arXiv subjects

Chuan-Hung Chen

Publications and source records attributed to Chuan-Hung Chen.

At least 37 records · Page 2Linked to original sources

arXiv2018

Charged Higgs boson contribution to $B^-_{q} \to \ell \bar ν$ and $\bar B\to (P, V) \ell \barν$ in a generic two-Higgs doublet model

We comprehensively study the charged-Higgs contributions to the leptonic $B^-_q \to \ell \bar ν$ ($q=u,c$) and semileptonic $\bar B \to X_q \ell \barν$ ($X_u=π, ρ; X_c=D,D^*$) decays in the type-III two-Higgs-doublet model (2HDM). We employ the Cheng-Sher ansatz to suppress the tree-level flavor-changing neutral currents (FCNCs) in the quark sector. When the strict constraints from the $ΔB=2$ and $b\to s γ$ processes are considered, parameters $χ^u_{tq}$ from the quark couplings and $χ^\ell_\ell$ from the lepton couplings dictate the leptonic and semileptonic $B$ decays. It is found that when the measured $B^-_u\to τ\bar ν$ and indirect bound of $B^-_c \to τ\bar ν$ obtained by LEP1 data are taken into account, $R(D)$ and $R(π)$ can have broadly allowed ranges; however, the values of $R(ρ)$ and $R(D^*)$ are limited to approximately the standard model (SM) results. We also find that the same behaviors also occur in the $τ$-lepton polarizations and forward-backward asymmetries ($A^{X_q,τ}_{FB}$) of the semileptonic decays, with the exception of $A^{D^*,τ}_{FB}$, for which the deviation from the SM due to the charged-Higgs effect is still sizable. In addition, the $q^2$-dependent $A^{π,τ}_{FB}$ and $A^{D,τ}_{FB}$ can be very sensitive to the charged-Higgs effects and have completely different shapes from the SM.

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arXiv2018

$ε_K$ and $ε'/ε$ in a diquark model

Based on the calculations using the lattice QCD by RBC-UKQCD collaboration and a large $N_c$ dual QCD, the resulted $ε'/ε$, which is less than the experimental data by more than a $2σ$ in the standard model (SM), suggests the necessity of a new physics effect. In order to complement the insufficient $ε'/ε$, we study the extension of the SM with a colored scalar in a diquark model. In addition to the pure diquark box diagrams, it is found that the box diagrams with one $W$-boson and one diquark, ignored in the literature, play an important role in the $ΔS=2$ process. The mass difference between $K_L$ and $K_S$ in the diquark model is well below the current data, whereas the Kaon indirect CP violation $ε_K$ gives a strict constraint on the new parameters. Three mechanisms are classified in the study of $ε'/ε$, and they include: tree-level diagram, QCD and electroweak penguins, and chromomagnetic dipole. Taking the Kobayashi-Maskawa phase as the unique CP source, we analyze each contribution of the three mechanisms in detail and conclude that these diquark-mediated effects can singly enhance $ε'/ε$ to be of ${\cal O}(10^{-3})$, depending on the values of free parameters, when the bound from $ε_K$ is satisfied.

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arXiv2018

$ε'_K/ε_K$ and $K \to πν\barν$ in a two-Higgs doublet model

The Kaon direct CP violation $Re(ε'_K/ε_K)$ below the experimental data of a $2σ$ in the standard model, which is calculated using the RBC-UKQCD lattice and a large $N_c$ dual QCD, indicates the necessity of a new physics effect. In order to resolve the insufficient $Re(ε'_K/ε_K)$, we study the charged-Higgs contributions in a generic two-Higgs-doublet model. If we assume that the origin of the CP-violation phase is uniquely from the Kobayashi-Maskawa (KM) phase when the constraints from the $B$- and $K$-meson mixings, $B\to X_s γ$, and Kaon indirect CP violating parameter $ε_K$ are simultaneously taken into account, it is found that the Kaon direct CP violation through the charged-Higgs effects can reach $Re(ε'_K/ε_K)_{H^\pm}\sim 8 \times 10^{-4}$. Moreover, with the constrained values of the parameters, the branching ratios of the rare $K\to πν\barν$ decays can be $BR(K^+\to π^+ ν\barν)\sim 14 \times 10^{-11}$ and $BR(K_L\to π^0 ν\barν)\sim 3.6 \times 10^{-11}$, where the results can be tested through the NA62 experiment at CERN and the KOTO experiment at J-PARC, respectively.

