arXiv subjects

Chuan-Hung Chen

Publications and source records attributed to Chuan-Hung Chen.

At least 55 records · Page 3Linked to original sources

arXiv2016

Higgs singlet boson as a diphoton resonance in a vector-like quark model

ATLAS and CMS recently show the first results from run 2 of the Large Hadron Collider (LHC) at $\sqrt{s}=13$ TeV. A resonant bump at a mass of around 750 GeV in the diphoton invariant mass spectrum is indicated and the corresponding diphoton production cross section is around 3-10 fb. Motivated by the LHC diphoton excess, we propose that the possible resonance candidate is a Higgs singlet. To produce the Higgs singlet via gluon-gluon fusion process, we embed the Higgs singlet in the framework of vector-like triplet quark (VLTQ) model. As a result, the Higgs singlet decaying to diphoton final state is via VLTQ loops. Using the enhanced number of new quarks and new Yukawa couplings of the VLTQs and Higgs singlet, we successfully explain the diphoton production cross section. We find that the width of the Higgs singlet is below 1 GeV, its production cross section can be of order of 1 pb at $\sqrt{s}=13$ TeV, and the branching ratio for it decaying to diphoton is around $0.017$ and is insensitive to the masses of VLTQs and new Yukawa couplings. We find a strong correlation between the Higgs Yukawa couplings to $s$-$b$ and $c$-$t$; the resulted branching ratio for $t \to c h$ can be $1.1\times 10^{-4}$ when the constraint from $B_s$ oscillation is applied. With the constrained parameter values, the signal strength for the SM Higgs decaying to diphoton is $μ_{γγ}< 1.18$, which is consistent with the current measurements at ATLAS and CMS.

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arXiv2016

Two-Higgs-Doublet type-II and -III models and $t\to c h$ at the LHC

We study the constraints of the generic two-Higgs-doublet model (2HDM) type-III and the impacts of the new Yukawa couplings. For comparisons, we revisit the analysis in the 2HDM type-II. To understand the influence of all involving free parameters and to realize their correlations, we employ $χ$-square fitting approach by including theoretical and experimental constraints, such as S, T, and U oblique parameters, the production of standard model Higgs and its decay to $γγ$, $WW^*/ZZ^*$, $τ^+τ^-$, etc. The errors of analysis are taken at $68\%$, $95.5\%$, and $99.7\%$ confidence levels. Due to the new Yukawa couplings being associated with $\cos(β-α)$ and $\sin(β-α)$, we find that the allowed regions for $\sinα$ and $\tanβ$ in the type-III model can be broader when the dictated parameter $χ_F$ is positive; however, for negative $χ_F$, the limits are stricter than those in the type-II model. By using the constrained parameters, we find that the deviation from the SM in the $h\to Zγ$ can be of ${\cal O}(10\%)$. Additionally, we also study the top-quark flavor-changing processes induced at the tree level in the type-III model and find that when all current experimental data are considered, we get $Br(t\to c(h, H) )< 10^{-3}$ for $m_h=125.36$ and $m_H=150$ GeV and $Br(t\to cA)$ slightly exceeds $10^{-3}$ for $m_A =130$ GeV.

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arXiv2015

Resolving Fermi, PAMELA and ATIC anomalies in split supersymmetry without R-parity

A long-lived decaying dark matter as a resolution to Fermi, PAMELA and ATIC anomalies is investigated in the framework of split supersymmetry (SUSY) without R-parity, where the neutralino is regarded as the dark matter and the extreme fine-tuned couplings for the long-lived neutralino are naturally evaded in the usual approach.

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arXiv2015

Diboson excess in the Higgs singlet and vector-like quark models

Diboson resonance with mass of $1.8-2$ TeV is reported successively by CMS and ATLAS experiments in proton-proton collisions at $\sqrt{s}=8$ TeV. We investigate the potentiality of Higgs singlet as the TeV resonance. The challenges of low production cross section and high width for a fundamental scalar could be got over by three factors: (1) larger Yukawa couplings, (2) larger number of heavy quarks and (3) smaller mixing angle with standard model Higgs. We find that the required factors could be realized in the framework of two vector-like triplet quarks (VLTQs) and the resulting production cross section and decay fraction of heavy Higgs $σ(pp\to H )\times {\rm BR}(H\to W^+ W^- + ZZ)$ can be of ${\cal O}(10)$ fb when masses of new heavy quarks are 1 TeV, the values of Yukawa couplings are around $3$ and the mixing angle is $\sinθ\sim 0.11$. Furthermore, we study the product of VLTQ-pair production cross section and the BRs of VLTQ decays, and find that the cross sections in the decay channels, such as $u_{4,5} \to b W^+$, $d_5 \to t W^-$ and $d_4 \to b h(Z)$ could be $7-17$ fb at 13 TeV LHC.

