arXiv subjects

Kingman Cheung

Publications and source records attributed to Kingman Cheung.

At least 73 records · Page 4Linked to original sources

arXiv2019

Vector-like Quark Interpretation of Excess in Higgs Signal Strength

There is a $+2σ$ deviation in the average Higgs-signal strength for all the $7+8+13$ TeV data up to Summer 2018. We find that a slight reduction of the bottom-Yukawa coupling can fit the data better than the standard model. We suggest an extension with a vector-like quark doublet, of which the right-handed component of $b'$ mixes nonnegligibly with the standard model $b$ quark. We show that the mixing would induce a reduction of the bottom Yukawa coupling. Simultaneously, the coupling of the $Z$ boson to the right-handed $b$ quark increases, which could reduce the forward-backward asymmetry of bottom production at LEP and bring it closer to the experimental value.

hep-ph↗

arXiv2019

Phenomenology of A Little Higgs Pseudo-Axion

In models where the Higgs is realized as a pseudo-Nambu-Goldstone boson (pNGB) of some global symmetry breaking, there are often remaining pNGBs of some $U(1)$ groups (called `pseudo-axions'), which could lead to smoking gun signatures of such scenarios and provide important clues on the electroweak symmetry breaking mechanism. As a concrete example, we investigate the phenomenology of the pseudo-axion in the anomaly-free Simplest Little Higgs (SLH) model. After clarifying a subtle issue related to the effect of symmetric vector-scalar-scalar (VSS) vertices (e.g. $Z_μ(H\partial^μη+η\partial^μH)$), we show that for natural region in the parameter space, the SLH pseudo-axion is top-philic, decaying almost exclusively to a pair of top quarks. The direct and indirect (i.e. via heavy particle decay) production of such a pseudo-axion at the $14\,\mbox{TeV}$ (HL-)LHC turn out to suffer from either large backgrounds or small rates, making its detection quite challenging. A $pp$ collider with higher energy and luminosity, such as the $27\,\mbox{TeV}$ HE-LHC, or even the $100\,\mbox{TeV}$ FCC-hh or SppC, is therefore motivated to capture the trace of such a pNGB.

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arXiv2018

Decaying warm dark matter and structure formation

We examine the cosmology of warm dark matter (WDM), both stable and decaying, from the point of view of structure formation. We compare the matter power spectrum associated to WDM masses of 1.5 keV and 0.158 keV, with that expected for the stable cold dark matter $Λ$CDM$\equiv$SCDM paradigm, taken as our reference model. We scrutinize the effects associated to the warm nature of dark matter, as well as the fact that it decays. The decaying warm dark matter (DWDM) scenario is well-motivated, emerging in a broad class of particle physics theories where neutrino masses arise from the spontaneous breaking of a continuous global lepton number symmetry. The majoron arises as a Nambu-Goldstone boson, and picks up a mass from gravitational effects, that explicitly violate global symmetries. The majoron necessarily decays to neutrinos, with an amplitude proportional to their tiny mass, which typically gives it cosmologically long lifetimes. Using N-body simulations we show that our DWDM picture leads to a viable alternative to the $Λ$CDM scenario, with predictions that can differ substantially on small scales.

astro-ph.CO↗

arXiv2018

The impact of EDGES 21-cm data on dark matter interactions

The recently announced results on the 21-cm absorption spectrum by the EDGES experiment can place very stringent limits on dark matter annihilation cross-sections. We properly take into account the heating energy released from dark matter annihilation from the radiation epoch to the 21-cm observation redshifts in the radiative transfer to compute the evolution of the gas temperature. Our results show that the global 21-cm absorption profile is a powerful cosmological probe of the dark matter interactions. For dark matter annihilating into electron-positron pairs, the EDGES results give a more stringent upper limit than the PLANCK result on the annihilation cross section at the lower dark matter mass region.

