arXiv subjects

Chuan-Hung Chen

Publications and source records attributed to Chuan-Hung Chen.

At least 19 recordsLinked to original sources

arXiv2025

Structure of leptonic Yukawa couplings in the Zee model

The radiative neutrino mass matrix $m^ν$ in the Zee model depends on leptonic Yukawa couplings $F$ to a singlet scalar and $Y^\ell$ to a new Higgs doublet. Leveraging the skew-symmetric structure of $F$, we derive a unique identity linking $F$ and $m^ν$ that is explicitly independent of $Y^\ell$. This relation implies that five entries of $Y^\ell$ can, in principle, be determined directly from $m^ν$ and $F$, while the remaining four can be selected based on phenomenological assumptions. As an illustration, we apply this framework to the two-zero texture $B2$, highlighting its enhancement of the muon $g-2$.

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arXiv2025

Leptoquark-mediated Dirac neutrino mass and its impact on $B \to K ν\barν$ and $K \to πν\barν$ decays

Right-handed neutrinos $ν_R$ play a crucial role in flavor-changing neutral-current processes with missing energy, such as $b\to s + \slashed{E} $ and $d\to s + \slashed{E} $, where Belle-II reports unexpectedly large branching fraction in $B\to K ν\barν$ decays. Assuming $ν_R$ is the partner of the active neutrino $ν_L$ in the standard model, a Dirac-type neutrino framework emerges. We investigate a scenario of radiative Dirac neutrino mass generation in a scalar leptoquark (LQ) model with a global $U(1)_X$ symmetry to suppress Majorana mass, tree-level Dirac mass, and diquark couplings. The simplest LQ realization consists of two $S_1 = (3,1,-1/3)$ LQs with distinct $U(1)_X$ charges. A non-Casas-Ibarra parametrization is proposed to match neutrino data with fewer model parameters. Imposing current experimental constraints from meson mixing and lepton flavor-violating processes, we find that right-handed neutrino effects can significantly enhance $B\to K^{(*)} ν\barν$ and $K^+\to π^+ ν\barν$. Additionally, the model predicts excesses in $R_D$ from $B\to Dτ\barν$ that remain within $1σ$ of current experimental data.

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arXiv2024

Phenomenological study of a gauged ${L_μ-L_τ}$ model with a scalar leptoquark

A $Z'$ gauge boson with sub-GeV mass has acquired a significant interest in phenomenology, particularly in view of the muon $g-2$ anomaly and coherent elastic neutrino-nucleon scattering. The latter is challenged by the nuclear recoil energy of a few tens of keV but has been observed by the COHERENT experiment. To further reconcile the observed excesses in $R(D^{(*)})$ from semileptonic charmful $B$ decays and in the $W$ boson mass, we investigate a model with a gauged $U(1)_{L_μ-L_τ}$ symmetry and a scalar leptoquark. In contrast to the mechanism that involves kinetic mixing between the gauge bosons of $U(1)_{\rm em}$ and $U(1)_{L_μ-L_τ}$, we adopt a dynamical symmetry breaking of $U(1)_{L_μ-L_τ}$ by incorporating an additional Higgs doublet. Through mixing with the $U(1)_{L_μ-L_τ}$-charged Higgs doublet, new Higgs decay channels $h\to Z_1 Z_1/Z_1 Z_2$ occur at percent-level branching ratios, which could be accessible at the LHC. The $W$-mass anomaly observed by CDF II can be potentially resolved through the enhancement in the oblique parameter $T$. Due to the flavored gauge symmetry, the introduced scalar leptoquark $S^{\frac{1}{3}}=(\bar{3},1,2/3)$ exhibits a unique coupling to the $τ$-lepton, offering an explanation for the excesses observed in $R(D^{(*)})$. Moreover, $τ\to μ(Z_1\to ) e^- e^+$ via the resonant light gauge boson decay can reach the sensitivity of Belle II at an integrated luminosity of 50 ab$^{-1}$.

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arXiv2024

Rare $B$ and $K$ decays in a scotogenic model

A scotogenic model can radiatively generate the observed neutrino mass, provide a dark matter candidate, and lead to rare lepton flavor-violating processes. We aim to extend the model to establish a potential connection to the quark flavor-related processes within the framework of scotogenesis, enhancing the unexpectedly large branching ratio (BR) of $B^+\to K^+ ν\barν$, observed by Belle II Collaboration. Meanwhile, the model can address tensions between some experimental measurements and standard model (SM) predictions in flavor physics, such as the muon $g-2$ excess and the higher BR of $B_s \to μ^- μ^+$. We introduce in the model the following dark particles: a neutral singlet Dirac-type lepton ($N$); two inert Higgs doublets ($η_{1,2}$), with one of which carrying a lepton number; a charged singlet dark scalar $(χ^+)$, and a singlet vector-like up-type dark quark ($T$). The first two entities are responsible for the radiative neutrino mass, and $χ^+$ couples to right-handed quarks and leptons and can resolve the tensions existing in muon $g-2$ and $B_s\to μ^- μ^+$. Furthermore, the BR of $B^+ \to K^+ ν\barν$ can be enhanced up to a factor of 2 compared to the SM prediction through the mediations of the dark $T$ and the charged scalars. In addition, we also study the impacts on the $K\to πν\barν$ decays.

