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Chung Kao

Publications and source records attributed to Chung Kao.

At least 19 recordsLinked to original sources
arXiv2025

Enhanced Charged Higgs Signal at the LHC

We investigate the discovery prospects of a charged Higgs boson ($H^\pm$) at the Large Hadron Collider (LHC) via the process $cg\to bH^\pm \to bc\bar{b}$ within the framework of a general two Higgs doublet model (G2HDM). In most two Higgs doublet models, the $H^+ cb$ coupling ($g_{H^+cb}$) is usually suppressed by the CKM matrix element $V_{cb}$. In G2HDM, there are additional Yukawa couplings, the process $cg\to bH^\pm \to bc\bar{b}$ is enhanced by the coupling $g_{H^+cb} \simeq \rtc V_{tb}$ in both the production and decay of the charged Higgs boson. We study possible physics backgrounds and evaluate the discovery potential with realistic acceptance cuts and tagging efficiencies at collider energies of $\sqrt{s}=$13 and 14 TeV. We apply $b$-tagging and $c$-tagging and show that $m_{H^+}$ can be extracted by pairing the tagged $b$ and $c$-jets. Our analysis leads to promising results for the current LHC and expected high-luminosity LHC.

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arXiv2025

Charming top interactions with a neutral Higgs Boson decaying into $\tau^+ \tau^-$ at the LHC

We investigate the discovery potential of flavor changing neutral Higgs (FCNH) interactions in top quark decays at the LHC. A general two Higgs doublet model (2HDM) is adopted to study the top quark decay $t \to c \phi^0$, where $\phi^0$ is either a CP-even heavy neutral scalar ($H^0$), or a CP-odd pseudoscalar ($A^0$), followed by the Higgs boson decaying into $\tau^+\tau^-$. The complete process is $pp \to t\bar{t} \to t c\phi^0 \to (bjj) (c\tau_{lep}\tau_{had}) +X$, where $t$ or $\bar{t}$ become virtual when $m_{\phi} > m_t -m_c$, $\tau_{lep}$ and $\tau_{had}$ represent the $\tau$ leptonic decay ($\tau^\pm \to \ell^\pm$ + MET) and the $\tau$ hadronic decay ($\tau^\pm \to j_\tau$ + MET), respectively. We evaluate the cross section applying realistic selection criteria for the FCNH signal and the dominant physics background at the parton-level. In addition, we employ \textsc{MadGraph}, \textsc{Pythia}~8, and \textsc{Delphes} to perform a realistic event-level Monte Carlo simulation analysis. The collinear approximation is applied since the $\tau$'s from Higgs boson decays are highly boosted such that $\tau_{lep}$ and $\tau_{had}$ momenta are approximately in the same direction as the $\tau$'s, which enables the reconstruction of the Higgs boson mass. The energy of the charm quark provides an additional kinematic variable that discriminates the signal from the background. In the alignment limit, the FCNH couplings for the heavy Higgs bosons ($H^0$ and $A^0$) will not be suppressed [$\lambda_{tcH} \propto \sin(\beta-\alpha) \approx 1$]. We study the discovery potential for the heavy neutral Higgs bosons at the LHC with collider energies of $\sqrt{s} = 13$ TeV, 13.6 TeV and 14 TeV, and find the High-Luminosity LHC offers great promise for the discovery of heavier Higgs bosons from $t \to c \phi^0$ or $t^* \to c \phi^0$.

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arXiv2023

Searching for extra Higgs bosons via $pp\to H,A\to τμ, ττ$ at the Large Hadron Collider

We extend heavy Higgs searches at the Large Hadron Collider for $H \to τμ$ by CMS, and $H \to ττ$ by ATLAS and CMS, to study discovery prospects of extra Higgs states in $pp \to H,A \to τμ, ττ$ with $eμ+ E_T$ and $j_τ μ+ E_T$ final states, where $j_τ = π\, , ρ\, , a_1$ and $E_T$ is missing transverse energy. In a general two Higgs doublet model without $Z_2$ symmetry, extra Yukawa couplings $ρ_{ττ}$ and $ρ_{τμ}$ can drive $H,A \to ττ$ and $τμ$ channels at hadron colliders, following gluon-gluon fusion production with extra $ρ_{tt}$ couplings. The light Higgs boson $h(125)$ is found to resemble closely the Standard Model Higgs boson; in the alignment limit of $\cosγ\to 0$ for $h$--$H$ mixing, flavor changing neutral Higgs couplings such as $h \to τμ$ are naturally suppressed, but the couplings of the heavier $H$ is optimized by $\sin γ\to 1$. We define various signal regions for $H,A \to τμ$ and $ττ$ and evaluate physics backgrounds from dominant processes with realistic acceptance cuts and tagging efficiencies. Two different studies are presented. We first perform a parton level study without any hadronization and with minimal detector smearing. We then include hadronization using PYTHIA 8.2 and fast detector simulation using DELPHES to give event level simulation. Results for $\sqrt{s}= 13$ TeV appear promising, which we extend further to $\sqrt{s} = 14$ TeV for the High Luminosity LHC.