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arXiv2015

2 TeV Higgs boson and diboson excess at the LHC

Diboson resonance with mass around 2 TeV in the dijet invariant mass spectrum is reported by ATLAS experiment in proton-proton collisions at $\sqrt{s}=8$ TeV. We propose that the candidate of resonance is a heavy neutral Higgs $H^0$ or charged Higgs $H^\pm$ and use the extended two-Higgs-doublet (THD) to demonstrate the potentiality. We find that the large Yukawa coupling to the first generation of quarks can be realized in THD and the required value for producing the right resonance production cross section is of ${\cal O}(0.06-0.2)$. Besides $WW/ZZ$ channels, we find that if the mass of pseudoscalar $A^0$ satisfies the jet mass tagging condition $|m_j - m_{Z/W}|< 13$ GeV, the diboson excess could be also caused by $ZA^0$ or $WA^0$ channel.

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arXiv2015

Dark matter for excess of AMS-02 positrons and antiprotons

We propose a dark matter explanation to simultaneously account for the excess of antiproton-to-proton and positron power spectra observed in the AMS-02 experiment while having the right dark matter relic abundance and satisfying the current direct search bounds. We extend the Higgs triplet model with a hidden gauge symmetry of $SU(2)_X$ that is broken to $Z_3$ by a quadruplet scalar field, rendering the associated gauge bosons stable weakly-interacting massive particle dark matter candidates. By coupling the complex Higgs triplet and the $SU(2)_X$ quadruplet, the dark matter candidates can annihilate into triplet Higgs bosons each of which in turn decays into lepton or gauge boson final states. Such a mechanism gives rise to correct excess of positrons and antiprotons with an appropriate choice of the triplet vacuum expectation value. Besides, the model provides a link between neutrino mass and dark matter phenomenology.

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arXiv2015

$SU(2)_X$ Vector DM and Galactic Center Gamma-Ray Excess

An unbroken $Z_3$ symmetry remains when local $SU(2)_X$ is broken spontaneously by one quadruplet. The gauge boson $χ_μ(\bar χ_μ)$ carries the dark charge and is the candidate of dark matter (DM). By the mixture of scalar boson $ϕ_r$ of quadruplet and standard model (SM) Higgs, the DM can annihilate to SM particles through Higgs portal. For investigating the implications of vector DM, we study the relic density of DM, the direct detection of DM-nucleon scattering and the excess of gamma-ray spectrum, which is supported by the data from {\it Fermi} Gamma-Ray Space Telescope. We find that with the DM mass of around $70$ GeV in our model, the excess of gamma-ray could be fitted well with the data.

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arXiv2015

Search for $δ^{\pm\pm}$ with new decay patterns at the LHC

A study of searching for doubly charged Higgs $(δ^{\pm \pm})$ is performed in two-Higgs-doublet extension of the conventional type-II seesaw model. We find that a fantastic mixing effect between singly charged Higgs of Higgs doublet and of triplet is arisen from the scalar potential. The mixing leads to following intriguing phenomena: (a) the mass splittings in triplet particles are magnified, (b) QCD processes dominate the production of $δ^{\pm \pm}$, and (c) new predominant decay channels of $δ^{\pm \pm}$ are $δ^{\pm \pm} \to W^{\pm^{[*]}} H^{\pm^{(*)}}_{1(2)}$, but not $δ^{\pm} \to (\ell^\pm \ell^\pm, W^\pm W^\pm)$ which are usually discussed in the literature. With luminosity of 40 fb$^{-1}$ and collision energy of 13 TeV, we demonstrate that $δ^{\pm \pm}$ with mass below $330$ GeV could be observed at the $5σ$ level. Moreover, when the luminosity approaches to 300 fb$^{-1}$, the observed mass of $δ^{\pm \pm}$ could reach up to 450 GeV.