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arXiv2023

Flavor anomalies in leptoquark model with gauged $U(1)_{L_μ-L_τ}$

Leptoquarks (LQs) have been extensively studied in the context of $B$ anomalies. When $U(1)_{L_μ-L_τ}$ is introduced to a scalar LQ model with the LQ $S_1$ charged under the new symmetry, $S_1$ primarily couples to the third-generation leptons while its couplings to first and second-generation leptons are naturally suppressed. Furthermore, only $S_1$ in the scalar LQ models has the feature that down-type quarks merely couple to neutrinos but not the charged leptons, avoiding strict restrictions from $b\to s μ^+ μ^-$. With this distinctive characteristic of $S_1$, we investigate its impact on rare processes involving the $d_i \to d_j ν\barν$ transitions. Under the dominant constraints from $ΔF=2$ processes, we find that the $S_1$ contributions to the branching ratios (BRs) of $B\to K(K^*) ν\barν$ and $K_L \to π^0 ν\barν$ can be factorized into the same multiplicative factor multiplying the standard model predictions. Enhancement in the BRs can possibly exceed a factor of 2. In particular, ${\cal B}(K^+\to π^+ ν\barν)$ can reach the upper $1σ$ error of the experimental value, i.e., $\simeq 15.4 \times 10^{-11}$. We also show that the model can fit the new world averages of $R(D)$ and $R(D^*)$.

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arXiv2023

Top-quark FCNC decays, LFVs, lepton $g-2$, and $W$ mass anomaly with inert charged Higgses

The observed flavor-changing neutral-current (FCNC) processes in the standard model (SM) arise from the loop diagrams involving the weak charged currents mediated by the $W$-gauge boson. Nevertheless, the top-quark FCNCs and lepton-flavor violating processes resulting from the same mechanism are highly suppressed. We investigate possible new physics effects that can enhance the suppressed FCNC processes, such as $t\to q(h,V)$ with $V=γ,Z,g$, $h\to \ell \ell'$, and $\ell\to \ell' γ$. To achieve the assumption that the induced-FCNCs are all from quantum loops, we consider the scotogenic mechanism, where a $Z_2$ symmetry is introduced and only new particles carry an odd $Z_2$ parity. With the extension of the SM to include an inert Higgs doublet, an inert charged Higgs singlet, a vector-like singlet quark, and two neutral leptons, it is found that, with relevant constraints taken into account, the $t\to c (h, Z)$, $h\to μτ$, and $τ\to \ell γ$ decays can be enhanced up to the expected sensitivities in experiments. The branching ratios of $h\to μ^+ μ^-/τ^+ τ^-$ from only new physics effects can reach up to ${\cal O}(10^{-3})$. Intriguingly, the resulting muon $g-2$ can fit the combined data within $2σ$ errors, whereas the electron $g-2$ can have either sign with a magnitude of ${\cal O}(10^{-13}-10^{-12})$. In addition, we examine the oblique parameters in the model and find that the resulting $W$-mass anomaly observed by CDF II can be accommodated.

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arXiv2022

Scotogenic top-quark FCNC decays

Flavor-changing neutral current (FCNC) top-quark decays are highly suppressed due to the Glashow-Iliopoulos-Maiani mechanism in the standard model (SM). If $t\to q h, qV$ with $V=g,γ, Z$ are all induced via quantum loop levels, then we investigate the effect that can enhance the top-FCNC up to the sensitivity designed at the high-luminosity (HL) LHC. Inspired by the mechanism of the scotogenic neutrino mass, we extend the SM by including $Z_2$-odd colored fermions when a $Z_2$ discrete is imposed. The results show that by taking $BR(t\to u g) \lesssim 0.61 \times 10^{-4}$ recently measured by ATLAS as an input, $t\to q γ$ can be indirectly bounded to be $BR(t\to q γ)\lesssim 3.2 \times 10^{-6}$, which is below the expected sensitivity at the HL LHC. After taking potential constraints from various experiments into account, the obtained branching ratios for the loop-induced $t\to q h$ and $t\to qZ$ decays can be $O(10^{-4})$, which falls within the sensitivity at the HL LHC.

