the LHC (HL-LHC).

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arXiv2023

Current Status of the Muon g-2 Interpretations within Two-Higgs-Doublet Models

In this article, we review and update implications of the muon anomalous magnetic moment (muon $g-2$) anomaly for two-Higgs-doublet models (2HDMs), which are classified according to imposed symmetries and their resulting Yukawa sector. In the minimal setup, the muon $g-2$ anomaly can be accommodated by the type-X (lepto-philic) 2HDM, flavor-aligned 2HDM (FA2HDM), muon-specific 2HDM ($\mu$2HDM), and $\mu\tau$-flavor violating 2HDM. We summarize all relevant experimental constraints from high-energy collider experiments and flavor experiments, as well as the theoretical constraints from the perturbative unitarity and vacuum stability bounds, to these 2HDMs in light of the muon $g-2$ anomaly. We clarify the available parameter spaces of these 2HDMs and investigate how to probe the remaining parameter regions in future experiments. In particular, we find that, due to the updated $B_s\to\mu^+ \mu^-$ measurement, the remaining parameter region of the FA2HDM is almost equivalent to the one of the type-X 2HDM. Furthermore, based on collider simulations, we find that the type-X 2HDM is excluded and the $\mu$2HDM scenario will be covered with the upcoming Run 3 data.

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arXiv2023

Probing chirality structure in lepton-flavour-violating Higgs decay $h\to\tau\mu$ at the LHC

A phenomenological study for determining the chirality structure in lepton-flavor-violating Higgs (hLFV) decays $h \to \tau\mu$ at the LHC is presented. We estimate the effects of the $\tau$ polarization in the analysis andthe importance of determining the relative visible momentum ratio $x$, and show the analysis with a collinear mass $m_{col1}$ by assuming one missing particle is appropriate. We find that the sensitivity would be generically affected up to $\pm$ $4-6$~\% in terms of the BR$(h\to \tau\mu)$ upper bound, and show the altered bounds on the $(|y_{\mu\tau}|, |y_{\tau\mu}|)$ plane. We further study the benchmark scenarios, and demonstrate the sensitivity study for the chirality structure using the relative visible momentum ratio. We find that the two fully polarized cases, the $\tau_R$ and $\tau_L$ scenarios consistent with the recently reported excess, are distinguishable at 2$\sigma$ level for 1000~fb$^{-1}$. We also show that a further improved study potentially provides a similar sensitivity already for 139~fb$^{-1}$.

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arXiv2021

Probing double-aligned two Higgs doublet models at LHC

We consider two Higgs doublet models (THDMs) with both the Higgs potential and Yukawa interactions being aligned, which we call "double-aligned THDMs". In this scenario, coupling constants of the discovered Higgs boson to the Standard Model (SM) particles are identical to those of the SM Higgs boson, and flavor changing neutral currents via neutral Higgs bosons do not appear at tree level. We investigate current constraints and future prospects of the model by using measurements from flavor experiments and data of multi-lepton final states at LHC. Especially, we focus on the electroweak pair production of the additional Higgs bosons with their masses below $2m_t$. We find that the most of the parameter space are already excluded by the current LHC data when the leptonic decays of the additional Higgs bosons are dominant, which can be interpreted to the scenario in the Type-X THDM as a special case. We also clarify the parameter region where the high-luminosity LHC can explore, and demonstrate the reconstruction of the masses of additional Higgs bosons from the $b\bar{b}\tau^+\tau^-$ final states in a few benchmark points.

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arXiv2021

Anomaly detection in high-energy physics using a quantum autoencoder

The lack of evidence for new interactions and particles at the Large Hadron Collider has motivated the high-energy physics community to explore model-agnostic data-analysis approaches to search for new physics. Autoencoders are unsupervised machine learning models based on artificial neural networks, capable of learning background distributions. We study quantum autoencoders based on variational quantum circuits for the problem of anomaly detection at the LHC. For a QCD $t\bar{t}$ background and resonant heavy Higgs signals, we find that a simple quantum autoencoder outperforms classical autoencoders for the same inputs and trains very efficiently. Moreover, this performance is reproducible on present quantum devices. This shows that quantum autoencoders are good candidates for analysing high-energy physics data in future LHC runs.

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arXiv2021

Non-resonant new physics search at the LHC for the $b \to c \tau \nu$ anomalies

Motivated by the $b \to c \tau \overline{\nu}$ anomalies, we study non-resonant searches for new physics at the large hadron collider (LHC) by considering final states with an energetic and hadronically decaying $\tau$ lepton, a $b$-jet and large missing transverse momentum ($pp \to \tau_h \overline{b} + E_{\rm T}^{\rm miss}$). Such searches can be useful to probe new physics contributions to $b \to c \tau \overline{\nu}$. They are analyzed not only within the dimension-six effective field theory (EFT) but also in explicit leptoquark (LQ) models with the LQ non-decoupled. The former is realized by taking a limit of large LQ mass in the latter. It is clarified that the LHC sensitivity is sensitive to the LQ mass for $O(1)$ TeV even in the search of $pp \to \tau_h \overline{b} + E_{\rm T}^{\rm miss}$. Although the LQ models provide a weaker sensitivity than the EFT limit, it is found that the non-resonant search of $pp \to \tau_h \overline{b} + E_{\rm T}^{\rm miss}$ can improve the sensitivity by $\approx 40\%$ versus a conventional mono-$\tau$ search ($pp \to \tau_h + E_{\rm T}^{\rm miss}$) in the whole LQ mass region. Consequently, it is expected that most of the parameter regions suggested by the $b \to c \tau \overline{\nu}$ anomalies can be probed at the HL-LHC. Also, it is shown that $\text{R}_2$ LQ scenario is accessible entirely once the LHC Run 2 data are analyzed. In addition, we discuss a charge selection of $\tau_h$ to further suppress the standard-model background, and investigate the angular correlations among $b,\, \tau$ and the missing transverse momentum to discriminate the LQ scenarios.