astro-ph.CO↗

arXiv2018

Sneutrino Dark Matter via pseudoscalar X-funnel meets Inverse Seesaw

In this paper we study sneutrino dark matter in a recently proposed supersymmetric electroweak-scale inverse seesaw model, in which the majority of the sneutrino dark matter particle is a mixture of the right-handed sneutrino $\tilde{N}^c$ and the singlet field $\tilde{S}$. The scalar field $X$ responsible for the generation of neutrino masses can simultaneously play a crucial role for sneutrino annihilation in the early Universe via the pseudoscalar mediator $A_X$ into neutrinos. We focus here on the dominant annihilation channels and provide all the formulas together with analytic estimates in order to identify the relevant parameters. Furthermore, we show that the direct detection scattering cross section is many orders of magnitude below the current limits, and estimate the indirect detection annihilation rate, which is only a few orders of magnitude below the current limits.

hep-ph↗

arXiv2018

Simplest Little Higgs Revisited: Hidden Mass Relation, Unitarity and Naturalness

We analyze the scalar potential of the Simplest Little Higgs (SLH) model in an approach consistent with the spirit of continuum effective field theory (CEFT). By requiring correct electroweak symmetry breaking (EWSB) with the $125\,\text{GeV}$ Higgs boson, we are able to derive a relation between the pseudo-axion mass $m_η$ and the heavy top mass $m_T$, which serves as a crucial test of the SLH mechanism. By requiring $m_η^2>0$ an upper bound on $m_T$ can be obtained for any fixed SLH global symmetry breaking scale $f$. We also point out that an absolute upper bound on $f$ can be obtained by imposing partial wave unitarity constraint, which in turn leads to absolute upper bounds of $m_T\lesssim 19\,\text{TeV}, m_η\lesssim 1.5\,\text{TeV}$ and $m_{Z'}\lesssim 48\,\text{TeV}$. We present the allowed region in the three-dimensional parameter space characterized by $f,t_β,m_T$, taking into account the requirement of valid EWSB and the constraint from perturbative unitarity. We also propose a strategy of analyzing the fine-tuning problem consistent with the spirit of CEFT and apply it to the SLH. We suggest that the scalar potential and fine-tuning analysis strategies adopted here should also be applicable to a wide class of Little Higgs and Twin Higgs models, which may reveal interesting relations as crucial tests of the related EWSB mechanism and provide a new perspective on assessing their degree of fine-tuning.

hep-ph↗

arXiv2018

The Importance of Quantum Pressure of Fuzzy Dark Matter on Lyman-Alpha Forest

With recent Lyman-alpha forest data from BOSS and XQ-100, some studies suggested that the lower mass limit on the fuzzy dark matter (FDM) particles is lifted up to $10^{-21}\,\mathrm{eV}$. However, such a limit was obtained by $Λ$CDM simulations with the FDM initial condition and the quantum pressure (QP) was not taken into account which could have generated non-trivial effects in large scales structures. We investigate the QP effects in cosmological simulations systematically, and find that the QP leads to further suppression of the matter power spectrum at small scales, as well as the halo mass function in the low mass end. We estimate the suppressing effect of QP in the 1D flux power spectrum of Lyman-alpha forest and compare it with data from BOSS and XQ-100. The rough uncertainties of thermal gas properties in the flux power spectrum model calculation were discussed. We conclude that more systematic studies, especially with QP taken into account, are necessary to constrain FDM particle mass using Lyman-alpha forest.

astro-ph.CO↗

arXiv2018

Vector Boson Fusion versus Gluon Fusion

Vector-boson fusion (VBF) is a clean probe of the electroweak-symmetry breaking (EWSB), which inevitably suffers from some level of contamination due to the gluon fusion (ggF). In addition to the jet variables used in the current experimental analysis, we analyze a few more jet-shape variables defined by the girth and integrated jet-shape. Taking $H\to W W^* \to e νμν$ and $H \to γγ$ as examples, we perform the analysis with a new technique of 2-step boosted-decision-tree method, which significantly reduces the contamination of the ggF in the VBF sample, thus, providing a clean environment in probing the EWSB sector.

hep-ph↗

arXiv2018

Measuring properties of a Heavy Higgs boson in the $H\to t\bar t \to bW^+\bar b W^-$ decay

