arXiv subjects

Cheng-Wei Chiang

Publications and source records attributed to Cheng-Wei Chiang.

At least 109 records · Page 6Linked to original sources

arXiv2016

Probing a dark photon using rare leptonic kaon and pion decays

Rare leptonic kaon and pion decays $K^+(π^+) \to μ^+\, ν_μ\, e^+ e^-$ can be used to probe a dark photon of mass ${\cal O}(10)$~MeV, with the background coming from the mediation of a virtual photon. This is most relevant for the 16.7-MeV dark photon proposed to explain a 6.8$σ$ anomaly recently observed in $^8$Be transitions by the Atomki Collaboration. We evaluate the reach of future experiments for different scenarios of how the dark photon couples with the standard model particles, and show that a great portion of the preferred 16.7-MeV dark photon parameter space can be decisively probed. We also show the use of angular distributions to further distinguish the signal from the background.

hep-ph↗

arXiv2016

Electroweak Baryogenesis with Lepton Flavor Violation

We investigate the feasibility of electroweak baryogenesis in a two-Higgs doublet model with lepton flavor violation. By scrutinizing the heavy Higgs boson mass spectrum, regions satisfying both strong first-order electroweak phase transition and the muon $g-2$ anomaly are identified. We also estimate the baryon number density by exploiting extra Yukawa couplings in the $μ$-$τ$ sector. It is found that a CP-violating source term can be enhanced by the $μ$-$τ$ flavor-violating coupling together with the extra $τ$ coupling. With $\mathcal{O}(1)$ Yukawa couplings and CP-violating phases, the observed baryon number density is marginally produced under a generous assumption for the bubble wall profile.

hep-ph↗

arXiv2016

750 GeV diphoton resonance in a visible heavy QCD axion model

In this paper, we revisit a visible heavy QCD axion model in light of the recent reports on the $750$ GeV diphoton resonance by the ATLAS and CMS experiments. In this model, the axion is made heavy with the help of the mirror copied sector of the Standard Model while the successful Peccei-Quinn mechanism is kept intact. We identify the $750$ GeV resonance as the scalar boson associated with spontaneous breaking of the Peccei-Quinn symmetry which mainly decays into a pair of the axions. We find that the mixings between the axion and $η$ and $η'$ play important roles in its decays and the resultant branching ratio into two photons. The axion decay length can be suitable for explaining the diphoton excess by the di-axion production when its decay constant $f_a \simeq 1$ TeV. We also find that our model allows multiple sets of the extra fermions without causing the domain wall problem, which is advantageous to explain the diphoton signal.

hep-ph↗

arXiv2016

Can the 750-GeV diphoton resonance be the singlet Higgs boson of custodial Higgs triplet model?

The observation of diphoton excess around the mass of 750 GeV in LHC Run-II motivates us to consider whether the singlet Higgs boson in the custodial Higgs triplet model can serve as a good candidate because an earlier study of comprehensive parameter scan shows that it can have the right mass in the viable mass spectra. By assuming the singlet Higgs mass at 750 GeV, its total width less than 50 GeV and imposing constraints from the LHC 8-TeV data, we identify an approximately linear region on the $(v_Δ, α)$ plane along which the exotic Higgs boson masses satisfy a specific hierarchy and have lower possible spectra, where $v_Δ$ denotes the triplet vacuum expectation value and $α$ is the mixing angle between the singlet Higgs boson and the standard model-like Higgs boson. Although the diphoton decay rate can be enhanced by charged Higgs bosons running in the loop in this region, it is mostly orders of magnitude smaller than that required for the observed production rate, except for the small $v_Δ$ region when the diphoton fusion production mechanism becomes dominant. Nonetheless, this part of parameter space suffers from the problems of breakdown of perturbativity and large uncertainties in the photon parton distribution function of proton.

hep-ph↗

arXiv2016

A global analysis of two-body D to VP decays within the framework of flavor symmetry

