arXiv subjects

Kingman Cheung

Publications and source records attributed to Kingman Cheung.

At least 91 records · Page 5Linked to original sources

arXiv2017

Entangling Higgs production associated with a single top and a top-quark pair in the presence of anomalous top-Yukawa coupling

The ATLAS and CMS collaborations observed a mild excess in the associated Higgs production with a top-quark pair ($t\bar t h$) and reported the signal strengths of $ μ_{tth}^{\rm ATLAS}=1.81\pm 0.80$ and $μ_{tth}^{\rm CMS}=2.75\pm 0.99 $ based on the data collected at $\sqrt{s}$= 7 and 8 TeV. Although, at the current stage, there is no obvious indication whether the excess is real or due to statistical fluctuations, here we perform a case study of this mild excess by exploiting the strong entanglement between the associated Higgs production with a single top quark ($thX$) and $t\bar t h$ production in the presence of anomalous top-Yukawa coupling. As well known, $t\bar t h$ production only depends on the absolute value of the top-Yukawa coupling. Meanwhile, in $thX$ production, this degeneracy is lifted through the strong interference between the two main contributions which are proportional to the top-Yukawa and the gauge-Higgs couplings, respectively. Especially, when the relative sign of the top-Yukawa coupling with respect to the gauge-Higgs coupling is reversed, the $thX$ cross section can be enhanced by more than one order of magnitude. We perform a detailed study of the influence of $thX$ production on $t\bar{t}h $ production in the presence of the anomalous top-Yukawa coupling and point out that it is crucial to include $thX$ production in the analysis of the $t\bar t h$ data to pin down the sign and the size of the top-Yukawa coupling in future. While assuming the Standard Model (SM) value for the gauge-Higgs coupling, we vary the top-Yukawa coupling within the range allowed by the current LHC Higgs data. We consider the Higgs decay modes into multileptons, $b\bar b$ and $γγ$ putting a particular emphasis on the same sign dilepton events. We also discuss ...

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arXiv2017

A Three-loop Neutrino Model with Leptoquark Triplet Scalars

We propose a three-loop neutrino mass model with a few leptoquark scalars in $SU(2)_L$-triplet form, through which we can explain the anomaly of $B \to K^{(*)} μ^+ μ^-$, a sizable muon $g-2$ and a bosonic dark matter candidate, and at the same time satisfying all the constraints from lepton flavor violations. We perform global numerical analyses and show the allowed regions, in which we find somewhat restricted parameter space, such as the mass of dark matter candidate and various components of the Yukawa couplings in the model.

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arXiv2017

Accommodation of the Dirac Phase in the Krauss-Nasri-Trodden Model

We investigate one of the radiative models, Kraus-Nasri-Trodden model, with the maximal value of Dirac CP violating phase, $δ$ is -π/2 (or equivalently 3π/2), which is preferred in not only recent long baseline experiments but also the global fit of neutrino oscillation data. We show that our predicted region of the μ-e conversion rate can be searched in the future experiments without conflicting lepton-flavor violation and dark matter constraints.

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arXiv2017

Detecting multimuon jets from the Higgs exotic decays in the Higgs portal framework

A muon jet ($μ$-jet) is a very special feature that consists of a cluster of collimated muons from the decay of a fast moving light particle of mass about $\mathcal{O}$(1 GeV). We will use this feature to search for very light particles from rare decays of the Higgs boson. For such a small angular separation of muons which might come from a long-lived particle, both ATLAS and CMS could have the displaced-vertexing-reconstruction capability. We use two simple models of the Higgs-portal type to explore the possibilities of event topologies with two $2μ$-jets, one $2μ$-jet $\&$ one $4μ$-jet, and two $4μ$-jets in the final state at LHC-14. We also summarize existing constraints on these models.

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arXiv2017

A Three-loop Neutrino Mass Model with a Colored Triplet Scalar

We study a variation of the Krauss-Nasri-Trodden (KNT) model with a colored triplet scalar field and a colored singlet scalar field, in which we discuss the anomaly coming from $b\to sμ\barμ$, fitting to the muon anomalous magnetic moment and the relic density of the Majorana-type dark matter candidate, as well as satisfying various constraints such as lepton-flavor violations and flavor-changing neutral currents. Also, we discuss the direct constraints from the collider searches and the possibilities of detecting the new fields at the LHC.