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arXiv2022

Detecting Heavy Higgs Bosons from Natural SUSY at a 100 TeV Hadron Collider

Supersymmetric models with radiatively-driven naturalness (RNS) enjoy low electroweak fine-tuning whilst respecting LHC search limits on gluinos and top squarks and allowing for $m_h\simeq 125$ GeV. While the heavier Higgs bosons $H,\ A$ may have TeV-scale masses, the SUSY conserving $μ$ parameter must lie in the few hundred GeV range. Thus, in natural SUSY models there should occur large heavy Higgs boson branching fractions to electroweakinos, with Higgs boson decays to higgsino plus gaugino dominating when they are kinematically accessible. These SUSY decays can open up new avenues for discovery. We investigate the prospects of discovering heavy neutral Higgs bosons $H$ and $A$ decaying into light plus heavy chargino pairs which can yield a four isolated lepton plus missing transverse energy signature at the LHC and at a future 100 TeV $pp$ collider. We find that discovery of heavy Higgs decay to electroweakinos via its $4\ell$ decay mode is very difficult at HL-LHC. For FCC-hh or SPPC, we study the $H,\ A \to $ SUSY reaction along with dominant physics backgrounds from the Standard Model and devise suitable selection requirements to extract a clean signal for FCC-hh or SPPC with $\sqrt{s}=100$ TeV, assuming an integrated luminosity of 15 $ab^{-1}$. We find that while a conventional cut-and-count analysis yields a signal statistical significance greater than $5σ$ for $m_{A,H}\sim 1.1-1.65$ TeV, a boosted-decision-tree analysis allows for heavy Higgs signal discovery at FCC-hh or SPPC for $m_{A,H}\sim 1-2$ TeV.

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arXiv2021

Collider Prospects for Muon $g-2$ in General Two Higgs Doublet Model

Recent progress on muon $g-2$ measurement prompts one to take it even more seriously. In the general two Higgs doublet model that allows extra Yukawa couplings, we take a simplified approach of single enhanced coupling. We fix the charged lepton flavor violating coupling, $ρ_{τμ} = ρ_{μτ}$, via the one-loop mechanism, for illustrative masses of the heavy scalar $H$ and pseudoscalar $A$, where we assume $m_A = m_{H^+}$. Since extra top Yukawa couplings are plausibly the largest, we turn on $ρ_{tt}$ and find that LHC search for $gg \to H,\,A \to τμ$ gives more stringent bound than from $τ\to μγ$ with two-loop mechanism. Turning on a second extra top Yukawa coupling, $ρ_{tc}$, can loosen the bound on $ρ_{tt}$, but LHC constraints can again be more stringent than from $B \to Dμν$ vs $Deν$ universality. This means that evidence for $H,\,A \to τμ$ may yet emerge with full LHC Run 2 data, while direct search for $τ^\pmμ^\mp bW^+$ or $t\bar cbW^+$ (plus conjugate) may also bear fruit.

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arXiv2021

Flavor changing top decays to charm and Higgs with $ττ$ at the LHC

We investigate the prospects of discovering the top quark decay into a charm quark and a Higgs boson ($t \to c h^0$) in top quark pair production at the CERN Large Hadron Collider (LHC). A general two Higgs doublet model is adopted to study flavor changing neutral Higgs (FCNH) interactions. We perform a parton level analysis as well as Monte Carlo simulations using \textsc{Pythia}~8 and \textsc{Delphes} to study the flavor changing top quark decay $t \to c h^0$, followed by the Higgs decaying into $τ^+ τ^-$, with the other top quark decaying to a bottom quark ($b$) and two light jets ($t\to bW\to bjj$). To reduce the physics background to the Higgs signal, only the leptonic decays of tau leptons are used, $τ^+τ^- \to e^\pmμ^\mp +\slashed{E}_T$, where $\slashed{E}_T$ represents the missing transverse energy from the neutrinos. In order to reconstruct the Higgs boson and top quark masses as well as to effectively remove the physics background, the collinear approximation for the highly boosted tau decays is employed. Our analysis suggests that a high energy LHC at $\sqrt{s} = 27$ TeV will be able to discover this FCNH signal with an integrated luminosity $\mathcal{L} = 3$ ab$^{-1}$ for a branching fraction ${\cal B}(t \to ch^0) \agt 1.4 \times 10^{-4}$ that corresponds to a FCNH coupling $|λ_{tch}| \agt 0.023$. This FCNH coupling is significantly below the current ATLAS combined upper limit of $|λ_{tch}| = 0.064$.