Suppose a heavy neutral Higgs or scalar boson $H$ is discovered at the LHC, it is important to investigate its couplings to the standard model particles as much as possible. Here in this work we attempt to probe the CP-even and CP-odd couplings of the heavy Higgs boson to a pair of top quarks, through the decay $H \to t\bar t \to b W^+ \bar b W^-$. We use the helicity-amplitude method to write down the most general form for the angular distributions of the final-state $b$ quarks and $W$ bosons. We figure out that there are 6 types of angular observables and, under CP$\widetilde{\rm T}$ conservation, one-dimensional angular distributions can only reveal two of them. Nevertheless, the $H$ couplings to the $t\bar t$ pair can be fully determined by exploiting the one-dimensional angular distributions. A Higgs-boson mass of 380 GeV not too far above the $t\bar t$ threshold is illustrated with full details. With a total of $10^4$ events of $H \to t\bar t \to bW^+ \bar b W^+$, one can determine the couplings up to 10-20 % uncertainties.

hep-ph↗

arXiv2018

$CP$ violating mode of the stoponium decay into $Zh$

We show that a novel decay mode $Zh$ of the bound state of stop-anti-stop pair in the ground state $^1S_0(\widetilde t_1\widetilde t_1^*)$ may have a significant branching ratio if the $CP$ violating mixing appears in the stop sector, even after we apply the stringent constraint from the measurement of the electric dipole moment (EDM) of the electron. We show that the branching ratio can be as large as 10% in some parameter space that it may be detectable at the LHC.

hep-ph↗

arXiv2018

A generalized one-loop neutrino mass model with charged particles

We propose a radiative neutrino-mass model by introducing 3 generations of fermion pairs $E^{-(N+1)/2} E^{+(N+1)/2}$ and a couple of multi-charged bosonic doublet fields $Φ_{N/2}, Φ_{N/2+1}$, where $N=1,3,5,7,9$. We show that the models can satisfy the neutrino masses and oscillation data, and are consistent with lepton-flavor violations, the muon anomalous magnetic moment, the oblique parameters, and the beta function of the $U(1)_Y$ hypercharge gauge coupling. We also discuss the collider signals for various $N$, namely, multi-charged leptons in the final state from the Drell-Yan production of $E^{-(N+1)/2} E^{+(N+1)/2}$. In general, the larger the $N$ the more charged leptons will appear in the final state.

hep-ph↗

arXiv2018

Neutrinophilic two Higgs doublet model with $U(1)$ global symmetry

We propose a neutrinophilic two-Higgs-doublet model, where the vacuum expectation value (VEV) of the second Higgs doublet is only induced at one-loop level via several neutral fermions. Thus, the masses of active neutrinos arising from the Higgs doublets are naturally small via such a tiny VEV. We discuss various phenomenology of the model, including the neutrino masses and oscillations, bounds on non-unitarity, lepton-flavor violations, the oblique parameters, the muon anomalous magnetic moment, the GeV-scale sterile neutrino candidate arising from the tiny VEV, and collider signatures. We finally discuss the possibility of detecting the sterile neutrino suggested in the experiment of Future Circular Collider (FCC).

hep-ph↗

arXiv2018

Ultra-Light Axion Dark Matter and its impacts on dark halo structure in $N$-body simulation

The Ultra-Light Axion (ULA) is a dark matter candidate with mass $\mathcal{O}(10^{-22})$eV and de Broglie wavelength of order kpc. Such an axion, also called the Fuzzy Dark Matter (FDM), thermalizes via the gravitational force and forms a Bose-Einstein condensate. Recent studies suggested that the quantum pressure from the FDM can significantly affect the structure formation in small scales, thus alleviating the so-called "small-scale crisis". In this paper, we develop a new technique to discretize the quantum pressure and illustrate the interactions among FDM particles in the $N$-body simulation, which accurately simulates the formation of the dark-matter halo and its inner structure in the region outside the softening length. In a self-gravitationally-bound virialized halo, we find a constant density, solitonic core, which is consistent with the theoretical prediction. The existence of the solitonic core reveals the non-linear effect of quantum pressure and impacts the structure formation in the FDM model.