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arXiv2018

Explaining the DAMPE $e^+ e^-$ excess using the Higgs triplet model with a vector dark matter

We explain the $e^+ e^-$ excess observed by the DAMPE Collaboration using a dark matter model based upon the Higgs triplet model and an additional hidden $SU(2)_X$ gauge symmetry. Two of the $SU(2)_X$ gauge bosons are stable due to a residual discrete symmetry and serve as the dark matter candidate. We search the parameter space for regions that can explain the observed relic abundance, and compute the flux of $e^+ e^-$ coming from a nearby dark matter subhalo. With the inclusion of background cosmic rays, we show that the model can render a good fit to the entire energy spectrum covering the AMS-02, Fermi-LAT and DAMPE data.

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arXiv2018

Penguin $b \to s\ell'^+ \ell'^-$ and $B$-meson anomalies in a gauged ${L_μ-L_τ}$

The $Z'$-gauge boson in an $U(1)_{L_μ-L_τ}$ gauge symmetry has two interesting features: one is its vector couplings to the charged leptons, and the other is the decoupling from the electron. Based on these properties, we investigate the feasibility to simultaneously resolve the $R_{K^{(*)}} = BR(B\to K^{(*)} μ^+ μ^-)/BR(B\to K^{(*)} e^+ e^-)$ and $R_{D^{(*)}} = BR(\bar B\to D^{(*)} τ\barν_τ)/BR(\bar B\to D^{(*)} \ell \barν_\ell)$ anomalies in an $U(1)_{L_μ-L_τ}$ model, where the former is expected to arise from the $Z'$-penguin-induced $b\to s μ^+ μ^-$ process and the latter from the tree-level $b\to c τ\barν_τ$ decay. In order to achieve the intended purpose, we employ one vector-like doublet lepton and one singlet scalar leptoquark (LQ), in which the new particles all carry the $U(1)_{L_μ-L_τ}$ charges; the $b\to sZ'$ effective interaction is generated from the vector-like lepton and LQ loop, and the $b\to c τ\barν_τ$ decay is induced from the LQ. When the constraints from the $b\to s γ$, $B^+\to K^+ ν\barν$, $B^-_c\to τ\bar ν_τ$, $ΔF=2$, and $τ\to μ\ell \bar \ell$ processes are included, it is found that $R_D$ and $R_{D^*}$ can be enhanced to fit the experimental data, and the Wilson coefficient $C_9$ from the LQ-loop can reach $C^{LQ,μ}_9\sim -1$, which can explain the $R_{K}$ and $R_{K^*}$ anomalies. In addition, in this simple model, the Higgs lepton-flavor violating $h\to μτ$ decay can occur at the tree level, and its branching ratio can be as large as the current experimental upper limit.

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arXiv2017

$L_μ-L_τ$ gauge-boson production from lepton flavor violating $τ$ decays at Belle II

$L_μ-L_τ$ gauge boson ($Z'$) with a mass in the MeV to GeV region can resolve not only the muon $g-2$ excess, but also the gap in the high-energy cosmic neutrino spectrum at IceCube. It was recently proposed that such a light gauge boson can be detected during the Belle II experiment with a luminosity of 50 ab$^{-1}$ by the $e^+ e^- \to γ+\slashed{E}$ process through the kinetic mixing with the photon, where the missing energy $\slashed{E}$ is from the $Z' \to \barνν$ decays. We study the phenomenological implications when a pair of singlet vector-like leptons carrying different $L_μ- L_τ$ charges are included, and a complex singlet scalar ($ϕ_S$) is introduced to accomplish the spontaneous $U(1)_{L_μ-L_τ}$ symmetry breaking. It is found that the extension leads to several phenomena of interest, including (i) branching ratio (BR) for $h\to μτ$ can be of the order of $10^{-3}$; (ii) $ϕ_S$-mediated muon $g-2$ can be of the order of $10\times 10^{-10}$; (iii) BR for $τ\to μϕ^*_S\to μZ'Z'$ can be $10^{-8}$, and (iv) kinetic mixing between the $Z'$ boson and photon is sensitive to the relative heavy lepton masses. The predicted BRs for $τ\to (3μ+\slashed{E}, 5μ$) through the leptonic $Z'$ decays can reach a level of $10^{-9}$, in which the results fall within the sensitivity of the Belle II in the search for the rare tau decays.