Two-body charmed meson decays $D\to VP$ are studied within the framework of the diagrammatic approach. Under flavor SU(3) symmetry, all the flavor amplitude sizes and their associated strong phases are extracted by performing a $χ^2$ fit. Thanks to the recent measurement of $D_s^+\toπ^+ρ^0$, the magnitudes and the strong phases of the $W$-annihilation amplitudes $A_{P,V}$ have been extracted for the first time. As a consequence, the branching fractions of all the $D\to VP$ decays are predicted, especially those modes that could not be predicted previously due to the unknown $A_{P,V}$. Our working assumption, the flavor SU(3) symmetry, is tested by comparing our predictions with experiment for the singly and doubly Cabibbo-suppressed decay modes based on the flavor amplitudes extracted from the Cabibbo-favored decays using the current data. The predictions for the doubly Cabibbo-suppressed channels are in good agreement with the data, while those for the singly Cabibbo-suppressed decay modes are seen to have flavor SU(3) symmetry breaking effects. We find that the inclusion of SU(3) symmetry breaking in color-allowed and color-suppressed tree amplitudes is needed in general in order to have a better agreement with experiment. Nevertheless, the exact flavor SU(3)-symmetric approach alone is adequate to provide an overall explanation for the current data.

hep-ph↗

arXiv2016

Phenomenology of the Georgi-Machacek model at future electron-positron colliders

We study the phenomenology of the exotic Higgs bosons in the Georgi-Machacek model at future electron-positron colliders such as the International Linear Collider (ILC), assuming the collision energies of 500 GeV and 1 TeV. We show that the existence of the neutral and singly-charged Higgs bosons in the 5-plet representation under the custodial $SU(2)_V$ symmetry can be readily identified by studying various energy and invariant mass distributions of the $W^+W^-Z$ final state. Moreover, their masses can be determined with sufficiently high precision to test the mass degeneracy, a feature due to the custodial symmetry of the model. A synergy between such searches at the ILC and the doubly-charged Higgs search at the LHC will make the 5-plet Higgs boson study more comprehensive.

hep-ph↗

arXiv2016

A $Z^\prime$ Model for $b\to s \ell\bar \ell$ Flavour Anomalies

We study the implications of flavour-changing neutral currents (FCNC's) in a model with the $SU(2)_l\times SU(2)_h\times U(1)_Y$ electroweak gauge symmetry for several anomalies appearing in $b\to s \ell\bar \ell$ induced $B$ decays in LHCb data. In this model, $SU(2)_l$ and $SU(2)_h$ govern the left-handed fermions in the first two generations and the third generation, respectively. The physical $Z$ and $Z'$ generate the $b\to s$ transition at tree level, leading to additional contributions to the $b \to s$ semileptonic operators ${\cal O}_{9,10}$. We find that although $B_s$-$\bar B_s$ mixing constrains the parameters severely, the model can produce values of ${\cal C}^{\rm NP}_{9,10}$ in the range determined by Descotes-Genon {\it et. al.} in Ref.~\cite{Descotes-Genon:2015uva} for this scenario to improve the global fit of observables in decays induced by the $b\to s μ\bar μ$ transition. The $Z'$ boson in this model also generates tree-level FCNC's for the leptonic interactions that can accommodate the experimental central value of $R_K = {\cal B}(B\to K μ\bar μ)/{\cal B}(B\to K e\bar e)=0.75$. In this case, the model predicts sizeable branching ratios for $B\to K e \bar τ$, $B\to K τ\bar e$, and an enhancement of $B\to K τ\bar τ$ with respect to its SM value.

hep-ph↗

arXiv2016

Searches of exotic Higgs bosons in general mass spectra of the Georgi-Machacek model at the LHC

We derive the most general sets of viable mass spectra of the exotic Higgs bosons in the Georgi-Machacek model that are consistent with the theoretical constraints of vacuum stability and perturbative unitarity and the experimental constraints of electroweak precision observables, $Zb \bar b$ coupling and Higgs boson signal strengths. Branching ratios of various cascade decay channels of the doubly-charged Higgs boson in the ${\bf 5}$ representation, the singly-charged Higgs boson in ${\bf 3}$, and the singlet Higgs boson are further computed. As one of the most promising channels for discovering the model, we study the prospects for detecting the doubly-charged Higgs boson that is produced via the vector boson fusion process and decays into final states containing a pair of same-sign leptons at the 14-TeV LHC and a 100-TeV future $pp$ collider. For this purpose, we evaluate acceptance times efficiency for signals of the doubly-charged Higgs boson with general viable mass spectra and compare it with the standard model background estimates.