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arXiv2022

Ultra-high-energy neutrino scattering in an anomalous U(1) effective field theory

A unique characteristic of exponentially growing scattering amplitude arises in an anomalous Abelian effective field theory when an extremely light Dirac neutrino mass is introduced to break the symmetry. We show that the low energy effective Lagrangian can be made explicitly gauge invariant with the help of a nonlinear representation of the Goldstone or Stueckelberg field. We study the peculiar feature of exponential growth in the ultra-high-energy neutrino-nucleon inelastic scattering. It is found that the inelastic scattering cross section is highly sensitive to the ratio of gauge coupling to the gauge boson mass, $g_X/m_X$. When the IceCube measurement of ultra-high-energy neutrinos, which is consistent with the standard model prediction up to $E_ν\sim 6$ PeV, is taken into account, the inferred constraint on $g_X/m_X$ is more severe than that obtained from the events of mono-lepton$+$missing transverse energy at the LHC. A muon collider with a collision energy of $10$ TeV can be a good environment other than hadron colliders to probe the novel effect.

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arXiv2022

Lepton flavor violation and scotogenic Majorana neutrino mass in a Stueckelberg $U(1)_X$ model

We construct a scotogenic Majorana neutrino mass model in a gauged $U(1)_X$ extension of the standard model, where the mass of the gauge boson and the unbroken gauge symmetry, which leads to a stable dark matter (DM), can be achieved through the Stueckelberg mechanism. It is found that the simplest version of the extended model consists of the two inert-Higgs doublets and one vector-like singlet fermion. In addition to the Majorana neutrino mass, we study the lepton flavor violation (LFV) processes, such as $\ell_i \to \ell_j γ$, $\ell_i \to 3 \ell_j$, $μ-e$ conversion rate in nucleus, and muonium-antimuonium oscillation. We show that the sensitivities of $μ\to 3e$ and $μ-e$ conversion rate designed in Mu3e and COMET/Mu2e experiments make both decays the most severe constraints on the $μ\to e$ LFV processes. It is found that $τ\to μγ$ and $τ\to 3μ$ can reach the designed significance level of Belle II. In addition to explaining the DM relic density, we also show that the DM-nucleon scattering cross section can satisfy the currently experimental limit of DM direct detection.

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arXiv2021

Two-loop radiative seesaw, muon $g-2$, and $τ$-lepton-flavor violation with DM constraints

The quartic scalar coupling $λ_5$ term, which violates the lepton-number by two units in the Ma-model, is phenomenologically small when the model is applied to the lepton-flavor violation (LFV) processes. In order to dynamically generate the $λ_5$ parameter through quantum loop effects and retain the dark matter (DM) candidate, we extend the Ma-model by adding a $Z_2$-odd vector-like lepton doublet and a $Z_2$-even Majorana singlet. With the new couplings to the Higgs and gauge bosons, the observed DM relic density can be explained when the upper limits from the DM-nucleon scattering cross sections are satisfied. In addition to the neutrino data and LFV constraints, it is found that the DM relic density can significantly exclude the free parameter space. Nevertheless, the resulting muon $g-2$ mediated by the inert charged-Higgs can fit the $4.2σ$ deviation between the experimental measurement and the SM result, and the branching ratio for $τ\to μγ$ can be as large as the current upper limit when the rare $μ\to (e γ, 3 e)$ decays are suppressed. In addition, it is found that the resulting $BR(τ\to μρ)$ can reach the sensitivity of Belle II with an integrated luminosity of 50 $ab^{-1}$.

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arXiv2021

Muon $g-2$ in a two-Higgs-doublet model with a type-II seesaw mechanism

We study the two-Higgs-doublet model with type-II seesaw mechanism. In view of constraints from the Higgs data, we consider the aligned two-Higgs-doublet scheme and its effects on muon anomalous magnetic dipole moment, $a_μ$, including both one-loop and two-loop Barr-Zee type diagrams. Thanks to a sizable trilinear scalar coupling, the Barr-Zee type diagrams mediated by the Higgs triplet fields have a dominant effect on $a_μ$. In particular, unlike the usual two-Higgs-doublet models that require exotic Higgs bosons light in mass, the masses of the corresponding particles in the model are of ${\cal O}(100)$~GeV. The doubly-charged Higgs boson presents a different decay pattern from the usual Higgs triplet model and thus calls for a new collider search strategy, such as multi-$τ$ searches at the LHC.