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arXiv2017

Excesses of muon $g-2$, $R_{D^{(\ast)}}$, and $R_K$ in a leptoquark model

In this study, we investigate muon $g-2$, $R_{K^{(*)}}$, and $R_{D^{(*)}}$ anomalies in a specific model with one doublet, one triplet, and one singlet scalar leptoquark (LQ). When the strict limits from the $\ell' \to \ell γ$, $ΔB=2$, $B_{s}\to μ^+ μ^-$, and $B^+ \to K^+ ν\barν$ processes are considered, it is difficult to use one scalar LQ to explain all of the anomalies due to the strong correlations among the constraints and observables. After ignoring the constraints and small couplings, the muon $g-2$ can be explained by the doublet LQ alone due to the $m_t$ enhancement, whereas the measured and unexpected smaller $R_{K^{(*)}}$ requires the combined effects of the doublet and triplet LQs, and the $R_D$ and $R_{D^*}$ excesses depend on the singlet LQ through scalar- and tensor-type interactions.

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arXiv2017

Study of $B^\pm_c \to (D^0 K^\pm, D^0 π^\pm)$ decays

LHCb observes the $B^+_c \to D^0 K^+$ decay with $R_{D^0 K} = f_c/f_u \times {\cal B}(B^+_c \to D^0 K^+)=(9.3^{+2.8}_{-2.5} \pm 0.6)\times 10^{-7}$. The corresponding branching ratio (BR) of the decay can be estimated as ${\cal B}(B^+_c \to D^0 K^+) \approx (10.01 \pm 3.40)\times 10^{-5}$; however, the theoretical estimates vary from $\sim 10^{-7}$ to $\sim 5\times 10^{-5}$. We phenomenologically investigate the $B^+_c \to (D^0 K^+, D^0 π^+)$ decays through the analysis of $B\to KK$, $B^+_u\to D^+ K^0$, and $B_d \to D^-_s K^+$. With the form factor of $f^{B_c D}_0\approx 0.2$, it is found that the tree-annihilation contribution dominates the $B^+_c \to D^0 K^+$ decay, and when ${\cal B}(B^+_u \to D^+ K^0) \approx (1-3.1) \times 10^{-7}$ is required, we obtain ${\cal B}(B^+_c \to D^0 K^+)\approx (4.4- 9) \times 10^{-5}$, and the magnitude of CP asymmetry is lower than approximately $10\%$. Although the $B^+_c \to D^0 π^+$ decay is dominated by the tree-transition effect, the tree-annihilation also makes an important contribution, where its effect could be around $70\%$ of the tree-transition. It is found that when ${\cal B}(B^+_c \to D^0 K^+)\approx (4.4- 9) \times 10^{-5}$ is taken, the BR and CP asymmetry for $B^+_c \to D^0 π^+$ with the common values of parameters can be ${\cal B}(B^+_c \to D^0 π^+)\approx (4.9-8)\times 10^{-6}$ and of the order of one, respectively. Moreover, we conclude ${\cal B}(B^+_c \to D^+ K^0)\approx {\cal B}(B^+_c \to D^0 K^+)$, and the BRs for $B^+_c \to K^+ \bar K^0$ and $B^+_c \to J/Ψπ^+$ are $(6.99 \pm 1.34) \times 10^{-7}$ and $(7.7 \pm 1.1)\times 10^{-4}$, respectively.