hep-ph↗

arXiv2016

Revisiting Scalar Quark Hidden Sector in Light of 750-GeV Diphoton Resonance

In this short note, we revisit the model of a CP-even singlet scalar resonance proposed in arXiv:1507.02483, where the resonance appears as the lightest composite state made of scalar quarks participating in hidden strong dynamics. We show that the model can consistently explain the excess of diphoton events with an invariant mass around 750GeV reported by both the ATLAS and CMS experiments. We also discuss the nature of the charged composite states in the TeV range which accompany to the neutral scalar. Due to inseparability of the dynamical scale and the mass of the resonance, the model also predicts signatures associated with the hidden dynamics such as leptons, jets along with multiple photons at future collider experiments. We also associate the TeV-scale dynamics behind the resonance with an explanation of dark matter.

hep-ph↗

arXiv2016

Diboson Resonance as a Portal to Hidden Strong Dynamics

We propose a new explanation for excess events observed in the search for a high-mass resonance decaying into dibosons by the ATLAS experiment. The resonance is identified as a composite spin-$0$ particle that couples to the Standard Model gauge bosons via dimension-5 operators. The excess events can be explained if the dimension-5 operators are suppressed by a mass scale of ${\cal O}(1$-$10$) TeV. We also construct a model of hidden strong gauge dynamics which realizes the spin-$0$ particle as its lightest composite state, with appropriate couplings to Standard Model gauge bosons.

hep-ph↗

arXiv2015

Revisiting R-invariant Direct Gauge Mediation

We revisit a special model of gauge mediated supersymmetry breaking, the "R-invariant direct gauge mediation." We pay particular attention to whether the model is consistent with the minimal model of the μ-term, i.e., a simple mass term of the Higgs doublets in the superpotential. Although the incompatibility is highlighted in view of the current experimental constraints on the superparticle masses and the observed Higgs boson mass, the minimal μ-term can be consistent with the R-invariant gauge mediation model via a careful choice of model parameters. We derive an upper limit on the gluino mass from the observed Higgs boson mass. We also discuss whether the model can explain the 3σexcess of the Z+jets+$E_T^{\rm miss}$ events reported by the ATLAS Collaboration.

hep-ph↗

arXiv2015

Electroweak interacting dark matter with a singlet scalar portal

We investigate an electroweak interacting dark matter (DM) model in which the DM is the neutral component of the SU$(2)_L$ triplet fermion that couples to the standard model (SM) Higgs sector via an SM singlet Higgs boson. In this setup, the DM can have a CP-violating coupling to the singlet Higgs boson at the renormalizable level. As long as the nonzero Higgs portal coupling (singlet-doublet Higgs boson mixing) exists, we can probe CP violation of the DM via the electric dipole moment of the electron. Assuming the $\mathcal{O}(1)$ CP-violating phase in magnitude, we investigate the relationship between the electron EDM and the singlet-like Higgs boson mass and coupling. It is found that for moderate values of the Higgs portal couplings, current experimental EDM bound is not able to exclude the wide parameter space due to a cancellation mechanism at work. We also study the spin-independent cross section of the DM in this model. It is found that although a similar cancellation mechanism may diminish the leading-order correction, as often occurs in the ordinary Higgs portal DM scenarios, the residual higher-order effects leave an $\mathcal{O}(10^{-47})~{\rm cm}^2$ correction in the cancellation region. It is shown that our benchmark scenarios would be fully tested by combining all future experiments of the electron EDM, DM direct detection and Higgs physics.

hep-ph↗

arXiv2015

Standard Model Effective Field Theory: Integrating out a Generic Scalar

We consider renormalisable models extended in the scalar sector by a generic scalar field in addition to the standard model Higgs boson field, and work out the effective theory for the latter in the decoupling limit. We match the full theory onto the effective theory at tree and one-loop levels, and concentrate on dimension-6 operators of the Higgs and electroweak gauge fields induced from such matching. The Wilson coefficients of these dimension-6 operators from tree-level matching are further improved by renormalisation group running. For specific $SU(2)_L$ representations of the scalar field, some "accidental" couplings with the Higgs field are allowed and can lead to dimension-6 operators at tree and/or one-loop level. Otherwise, two types of interaction terms are identified to have only one-loop contributions, for the Wilson coefficients of which we have obtained a general formula. Using the obtained results, we analyse constraints from electroweak oblique parameters and the Higgs data on several phenomenological models.