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arXiv2014

Inert Dark Matter in Type-II Seesaw

Weakly interacting massive particle (WIMP) as a dark matter (DM) candidate is further inspired by recent AMS-02 data, which confirm the excess of positron fraction observed earlier by PAMELA and Fermi-LAT experiments. Additionally, the excess of positron+electron flux is still significant in the measurement of Fermi-LAT. For solving the problems of massive neutrinos and observed excess of cosmic-ray, we study the model with an inert Higgs doublet (IHD) in the framework of type-II seesaw model by imposing a $Z_2$ symmetry on the IHD, where the lightest particle of IHD is the DM candidate and the neutrino masses originate from the Yukawa couplings of Higgs triplet and leptons. We calculate the cosmic-ray production in our model by using three kinds of neutrino mass spectra, which are classified by normal ordering, inverted ordering and quasi-degeneracy. We find that when the constraints of DM relic density and comic-ray antiproton spectrum are taken into account, the observed excess of positron/electron flux could be explained well in normal ordered neutrino mass spectrum. Moreover, excess of comic-ray neutrinos is implied in our model. We find that our results on $\langle σv \rangle$ are satisfied with and close to the upper limit of IceCube analysis. More data from comic-ray neutrinos could test our model.

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arXiv2014

Two-Higgs-Doublet Type-II Seesaw Model

Motivated by the new observed scalar boson of 126 GeV at ATLAS and CMS, various phenomena in two-Higgs-doublet model (THDM) are investigated broadly in the literature. For considering the model that possesses a solution to the massive neutrinos, we study the simplest extension of conventional type-II seesaw model to two Higgs doublets. We find that the new interactions in the scalar potential cause the sizable mixture of charged Higgses in triplet and doublet. As a result, we have a completely different decay pattern for doubly charged Higgs ($δ^{\pm\pm}$), even the vacuum expectation value (VEV) of Higgs triplet is at GeV level, which is limited by the precision measurement for $ρ$-parameter. For illustrating the new characters of the model, we study the influence of new interactions on the new open channels $δ^{++}\to ( H^+_1 W^{+^{(*)}}, H^+_1 H^+_1)$ with $H^+_1$ being the lightest charged Higgs. Additionally, due to the new mixing effect, the triplet charged Higgs could couple to quarks in the model; therefore, the search for $δ^{++}$ via $δ^{++}\to tb W^+ \to b \bar b W^+ W^+$ by mediated $H^+_{1}$ becomes significant.

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arXiv2013

Colored bosons on top FBA and angular cross section for $t \bar t$ production

With full data set that corresponds to an integrated luminosity of 9.4 fb$^{-1}$, CDF has updated the top quark forward-backward asymmetry (FBA) as functions of rapidity difference $|Δy|$ and $t\bar t$ invariant mass $M_{t\bar t}$. Beside the sustained inconsistency between experiments and standard model (SM) predictions at large $|Δy|$ and $M_{t\bar t}$, an unexpected large first Legendre moment with $a_1= 0.39\pm 0.108 $ is found. In order to solve the large top FBA, we study the contributions of color triplet scalar and color octet vector boson. We find that the top FBA at $|Δy| >1$ and $M_{t\bar t} > 450$ GeV in triplet and octet model could be enhanced to be around 30% and 20%, whereas the first Legendre moment is $a^{\bf Di}_1= 0.38$ and $a^{\bf Axi}_1= 0.23$, respectively.

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arXiv2012

Direct CP Violation in Charm Decays due to Left-Right Mixing

Motivated by the $3.8σ$ deviation from no CP violation hypothesis for the CP asymmetry (CPA) difference between $D^0\to K^+ K^-$ and $D^0\to π^+ π^-$, reported recently by LHCb and CDF, we investigate the CP violating effect due to the left-right (LR) mixing in the general LR symmetric model. In particular, in the non-manifest LR model we show that the large CPA difference could be explained, while the constraints from $(ε'/ε)_K$ and $D^0$-$\bar D^0$ are satisfied.

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arXiv2012

H\toγγin the Complex Two Higgs Doublet Model

We explore the parameter regions in the complex Two Higgs Doublet Model (c2HDM) with explicit CP violation that can describe current LHC hints for one or more Higgs boson signals at 125 GeV. Such a simple extension of the Standard model has three neutral Higgs bosons and a pair charged Higgs and leads to rich CP-violating sources including the CP-even CP-odd mixing of the neutral Higgs bosons. Within this model we present the production of light Higgs boson at the LHC followed by its decay into two photons. Our numerical study takes into account theoretical and experimental constraints on the Higgs potential like positivity, unitarity and perturbativity as well as $ρ$-parameter, $b\to sγ$ and $R_b$. These requirement together with the minimum conditions could explain the di-photon excess observed at the LHC. We also discuss the effects of the CP violating phases on the CP violation observable which is the difference between left-circular and right-circular polarization.