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arXiv2017

Charged-Higgs on $R_{D^{(*)}}$, $τ$ polarization, and FBA

We study the influence of a charged-Higgs on the excess of branching fraction ratio, $R_M = BR(\bar B \to M τ\barν_τ)/BR(\bar B \to M \ell \bar ν_\ell)$ $(M=D, D^*)$, in a generic two-Higgs-doublet model. In order to investigate the lepton polarization, the detailed decay amplitudes with lepton helicity are given. When the charged-Higgs is used to resolve excesses, it is found that two independent Yukawa couplings are needed to explain the $R_D$ and $R_{D^*}$ anomalies. We show that when the upper limit of $BR(B_c \to τ\bar ν_τ)<30\%$ is included, $R_D$ can be significantly enhanced while $R_{D^*}<0.27$. With the $BR(B_c\to τ\bar ν_τ)$ constraint, we find that the $τ$-lepton polarizations can be still affected by the charged-Higgs effects, where the standard model (SM) predictions are obtained as: $P^τ_{D} \approx 0.324$ and $P^τ_{D^*}\approx -0.500$, and they can be enhanced to be $P^τ_{D} \approx 0.5$ and $P^τ_{D^*} \approx -0.41$ by the charged-Higgs. The integrated lepton froward-backward asymmetry (FBA) is also studied, where the SM result is $\bar A^{D^{(*)},τ}_{FB} \approx -0.359(0.064)$, and they can be enhanced (decreased) to be $\bar A^{D^{(*)},τ}_{FB} \approx -0.33 (0.02)$.

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arXiv2017

Constraint on the branching ratio of B_c \to tau nu from LEP1 and consequences for R(D(*)) anomaly

Recently there has been interest in the correlation between R(D*) and the branching ratio (BR) of $B_c \to τν$ in models with a charged scalar H^\pm. Any enhancement of R(D*) by $H^\pm$ alone (in order to agree with current data) also enhances $BR(B_c \to τν$), for which there has been no direct search at hadron colliders. We show that LEP data taken at the Z peak requires BR($B_c \to τν$) < 10%, and this constraint is significantly stronger than the recent constraint BR($B_c \to τν$) < 30% from considering the lifetime of B_c. In order to respect this new constraint, any explanation of the R(D) and R(D*) anomaly in terms of $H^\pm$ alone would require the future measurements of R(D*) to be even closer to the Standard Model prediction. A stronger limit on BR($B_c \to τν$) (or its first measurement) would be obtained if the L3 collaboration used all its data taken at the Z peak.

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arXiv2017

Search for a light-charged Higgs in a two-Higgs-doublet type II seesaw model at the LHC

A charged Higgs in the type II two-Higgs-doublet model (THDM) has been bounded to be above a few hundred GeV by the radiative $B$ decays. A Higgs triplet extension of the THDM not only provides an origin of neutrino masses and a completely new doubly-charged Higgs decay pattern, but it also achieves a light-charged Higgs with a mass of ${\cal O}(100)$ GeV through the new scalar couplings in the scalar potential. It was found that these light-charged Higgs decays depend on its mass $m_{H^\pm}$, $\tanβ$, and mixing effect $\sinθ_\pm$: at $\tanβ=1$, if $m_{H^\pm} > m_W + m_Z$, $\bar b b W^\pm$, $W^\pm Z$, and $τν$ are the main decay modes; however, if $m_{H^\pm} < m_W + m_Z$, the main decay modes are then $\bar b b W$ and $τν$, and at $\tanβ=30$, the $τν$ mode dominates the other decays. When $m_t > m_{H^\pm} + m_b$, we found that the ATLAS and CMS recent upper bounds on the product of $BR(t\to H^+ b)BR(H^+\to τ^+ ν)$ can be directly applied and will give a strict constraint on the correlation of $m_{H^\pm}$ and $\sinθ_\pm$. If the upper bound of $BR(t\to H^+ b)BR(H^+\to τ^+ ν)$ is satisfied (escaped) for $m_t > (<) m_{H^\pm} + m_b$, it was found that the significance of discovering the charged Higgs through $H^\pm \to W^\pm Z$ is much lower than that through $H^\pm \to \bar b b W^\pm$. With a luminosity of 100 fb$^{-1}$ at $\sqrt{s}=13$ TeV and including the experimental bounds, the significance of the $H^\pm \to \bar b b W^\pm$ signal can reach around $6.2 (2.4)σ$ for $m_{H^\pm} <(>) m_W + m_Z$.