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arXiv2021

Electron and muon $g-2$, radiative neutrino mass, and $\ell' \to \ell γ$ in a $U(1)_{e-μ}$ model

A nonconventional $U(1)_{e-μ}$ gauge model is proposed to explain the unexpected anomalous magnetic moments of the electron and muon (lepton $g-2$), where only the right-handed electron and muon in the standard model carry the $U(1)_{e-μ}$ charge. Although the light lepton masses are suppressed when the gauge symmetry is spontaneously broken, they can be generated through the Yukawa couplings to newly introduced particles, such as vector-like lepton doublets and singlet, and scalar singlets. It is found that the same Yukawa couplings combined with the new scalar couplings to the Higgs can induce the radiative lepton-flavor violation processes $\ell' \to \ell γ$ and lepton $g-2$, where the lepton $g-2$ is proportional to $m_{\ell}$. When Majorana fermions and a scalar singlet are further added into the model, the active neutrinos can obtain masses via the radiative seesaw mechanism. When the bounds from the $m_e$ and $m_μ$ and the neutrino data are satisfied, we find that the electron $g-2$ can reach an order of $-10^{-12}$, and the muon $g-2$ can be an order of $10^{-9}$. In addition, when the $μ\to e γ$ decay is suppressed, the resulting branching ratio for $τ\to e γ$ can be of $O(10^{-8})$, and that for $τ\to μγ$ can be as large as the current upper limit.

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arXiv2019

Radiatively scotogenic type-II seesaw and a relevant phenomenological analysis

When a small vacuum expectation value of Higgs triplet ($v_Δ$) in the type-II seesaw model is required to explain neutrino oscillation data, a fine-tuning issue occurs on the mass-dimension lepton-number-violation (LNV) scalar coupling. Using the scotogenic approach, we investigate how a small LNV term is arisen through a radiative correction when an $Z_2$-odd vector-like lepton ($X$) and an $Z_2$-odd right-handed Majorana lepton ($N$) are introduced to the type-II seesaw model. Due to the dark matter (DM) direct detection constraints, the available DM candidate is the right-handed Majorana particle, whose mass depends on and is close to the $m_X$ parameter. Combing the constraints from the DM measurements, the $h\to γγ$ decay, and the oblique $T$-parameter, it is found that the preferred range of $v_Δ$ is approximately in the region of $10^{-5}-10^{-4}$ GeV; the mass difference between the doubly and the singly charged Higgs is less than 50 GeV, and the influence on the $h\to Zγ$ is not significant. Using the constrained parameters, we analyze the decays of each Higgs triplet scalar in detail, including the possible three-body decays when the kinematic condition is allowed. It is found that with the exception of doubly charged Higgs, scalar mixing effects play an important role in the Higgs triplet two-body decays when the scalar masses are near-degenerate. In the non-degenerate mass region, the branching ratios of the Higgs triplet decays are dominated by the three-body decays.

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arXiv2019

Top-quark flavor-changing $tqZ$ couplings and rare $ΔF=1$ processes

We model-independently study the impacts of anomalous $tqZ$ couplings ($q=u,c$), which lead to the $t\to q Z$ decays, on low energy flavor physics. It is found that the $tuZ$-coupling effect can significantly affect the rare $K$ and $B$ decays, whereas the $tcZ$-coupling effect is small. Using the ATLAS's branching ratio (BR) upper bound of $BR(t\to uZ) < 1.7\times 10^{-4}$, the influence of the anomalous $tuZ$-coupling on the rare decays can be found as follows: (a) The contribution to the Kaon direct CP violation can be up to $Re(ε'/ε) \lesssim 0.8 \times 10^{-3}$; (b) $BR(K^+\to π^+ ν\bar ν) \lesssim 12 \times 10^{-11}$ and $BR(K_L \to π^0 ν\bar ν)\lesssim 7.9 \times 10^{-11}$; (c) the BR for $K_S \to μ^+ μ^-$ including the long-distance effect can be enhanced by $11\%$ with respect to the standard model result, and (d) $BR(B_d \to μ^+ μ^-) \lesssim 1.97 \times 10^{-10}$. In addition, although $Re(ε'/ε)$ cannot be synchronously enhanced with $BR(K_L\to π^0 ν\barν)$ and $BR(K_S\to μ^+ μ^-)$ in the same region of the CP-violating phase, the values of $Re(ε'/ε)$, $BR(K^+ \to π^+ ν\barν)$, and $BR(B_d \to μ^+ μ^-)$ can be simultaneously increased.