hep-ph↗

arXiv2015

Flavor-Changing Neutral-Current Decays in Top-Specific Variant Axion Model

The invisible variant axion model is very attractive as it is free from the domain wall problem. This model requires two Higgs doublets at the electroweak scale where one Higgs doublet carries a nonzero Peccei-Quinn (PQ) charge and the other is neutral under the PQ $\text U(1)$ symmetry. We consider the most interesting and less constrained scenario of the variant axion model, where only the right-handed top quark is charged under the PQ symmetry and couples with the PQ-charged Higgs doublet. As a result, the top quark can decay to the observed standard-model-like Higgs boson $h$ and the charm or up quark, $t\to h~ c/u$, which is testable soon at the LHC Run-II. Moreover, we propose a method to probe the chiral nature of the Higgs flavor-changing interaction using the angular distribution of $t \to ch$ decays if a sufficient number of such events are observed. We also show that our model has the capacity to explain the $h\toτμ$ decay reported by the CMS Collaboration, if the right-handed tau lepton also carries a PQ charge and couples to the PQ-charged Higgs boson.

hep-ph↗

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.

hep-ph↗

arXiv2015

Constraints on the Georgi-Machacek Model by Current LHC Data

In the Georgi-Machacek model with a custodial symmetry in the Higgs potential and vacuum alignment, the triplet vacuum expectation value is allowed to be of ${\cal O}(10)$ GeV, which leads to the possibility of significant modifications in the couplings of the SM-like Higgs bosons $h$ with other SM particles. In this talk given at the HPNP2015 conference held at Toyama University, we review constraints on the model based on the latest LHC data of the SM-like Higgs boson, like-sign $W$ boson events, and searches for additional neutral Higgs bosons. In particular, we concentrate on the parameter space for small mixing angle $α$ between the two custodial singlets. It is pointed out that constraints from the non-SM custodial singlet are most constraining and those from the 5-plet are independent of $α$. While currently there is no constraint from the 3-plet, we show that its $f \bar f$ and $γγ$ channels through the gluon fusion production can be very promising for searches or constraining in the mass range between 160 GeV and 350 GeV because of its gauge-phobic property.

hep-ph↗

arXiv2015

Leptophobic $Z'$ in Models with Multiple Higgs Doublet Fields

We study the collider phenomenology of the leptophobic $Z'$ boson from an extra $U(1)'$ gauge symmetry in models with $N$-Higgs doublet fields. We assume that the $Z'$ boson at tree level has (i) no $Z$-$Z'$ mixing, (ii) no interaction with charged leptons, and (iii) no flavour-changing neutral current. Under such a setup, it is shown that in the $N=1$ case, all the $U(1)'$ charges of left-handed quark doublets and right-handed up- and down-type quarks are required to be the same, while in the $N \ge 3$ case one can take different charges for the three types of quarks. The $N=2$ case is not well-defined under the above three requirements. We study the $pp\to Z'V\to b\bar{b}V$ processes ($V=γ,~Z$ and $W^\pm$) with the leptonic decays of $Z$ and $W^\pm$ at the LHC. The most promising discovery channel or the most stringent constraint on the $U(1)'$ gauge coupling constant comes from the $Z'γ$ process below the $t\bar{t}$ threshold and from the $t\bar{t}$ process above the threshold. Assuming the collision energy of 8 TeV and the integrated luminosity of 19.6 fb$^{-1}$, we find that the constraint from the $Z'γ$ search in the lower mass regime can be stronger than that from the UA2 experiment. In the $N \ge 3$ case, we consider four benchmark points for the $Z'$ couplings with quarks. It is noted that if such a $Z'$ is discovered, a careful comparison between the $Z'γ$ and $Z'W$ signals is crucial to reveal the nature of $Z'$ couplings with quarks. We also present the discovery reach of the $Z'$ boson at the 14-TeV LHC in both $N=1$ and $N\geq 3$ cases.

hep-ph↗

arXiv2015

Properties and searches of the exotic neutral Higgs bosons in the Georgi-Machacek model

The Georgi-Machacek model predicts the existence of four neutral Higgs bosons, one of which can be identified as the 125-GeV Higgs boson. The latest Higgs data favor the parameter space of small mixing angle alpha between the two custodial singlets of the model. The other two neutral Higgs bosons belong respectively to the custodial triplet and quintet. We study the general decay and production properties of these particles in the small-alpha scenario. Constraints on the SU(2)_L triplet vacuum expectation value are obtained as a function of the exotic Higgs boson masses using latest ATLAS data of various search channels for additional neutral Higgs bosons.

hep-ph↗