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arXiv2016

A testable radiative neutrino mass model without additional symmetries and Explanation for the $b \to s \ell^+ \ell^-$ anomaly

We propose a one-loop radiative Majorana-type neutrino-mass matrix without any kind of additional symmetries by introducing two leptoquark-like bosons only. In this scenario, we show that the anomaly appearing in the process $b\to s\ell\bar\ell$ can be explained without any conflicts against various constraints such as lepton-flavor violations, flavor-changing neutral currents, oblique parameters $ΔS,\ ΔT$, and the Drell-Yan process. We make the predictions for the flavor-violating lepton-pair production ($eμ$, $eτ$, and $μτ$) at the LHC, as well as the cross sections for pair production of these leptoquark-like bosons.

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arXiv2016

Double Higgcision: 125 GeV Higgs boson and a potential diphoton Resonance

Searches for diphoton resonance have been shown to be very useful in discovering new heavy spin-0 or spin-2 particles. Supposing that a new heavy particle shows up in the diphoton channel and it points to a spin-0 boson, it can be allowed to have a small mixing with the observed 125 GeV Higgs-like boson. We borrow the example of the 750 GeV particles hinted with 3.2 fb$^{-1}$ data at the end of 2015 (though it did not appear in the 2016 data) to perform an analysis of "double Higgcision". In this work, we perform a complete Higgs-signal strength analysis in the Higgs-portal type framework, using all the existing 125 GeV Higgs boson data as well as the diphoton signal strength of the 750 GeV scalar boson. The best fit prefers a very tiny mixing between two scalar bosons, which has to be accommodated in models for the 750 GeV scalar boson.

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arXiv2016

Higgs precision study of the 750 GeV diphoton resonance and the 125 GeV standard model Higgs boson with Higgs-singlet mixing

We interpret the potential observation of the 750 GeV di-photon resonance at the LHC in models, in which an $SU(2)$ isospin-singlet scalar boson mixes with the Standard Model (SM) Higgs boson through an angle $α$. Allowing the singlet scalar boson to have renormalizable couplings to vector-like leptons and quarks and introducing sizable decay width of the 750 GeV di-photon resonance into non-SM particles such as dark matters, we can explain the large production cross section $σ(H_2) \times B(H_2 \to γγ)$ as well as the apparent large total width of the boson without conflicts from the results obtained by previous global fits to the SM Higgs boson data.

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arXiv2016

Enhanced Charged Higgs Production through W-Higgs Fusion in W-b Scattering

We study the associated production of a charged Higgs boson with a bottom quark and a light quark at the LHC via p p \to H^\pm\,b\,j in the Two Higgs Doublet Models (2HDMs). Using the effective W approximation, we show that there is exact cancellation among various Feynman diagrams in high energy limit. This may imply that the production of charged Higgs can be significantly enhanced in the presence of large mass differences among the neutral Higgs bosons via W^\pm-Higgs fusion in the p p \to H^\pm\,b\,j process. Particularly, we emphasize the potential enhancement due to a light pseudoscalar boson $A$, which is still allowed by the current data by which we explicitly calculate the allowed regions in (M_A,\,\tanβ) plane, and show that the production cross section can be as large as 0.1 pb for large $\tanβ$. We also show that the transverse momentum distribution of the b quark can potentially distinguish the W^\pm-A fusion diagram from the top diagram. Finally, we point out further enhancement when we go beyond the 2HDMs.

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arXiv2016

Resurgence of $Z'$ from the single electron-muon event at ATLAS

Inspired by the recent single $e^{\pm}μ^{\mp}$ event at 2.1 TeV invariant mass from the ATLAS at $\sqrt{s}=13$ TeV with 3.2 fb$^{-1}$ luminosity, we propose an explanation using a $Z'$ gauge boson, which possesses lepton-flavor-changing neutral currents originated from non-universal couplings to charged leptons. We assume that the left-handed charged-lepton mixing matrix equals to the PMNS matrix and no mixing in the neutrino sector to make this phenomenological $Z'$ model more predictive. There are indeed some parameter regions, where the $Z'$ can generate a large enough $e^{\pm}μ^{\mp}$ production cross section, while at the same time satisfies various observables from lepton-flavor violation and other constraints from the LHC.