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arXiv2017

Phenomenology of an $SU(2)_1 \times SU(2)_2 \times U(1)_Y$ model at the LHC

We investigate the implications of a minimal $SU(2)$ gauge symmetry extension of the standard model at the LHC. To achieve the spontaneous symmetry breaking, a heavy Higgs doublet of the $SU(2)$ is introduced. To obtain an anomaly free model and the decays of new charged gauge bosons, we include a vector-like quark doublet. We also employ a real scalar boson to dictate the heavy Higgs production via the gluon-gluon fusion processes. It is found that the new gauge coupling and the masses of new gauge bosons can be strictly bounded by the electroweak $ρ$-parameter and dilepton resonance experiments at the LHC. It is found that due to the new charged gauge boson enhancement, the cross sections for a heavy scalar boson to diphoton channel measured by ATLAS and CMS can be easily satisfied when the values of Yukawa couplings are properly taken. Furthermore, by adopting event simulation, we find that the significance of $pp\to (γγ)_H+{\rm jet}$, where the diphoton is from the heavy Higgs decay, can be over $4σ$ when the luminosity is above 60 fb$^{-1}$.

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arXiv2016

Explanation of $B \to K^{(*)} \ell^+ \ell^-$ and muon $g-2$, and implications at the LHC

More than $3σ$ deviations from the standard model are observed in the angular observable $P'_5$ of $B\to K^* μ^+ μ^-$ and muon $g-2$. To resolve these anomalies, we extend the standard model by adding two leptoquarks. It is found that the signal strength of the diphoton Higgs decay can exhibit a significant deviation from unity and is within the data errors. Although $\ell_i \to \ell_j γ$ puts severe bounds on some couplings, it is found that the excesses of $P'_5$ and muon $g-2$ can still be explained and can be accommodated to the measurement of $B_s\to μ^+ μ^-$ in this model. In addition, the leptoquark effects can also explain the LHCb measurement of $R_K = BR(B^+ \to K^+ μ^+μ^-)/BR(B^+ \to K^+ e^+e^-)=0.745^{+0.090}_{-0.074} \pm 0.036$, which shows a $2.6σ$ deviation from the standard model prediction.

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arXiv2016

Light gauge boson in rare $K$ decay

We study the production of a light gauge boson in $K^- \to π^- X$ decay, where the associated new charge current is not conserved. It is found that the process can be generated by the tree-level $W$-boson annihilation and loop-induced $s\to dX$. We find that it strongly depends on the $SU(3)$ limit or the unique gauge coupling to the quarks, whether the decay amplitude of $K^-\to π^- X$ in the $W$-boson annihilation is suppressed by $m^2_X ε_X \cdot p_K$; however, no such suppression is found via the loop-induced $s\to d X$. The constraints on the relevant couplings are studied.

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arXiv2016

Single production of $X_{\pm 5/3}$ and $Y_{\mp 4/3}$ vectorlike quarks at the LHC

Two triplet vector-like quarks (VLQs) with hypercharges of $Y=2/3, -1/3$ and one singlet scalar boson are embedded in the standard model (SM) to resolve the 750 GeV diphoton excess. The constraints on the tree-level Higgs- and $Z$-mediated flavor-changing neutral currents are discussed in detail. Besides the resolution of excess, it is found that the signal strength of diphoton Higgs decay can have a $10\%$ deviation from the SM prediction and that the upper limits of the branching ratios for rare top-quark decays are $BR(t\to c (h, Z)) < (6.8, 0.48) \times 10^{-5}$. We find that the production cross section of a single VLQ by electroweak processes is larger than that of VLQ-pair by QCD processes. To explore the signals of the heavy VLQs at the LHC, we throughly analyze the production of single $X_{\pm 5/3}$ and $Y_{\mp 4/3}$ via $q_i q'_j$ annihilations in $pp$ collisions at $\sqrt{s}=13$ TeV. It is found that the electroweak production cross sections for $d X_{5/3}$, $u Y_{-4/3} $, and $d Y_{4/3} $ channels with $m_X=m_Y=1$ TeV can be $84.3$, $72.3$, and $157.8$ fb, respectively; and the dominant decay modes are $X_{5/3} \to (c,t) W^+$ and $Y_{-4/3} \to (s,b) W^-$. With adopting kinematic cuts, the significance for $pp\to d W^+ t$ channel can be over $5σ$.