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arXiv2021

Charged lepton flavor violation associated with heavy quark production in deep inelastic lepton-nucleon scattering via scalar exchange

We study charged lepton flavor violation (CLFV) associated with heavy quark pair production in lepton-nucleon deep-inelastic scattering $\ell_i N \to \ell_j q\bar{q} X$. Here $\ell_i$ and $\ell_j$ denote the initial and final leptons; $N$ and $X$ are respectively the initial nucleon and arbitrary final hadronic system. We employ a model Lagrangian in which a scalar and pseudoscalar mediator generates the CLFV. We derive heavy quark structure functions for scalar and pseudoscalar currents and compute momentum distributions of the final lepton for the process. Our focus is on the heavy quark mass effects in the final lepton momentum distribution. We clarify the necessity of inclusion of the heavy quark mass to obtain reliable theory predictions for the CLFV signal searches in the deep-inelastic scattering.

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arXiv2020

Testing Leptoquark/EFT in $\bar B \to D^{(*)} l\bar\nu$ at the LHC

We investigate the current LHC bounds on New Physics (NP) that contributes to $\bar B \to D^{(*)} l\bar\nu$ for $l = (e,\mu,\tau)$ by considering both leptoquark (LQ) models and an effective-field-theory (EFT) Hamiltonian. Experimental analyses from $l+\text{missing}$ searches with high $p_T$ are applied to evaluate the NP constraints with respect to the Wilson coefficients. A novel point of this work is to show difference between LQs and EFT for the applicable LHC bound. In particular, we find that the EFT description is not valid to search for LQs with the mass less than $\lesssim 10\,\text{TeV}$ at the LHC and leads to overestimated bounds. We also discuss future prospects of high luminosity LHC searches including the charge asymmetry of background and signal events. Finally, a combined summary for the flavor and LHC bounds is given, and then we see that in several NP scenarios the LHC constraints are comparable with the flavor ones.

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arXiv2020

Neural Network-based Top Tagger with Two-Point Energy Correlations and Geometry of Soft Emissions

Deep neural networks trained on jet images have been successful in classifying different kinds of jets. In this paper, we identify the crucial physics features that could reproduce the classification performance of the convolutional neural network in the top jet vs. QCD jet classification. We design a neural network that considers two types of substructural features: two-point energy correlations, and the IRC unsafe counting variables of a morphological analysis of jet images. The new set of IRC unsafe variables can be described by Minkowski functionals from integral geometry. To integrate these features into a single framework, we reintroduce two-point energy correlations in terms of a graph neural network and provide the other features to the network afterward. The network shows a comparable classification performance to the convolutional neural network. Since both networks are using IRC unsafe features at some level, the results based on simulations are often dependent on the event generator choice. We compare the classification results of Pythia 8 and Herwig 7, and a simple reweighting on the distribution of IRC unsafe features reduces the difference between the results from the two simulations.

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arXiv2020

Probing $μτ$ flavor-violating solutions for the muon $g-2$ anomaly at Belle II

The discrepancy between the measured value and the Standard Model prediction of the muon anomalous magnetic moment is one of the most important issues in the particle physics. It is known that introducing a mediator boson X with the $μτ$ lepton flavor violating (LFV) couplings is one good solution to explain the discrepancy, due to the $τ$ mass enhancement in the one-loop correction. In this paper, we study the signal of this model, i.e. the same-sign leptons, in the Belle II experiment, assuming the flavor-diagonal couplings are suppressed. We show that the Belle II experiment is highly sensitive to the scenario in the mediator mass range of ${\cal O}(1-10)$~GeV, using the $e^+e^- \to μ^\pmτ^\mp X \toμ^\pm μ^\pm τ^\mp τ^\mp$ process induced by the $X$.

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arXiv2019

Testing the 2HDM explanation of the muon g-2 anomaly at the LHC

The discrepancy between the measured value and the Standard Model prediction for the muon anomalous magnetic moment is one of the important issues in the particle physics. In this paper, we consider a two Higgs doublet model (2HDM) where the extra Higgs doublet couples to muon and tau in lepton flavor violating (LFV) way and the one-loop correction involving the scalars largely contributes to the muon anomalous magnetic moment. The couplings should be sizable to explain the discrepancy, so that the extra Higgs bosons would dominantly decay into $μτ$ LFV modes, which makes the model testable at the LHC through multi-lepton signatures even though they are produced via the electroweak interaction. We discuss the current status and the future prospect for the extra Higgs searches at the LHC, and demonstrate the reconstruction of the mass spectrum using the multi-lepton events.

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arXiv2019

Test of the $R(D^{(*)})$ anomaly at the LHC

There are discrepancies between the experimental results and the Standard Model predictions, in the lepton flavor universality of the semileptonic $B$ decays: $B \to D^{(*)} \ell ν$. As the new physics interpretations, new charged vector and charged scalar fields, that dominantly couple to the second and third generations, have been widely discussed. In this paper, we study the signals of the new particles at the LHC, and test the interpretations via the direct search for the new resonances. In particular, we see that the $τν$ resonance search at the LHC has already covered most of the parameter regions favored by the Belle and BaBar experiments. We find that the bound is already stronger than the one from the $B_c$ decay depending on the mass of charged scalar.

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