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arXiv2012

CP violation in $D^0 \to (K^- K^+, π^- π^+)$ from diquarks

The explanation of the large CP asymmetries in $D^0\to (π^+π^-, K^+K^-)$ decays observed by the LHCb collaboration is likely to call for new physics beyond the CKM paradigm. We explore new contributions caused by the color-sextet scalar diquark, and demonstrate that the diquark with the mass of order 1 TeV and nominal couplings with quarks can generate the CP asymmetries at the percent level. Using the experimental data on branching ratios and CP asymmetries of $D^0\to (π^+π^-, K^+K^-)$, we derive the constraints on the diquark mass and couplings, which can be further examined on hadron colliders in the dijet final states.

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arXiv2012

Exotic fermion multiplets as a solution to baryon asymmetry, dark matter and neutrino masses

We propose an extension to the standard model where three exotic fermion 5-plets and one scalar 6-plet are added to the particle content. By demanding that all interactions are renormalizable and standard model gauge invariant, we show that the lightest exotic particle in this model can be a dark matter candidate as long as the new 6-plet scalar does not develop a nonzero vacuum expectation value. Furthermore, light neutrino masses are generated radiatively at one-loop while the baryon asymmetry is produced by the CP-violating decays of the second lightest exotic particle. We have demonstrated using concrete examples that there is a parameter space where a consistent solution to the problems of baryon asymmetry, dark matter and neutrino masses can be obtained.

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arXiv2012

A light charged Higgs boson in two-Higgs doublet model for CDF $Wjj$ anomaly

Motivated by recent anomalous CDF data on $Wjj$ events, we study a possible explanation within the framework of the two-Higgs doublet model. We find that a charged Higgs boson of mass $\sim$ 140 GeV with appropriate couplings can account for the observed excess. In addition, we consider the flavor-changing neutral current effects induced at loop level by the charged Higgs boson on the $B$ meson system to further constrain the model. Our study shows that the like-sign charge asymmetry $A_{s\ell}^b$ can be of ${\cal O}(10^{-3})$ in this scenario.

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arXiv2011

Pair production of neutralinos and charginos at the LHC: the role of Higgs bosons exchange

We analyze the effects of the s-channel Higgs bosons exchange on the charginos and neutralinos-pair production in proton-proton collision at the CERN Large Hadron Collider (LHC) in the following channels: $pp \to \Cha^+\Cha^-/\Neu^0\Neu^0 + X$, within the minimal supersymmetric standard model (MSSM). Assuming the usual GUT relation between $M_1$ and $M_2$ at the weak scale, we found that substantial enhancement can be obtained through s-channel Higgs bosons exchange in the mixed regime where $M_2 \sim |μ|$ with moderate to large $\tanβ$ at the resonance of the heavy Higgs bosons. By Combining the phenomenological constraints on neutralinos and charginos, we may still find regions of parameter space where charginos and neutralinos-pair production at the LHC from $b\bar b$ initial state can be large and observable at LHC. We also compute the full complete set of electroweak (EW) contributions to $pp\to gg\to \Cha^+\Cha^-/\Neu^0\Neu^0 + X$ at one loop level in the general MSSM. The analytical computation of the complete tree level amplitude for $b\bar{b} \to \Cha^+\Cha^-/\Neu^0\Neu^0 + X$, including s-channel Higgs exchange, is given.

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arXiv2011

Rare B decays and Tevatron top-pair asymmetry

The recent Tevatron result on the top quark forward-backward asymmetry, which deviates from its standard model prediction by 3.4$σ$, has prompted many authors to build new models to account for this anomaly. Among the various proposals, we find that those mechanisms which produce $t\bar t$ via $t$- or $u$-channel can have a strong correlation to the rare B decays. We demonstrate this link by studying a model with a new charged gauge boson, $W'$. In terms of the current measurements on $B\to πK$ decays, we conclude that the branching ratio for $B^-\to π^- \bar K^0$ is affected most by the new effects. Furthermore, using the world average branching ratio for the exclusive B decays at $2σ$ level, we discuss the allowed values for the new parameters. Finally, we point out that the influence of the new physics effects on the direct CP asymmetry in B decays is insignificant.

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