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arXiv2016

$h,Z\to \ell_i \bar\ell_j$, $Δa_μ$, $τ\to (3μ,μγ)$ in generic two-Higgs-doublet models

Inspired by a significance of $2.4σ$ in $h\to μτ$ decay observed by CMS at $\sqrt{s} = 8$ TeV, we investigate the Higgs lepton flavor violating effects in the generic two-Higgs-doublet model (GTHDM), where the lepton flavor changing neutral currents are induced at tree level and arisen from Yukawa sector. We revisit the constraints for GTHDM by considering theoretical requirements, precision measurements of $δρ$ and oblique parameters $S$, $T$, $U$ and Higgs measurements. The bounds from Higgs data now play the major role. With the values of parameters that simultaneously satisfy the Higgs bounds and the excess of Higgs coupling to $μ$-$τ$ at CMS, we find that the tree-level $τ\to 3μ$ and loop-induced $τ\to μγ$ could be consistent with current experimental upper limits; the discrepancy in muon $g-2$ between experiment and standard model prediction could be solved; and an interesting relation between muon $g-2$ and branching ratio (BR) for $μ\to e γ$ is found. The GTHM results that the ratio $BR(h\to eτ)/BR(h\to μτ)$ should be smaller than $10^{-4}$ in order of magnitude. Additionally, we also study the rare decay $Z\to μτ$ and get $BR(Z\to μτ)< 10^{-6}$.

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arXiv2016

Bounds on LFV Higgs decays in a vector-like lepton model and searching for doubly charged leptons at the LHC

The Higgs-portal lepton flavor violation is studied in a vectorlike lepton model. For evading the constraints from rare $Z\to \ell^\pm_i \ell^\mp_j$ decays, we introduce two triplet vectorlike leptons, $(1,3)_{-1}$ and $(1,3)_{0}$. The resultant branching ratio for $h\to μτ$ can be up to $10^{-4}$ when the constraints from the invisible $Z$ decays are applied. As a result, the signal strength for $ττ$ channel has a $12\%$ deviation from the standard model prediction, while the muon $g-2$ is two-order of magnitude smaller than the data and $BR(τ\to μγ)$ is of order of $10^{-12}$. A predicted doubly charged lepton in $pp$ collisions at $\sqrt{s}=13$ TeV is analyzed and it is found that the interesting production channels are $pp\to (Ψ^{--}_{1} Ψ^{++}_1, Ψ^{\pm\pm}_1 Ψ^\mp_1)$. Both single and pair production cross sections of $Ψ^{++}_1$ are comparable and can be a few hundred fb. The main decay channels for the doubly charged lepton are $Ψ^{\pm\pm} \to \ell^\pm W^\pm$ and for the heavy singly charged lepton are $Ψ^\pm_1 \to νW^\pm, \ell^\pm Z$. The numerical analysis is given at $13$ TeV LHC with $100$ fb$^{-1}$ luminosity.

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arXiv2016

Searching for Vector Dark Matter by Higgs Portal at the LHC

A Higgs portal dark matter model for explaining the gamma-ray excess from the galactic center can be realized with the extension of local $SU(2)_X$ gauge symmetry with one quadruplet. Due to the residual $Z_3$ discrete symmetry of $SU(2)_X$, the new gauge bosons are the stable dark matter candidates. Due to the mixture of the standard model Higgs doublet and the introduced quadruplet, dark matter could annihilate into the standard model particles through the Higgs portal and new scalar portal. We study the discovery significance of the vector dark matter at the Large Hadron Collider. The involved parameters are consistent with the constraints from relic density and direct detection and with the data of the galactic center gamma-ray excess. With $\sqrt{s}=14$ TeV and luminosities of $100$ and $300$ fb$^{-1}$, we find that a discovery significance of $S/\sqrt{B}=5$ can be easily reached if the production of dark matter is through the invisible decays of the Higgs boson and a new scalar boson.

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