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arXiv2019

Influence of an inert charged Higgs boson on the muon $g-2$ and radiative neutrino masses in a scotogenic model

A simple extension of Ma's approach in a scotogenic model is studied for the purpose of simultaneously interpreting the neutrino data and the excess of muon anomalous magnetic moment (muon $g-2$). The feasible minimal extension is to add an $Z_2$-odd vector-like lepton doublet to the Ma's model. It is found that in addition to the neutrino data, the strict constraints on the relevant parameters are from the electroweak oblique parameters and the induced lepton-flavor violation processes, such as $\ell_i \to \ell_j γ$ and $\ell_i \to \ell^-_j \ell^-_j \ell^+_j$. Performing parameter scan, we numerically demonstrate that when the constraint conditions are satisfied, the muon $g-2$ of $O(10^{-9})$ can be achieved, where it can be expected that with a $5σ$ observation, the Muon $g-2$ experiment at Fermilab can observe $a_μ\approx 13.31 \times 10^{-10}$ when the current experiment and the SM errors are reduced by a factor of 4 and 2, respectively. Moreover, the branching ratio of the $τ\to μγ$ decay can match the Belle II sensitivity of $O(10^{-9})$ with an integrated luminosity of 50 ab$^{-1}$.

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arXiv2019

Left-handed color-sextet diquark in the Kaon system

We investigate whether a color-sextet scalar diquark (${\bf H}_6$) coupling to the left-handed quarks contributes to the $ΔS=2$ process. It is found that the box diagrams mediated by $W$ and ${\bf H}_6$ bosons have no contributions to $ΔS=2$ when the limit of $m_t=0$ is used, and the flavor mixing matrices for diagonalizing quark mass matrices are introduced at the same time. When the heavy top-quark mass effects are taken into account, it is found that in addition to the $W-{\bf H}_6$ box diagrams significantly contributing to $ΔS=2$, their effects can be as large as those from the ${\bf H}_6-{\bf H}_6$ box diagrams. Using the parameters that are constrained by the $K^0-\bar K^0$ mixing parameter $ΔM_K$ and the Kaon indirect CP violation $ε_K$, we find that the left-handed color-sextet diquark can lead to the Kaon direct CP violation being $Re(ε'/ε) \sim 0.4 \times 10^{-3}$. In the chosen scheme, although the diquark contribution to $K_L\to π^0 ν\barν$ is small, the branching ratio of $K^+ \to π^+ ν\barν$ can reach the current experimental upper bound.

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arXiv2019

Neutrino mass in a gauged $L_μ- L_τ$ model

We study the origin of neutrino mass through lepton-number violation and spontaneous $U(1)_{L_μ-L_τ}$ symmetry breaking. To accomplish the purpose, we include one Higgs triplet, two singlet scalars, and two vector-like doublet leptons in the $U(1)_{L_μ-L_τ}$ gauge extension of the standard model. To completely determine the free parameters, we employ the Frampton-Glashow-Marfatia (FGM) two-zero texture neutrino mass matrix as a theoretical input. It is found that when some particular Yukawa couplings vanish, an FGM pattern can be achieved in the model. Besides the explanation of neutrino data, we find that the absolute value of neutrino mass $m_j$ can be obtained in the model, and their sum can satisfy the upper bound of the cosmological measurement with $\sum_j |m_j| < 0.12$ eV. The effective Majorana neutrino mass for neutrinoless double-beta decay is below the current upper limit and is obtained as $\langle m_{ββ} \rangle =(0.34,\, 2.3)\times 10^{-2}$ eV. In addition, the doubly charged Higgs $H^{\pm\pm}$ decaying to $μ^\pm τ^\pm$ final states can be induced from a dimension-6 operator and is not suppressed, and its branching ratio is compatible with the $H^{\pm \pm}\to W^\pm W^\pm$ decay when the vacuum expectation value of Higgs triplet is $O(0.01)$ GeV.

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arXiv2018

$ε'/ε$ from charged-Higgs-induced gluonic dipole operators

We study the effect of charged-Higgs-induced chromomagnetic operator, $Q_{8G}(-) \equiv \bar s σ^{μν} T^a γ_5 d G^a_{μν}$, on the Kaon direct CP violation $Re(ε'/ε)$. Using the matrix element $\langle ππ| O_{8G}(-) | K^0 \rangle$ recently obtained by a large $N_c$ dual QCD approach, we find that if the Kobayashi-Maskawa phase is the origin of CP violation, the charged-Higgs-induced gluon penguin dipole operator in the type-III two-Higgs-doublet model can explain the measured $Re(ε'/ε)$ when the constraints from the relevant low energy flavor physics, such as $ΔB(K)=2$, $B\to X_s γ$, and Kaon indirect CP violation parameter $ε$, are included.

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