astro-ph.CO↗

arXiv2017

Measuring properties of a Heavy Higgs boson in the $H\to ZZ \to 4\ell$ decay

In many extensions of the standard model, there exist a few extra Higgs bosons. Suppose a heavy neutral Higgs boson H is discovered at the LHC, one could then investigate CP and CPT~ properties of its couplings to a pair of $Z$ bosons through $H \to ZZ \to 4\ell$. We use the helicity-amplitude method to write down the most general form for the angular distributions of the four final-state leptons, which can cover the case of CP-even, -odd, and -mixed state for the Higgs boson. We figure out there are 9 types of angular observables and all the $H$ couplings to $Z$ bosons can be fully determined by exploiting them. A Higgs-boson mass of 260 GeV below the $t\bar t$ threshold is illustrated with full details. With a total of $10^3$ events of $H \to ZZ \to 4\ell$, one can determine the couplings up to 12-20\% uncertainties.

hep-ph↗

arXiv2017

A supersymmetric electroweak scale seesaw model

In this paper we propose a novel supersymmetric inverse seesaw model which has only one additional $Z_6$ symmetry. The field content is minimal to get a viable neutrino spectrum at tree-level. Interestingly, the inverse seesaw scale in our model is related to the scale of electroweak symmetry breaking. Due to that origin we are less biased about hierarchies and discuss three different types of the inverse seesaw mechanism with different phenomenologies. We can successfully reproduce neutrino masses and mixing and our model is consistent with current bounds on neutrinoless double beta decay, non-unitarity of the PMNS matrix and charged lepton flavor violation.

hep-ph↗

arXiv2017

A testable radiative neutrino mass model with multi-charged particles

We propose a radiatively-induced neutrino mass model at one-loop level by introducing a pair of doubly-charged fermions and a few multi-charged bosons. We investigate the contributions of the model to neutrino masses, lepton-flavor violations, muon $g-2$, oblique parameters, and collider signals, and find a substantial fraction of the parameter space that can satisfy all the constraints. Furthermore, we discuss the possibility of detecting the doubly-charged fermions at the LHC.

hep-ph↗

arXiv2017

Interpreting the 3 TeV $WH$ Resonance as a $W'$ boson

Motivated by a local $3.2-3.4$ sigma resonance in $WH$ and $ZH$ in the ATLAS Run 2 data, we attempt to interpret the excess in terms of a $W'$ boson in a $SU(2)_1 \times SU(2)_2 \times U(1)_X$ model. We stretch the deviation from the alignment limit of the Equivalence Theorem, so as to maximize $WH$ production while keeping the $WZ$ production rate below the experimental limit. We found a viable though small region of parameter space that satisfies all existing constraints on $W' \to jj, t \bar b, WZ$, as well as the precision Higgs data. The cross section of $W' \to WH$ that we obtain is about $5-6$ fb.

hep-ph↗

arXiv2017

Radiative decays of a singlet scalar boson through vector-like quarks

If the standard model Higgs boson were much heavier, it would appear as a broad resonance since its decay into a pair of longitudinally polarized gauge bosons is highly enhanced. We study whether the same enhancement happens at loop level in a simple extension of the standard model with a singlet scalar boson $S$. In order to focus on the loop effects, we assume that $S$ does not interact with the standard model particles at tree level. The singlet scalar $S$ is linked to the standard model world by vector-like quarks running in the loop. We introduce three vector-like quark multiplets, an $SU(2)_L$ doublet, an up-type singlet, and a down-type singlet. There are two kinds of loop effects in the $S$ phenomenology, the mixing with the Higgs boson and the radiative decays into $hh$, $WW$, $ZZ$, $gg$, and $γγ$ through the triangle loops. We show that the crucial condition for enhancing loop effects including the longitudinal polarization enhancement is the large mass differences among vector-like quarks. The current LHC constraints on $S$ from the heavy scalar searches and the Higgs precision data are shown to be very significant: the mixing angle with the Higgs boson should be smaller than about 0.1 for $m_S= 750$ GeV.

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