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arXiv2016

Interpreting the 750 GeV Di-photon Resonance using photon-jets in Hidden-Valley-like models

Motivated by the di-photon resonance recently reported by the ATLAS and CMS collaborations at $\sqrt{s} =13$ TeV, we interpret the resonance as a scalar boson $X (750)$ in hidden-valley-like models. The scalar boson $X$ can mix with the standard model Higgs boson and thus can be produced via gluon fusion. It then decays into a pair of very light hidden particles $Y$ of sub-GeV, each of which in turn decays to a pair of collimated $ π^{0} $'s, and these two $ π^{0} $'s decay into photons which then form photon-jets. A photon-jet ($ γ$-jet) is a special feature that consists of a cluster of collinear photons from the decay of a fast moving light particle (sub-GeV). Because these photons inside the photon-jet are so collimated that it cannot be distinguished from a single photon, and so in the final state of the decay of $X(750)$ a pair of photon-jets look like a pair of single photons, which the experimentalists observed and formed the 750 GeV di-photon resonance. Prospects for the LHC Run-2 about other new and testable features are also discussed.

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arXiv2016

Dark Matter with multi-annihilation channels and AMS-02 positron excess and antiproton

AMS-02 provided the unprecedented statistics in the measurement of the positron fraction from cosmic rays. That may offer a unique opportunity to distinguish the positron spectrum coming from various dark matter (DM) annihilation channels, if DM is the source of this positron excess. Therefore, we consider the scenario that the DM can annihilate into leptonic, quark, and massive gauge boson channels simultaneously with floating branching ratios to test this hypothesis. We also study the impacts from MAX, MED, MIN, and DC diffusion models as well as from isothermal, NFW, and Einasto DM density profiles on our results. We found two parameter regions that can satisfy both AMS-02 $\frac{e^+}{e^++e^-}$ and $\bar{p}/p$ datasets at 95\% CL. i) Under the NFW-MIN combination with $M_χ\subset[10,30]$ TeV. ii) Under the Einasto-DC combination with $M_χ\subset[500,1500]$ GeV.

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arXiv2015

Interpretations of the ATLAS Diboson Anomaly

Recently, the ATLAS Collaboration recorded an interesting anomaly in diboson production with excesses at the diboson invariant mass around 2 TeV in boosted jets of all the $WZ$, $W^+ W^-$, and $ZZ$ channels. We offer a theoretical interpretation of the anomaly using a phenomenological right-handed model with extra $W'$ and $Z'$ bosons. Constraints from narrow total decay widths, dijet cross sections, and $W/Z + H$ production are taken into account. We also comment on a few other possibilities.

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arXiv2015

Bounds on Higgs-Portal models from the LHC Higgs data

In a number of Higgs-portal models, an $SU(2)$ isospin-singlet scalar boson generically appears at the electroweak scale and can mix with the Standard Model (SM) Higgs boson with a mixing angle $α$. This singlet scalar boson can have renormalizable couplings to a pair of dark matter particles, vectorlike leptons or quarks, or new gauge bosons, thereby modifying the Higgs signal strengths in a nontrivial way. In this work, we perform global fits to such models using the most updated LHC Higgs-boson data and discuss the corresponding implications on Higgs-portal-type models. In particular we find that the current LHC Higgs-boson data slightly favors the SM over the Higgs-portal singlet-scalar models, which has to be further examined using the upcoming LHC Higgs-boson data. Finally the Higgs-portal models are constrained as follows: $\cosα\gtrsim 0.86$ and $ΔΓ_{\rm tot} \lesssim 1.9$ MeV at 95 \% CL.

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arXiv2015

Higgs data constraints on the minimal supersymmetric standard model

We perform global fits to the most recent data (after summer 2014) on Higgs boson signal strengths in the framework of the minimal supersymmetric standard model (MSSM). We further impose the existing limits on the masses of charginos, staus, stops and sbottoms together with the current Higgs mass constraint $|M_{H_1} - 125.5\,{\rm GeV}| < 6$ GeV. The heavy supersymmetric (SUSY) particles such as squarks enter into the loop factors of the $Hgg$ and $Hγγ$ vertices while other SUSY particles such as sleptons and charginos also enter into that of the $Hγγ$ vertex. We also take into account the possibility of other light particles such as other Higgs bosons and neutralinos, such that the 125.5 GeV Higgs boson can decay into. We use the data from the ATLAS, CMS, and the Tevatron, with existing limits on SUSY particles, to constrain on the relevant SUSY parameters. We obtain allowed regions in the SUSY parameter space of squark, slepton and chargino masses, and the $μ$ parameter. We find that $|ΔS^γ/S^γ_{\rm SM}|\lsim 0.1$ at $68\%$ confidence level when $M_{\tildeχ^{\pm}_1}>300$ GeV and $M_{\tildeτ_1}>300$ GeV, irrespective of the squarks masses. Furthermore, $|ΔS^γ/S^γ_{\rm SM}|\lsim 0.03 $ when $M_{\tildeχ^\pm_1,{\tildeτ_1}} > 500$ GeV and $M_{{\tilde t}_1,{\tilde b}_1} \gsim 600$ GeV.

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arXiv2015

Leptoquark induced rare decay amplitudes $h \to τ^\mp μ^\pm$ and $τ\to μγ$

Rare decay modes of the newly discovered standard-model-like Higgs boson $h$ may test the flavor changing couplings in the leptoquark sector through the process $h \to τ^\mpμ^\pm$. Motived by the recently reported excess in LHC data from the CMS detector, we found that a predicted branching fraction Br($h \to τ^\mpμ^\pm$) at the level of 1\% is possible even though the coupling parameters are subjected to the stringent constraint from the null observation of $τ\toμγ$, where the destructive cancellation among amplitudes is achievable by fine tuning.

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arXiv2015

An Exploratory study of Higgs-boson pair production

Higgs-boson pair production is well known being capable to probe the trilinear self-coupling of the Higgs boson, which is one of the important ingredients of the Higgs sector itself. Pair production then depends on the top-quark Yukawa coupling $g_t^{S,P}$, Higgs trilinear coupling $λ_{3H}$, and a possible dim-5 contact-type $ttHH$ coupling $g_{tt}^{S,P}$, which may appear in some higher representations of the Higgs sector. We take into account the possibility that the top-Yukawa and the $ttHH$ couplings involved can be CP violating. We calculate the cross sections and the interference terms as coefficients of the square or the 4th power of each coupling $(g_t^{S,P}, λ_{3H}, g_{tt}^{S,P})$ at various stages of cuts, such that the desired cross section under various cuts can be obtained by simply inputing the couplings. We employ the $H H \to γγb \bar b$ decay mode of the Higgs-boson pair to investigate the possibility of disentangle the triangle diagram from the box digram so as to have a clean probe of the trilinear coupling at the LHC. We found that the angular separation between the $b$ and $\bar b$ and that between the two photons is useful. We obtain the sensitivity reach of each pair of couplings at the 14 TeV LHC and the future 100 TeV pp machine. Finally, we also comment on using the $b\bar b τ^+ τ^-$ decay mode in Appendix.

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arXiv2015

Non-abelian Dark Matter Solutions for Galactic Gamma-ray Excess and Perseus 3.5 keV X-ray Line

We attempt to explain simultaneously the Galactic center gamma-ray excess and the 3.5 keV X-ray line from the Perseus cluster based on a class of non-abelian $SU(2)$ DM models, in which the dark matter and an excited state comprise a "dark" $SU(2)$ doublet. The non-abelian group kinetically mixes with the standard model gauge group via dimensions-5 operators. The dark matter particles annihilate into standard model fermions, followed by fragmentation and bremsstrahlung, and thus producing a continuous spectrum of gamma-rays. On the other hand, the dark matter particles can annihilate into a pair of excited states, each of which decays back into the dark matter particle and an X-ray photon, which has an energy equal to the mass difference between the dark matter and the excited state, which is set to be 3.5 keV. The large hierarchy between the required X-ray and $γ$-ray annihilation cross-sections can be achieved by a very small kinetic mixing between the SM and dark sector, which effectively suppresses the annihilation into the standard model fermions but not into the excited state.

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