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arXiv2021

Self-interacting Dark Matter with Scalar Dilepton Mediator

The cold dark matter (CDM) candidate with weakly interacting massive particles can successfully explain the observed dark matter relic density in cosmic scale and the large-scale structure of the Universe. However, a number of observations at the satellite galaxy scale seem to be inconsistent with CDM simulation. This is known as the small-scale problem of CDM. In recent years, it has been demonstrated that self-interacting dark matter (SIDM) with a light mediator offers a reasonable explanation for the small-scale problem. We adopt a simple model with SIDM and focus on the effects of Sommerfeld enhancement. In this model, the dark matter candidate is a leptonic scalar particle with a light mediator. We have found several regions of the parameter space with proper masses and coupling strength generating a relic density that is consistent with the observed CDM relic density. Furthermore, this model satisfies the constraints of recent direct searches and indirect detection for dark matter as well as the effective number of neutrinos and the observed small-scale structure of the Universe. In addition, this model with the favored parameters can resolve the discrepancies between astrophysical observations and $N$-body simulations.

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arXiv2019

Flavor Changing Heavy Higgs Interactions with Leptons at Hadron Colliders

In a general two Higgs doublet model, we study flavor changing neutral Higgs (FCNH) decays into leptons at hadron colliders, $pp \to ϕ^0 \to τ^\mpμ^\pm +X$, where $ϕ^0$ could be a CP-even scalar ($h^0$, $H^0$) or a CP-odd pseudoscalar ($A^0$). The light Higgs boson $h^0$ is found to resemble closely the Standard Model Higgs boson at the Large Hadron Collider. In the alignment limit of $\cos(β-α) \cong 0$ for $h^0$--$H^0$ mixing, FCNH couplings of $h^0$ are naturally suppressed, but such couplings of the heavier $H^0, A^0$ are sustained by $\sin(β-α) \simeq 1$. We evaluate physics backgrounds from dominant processes with realistic acceptance cuts and tagging efficiencies. We find promising results for $\sqrt{s} = 14$ TeV, which we extend further to $\sqrt{s} = 27$ TeV and 100 TeV future pp colliders.

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arXiv2019

Charming Top Decays with Flavor Changing Neutral Higgs Boson and WW at Hadron Colliders

We investigate the prospects for discovering a top quark decaying into one light Higgs boson ($h^0$) along with a charm quark ($c$) in top quark pair production at the CERN Large Hadron Collider (LHC) and future hadron colliers. A general two Higgs doublet model is adopted to study the signature of flavor changing neutral Higgs (FCNH) interactions with $t \to c h^0$, followed by $h^0 \to WW^* \to \ell^+ \ell^- +\notE_T$, where $h^0$ is the CP-even Higgs boson and $\notE_T$ stands for missing transverse energy from neutrinos. We study the discovery potential for this FCNH signal and physics background from dominant processes with realistic acceptance cuts as well as tagging and mistagging efficiencies. Promising results are found for the LHC running at 13 TeV and 14 TeV center-of-mass energy as well as future pp colliders at 27 TeV and 100 TeV.

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arXiv2015

Interference effects and the use of Higgs boson pair production to study the Higgs trilinear self coupling

We show that the dominant channel proposed for the determination of the Higgs boson trilinear coupling, $pp\to HH+X$ via gluon fusion, exhibits an interference structure that is independent of the collider energy for collider energies in the range $8\,{\rm TeV}\leq \sqrt{s}\leq 100\,{\rm TeV}$ and is almost maximally destructive. This insensitivity to the collider energy remains approximately true for a variety of other two Higgs production mechanisms although the magnitude of the interference varies widely.

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arXiv2015

Flavor Changing Heavy Higgs Interactions at the LHC

A general two Higgs doublet model (2HDM) is adopted to study the signature of flavor changing neutral Higgs (FCNH) decay $ϕ^0 \to t\bar{c}+\bar{t}c$, where $ϕ^0$ could be a CP-even scalar ($H^0$) or a CP-odd pseudoscalar ($A^0$). Measurement of the light 125 GeV neutral Higgs boson ($h^0$) couplings at the Large Hadron Collider (LHC) favor the decoupling limit or the alignment limit of a 2HDM, in which gauge boson and diagonal fermion couplings of $h^0$ approach Standard Model values. In such limit, FCNH couplings of $h^0$ are naturally suppressed by a small mixing parameter $\cos(β-α)$, while the off-diagonal couplings of heavier neutral scalars $ϕ^0$ are sustained by $\sin(β-α) \sim 1$. We study physics background from dominant processes with realistic acceptance cuts and tagging efficiencies. Promising results are found for the LHC running at 13 or 14 TeV collision energies.

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arXiv2013

Unveiling the MSSM Neutral Higgs Bosons with Leptons and a Bottom Quark

We investigate the prospects for the discovery of neutral Higgs bosons produced with a bottom quark where the Higgs decays into a pair of tau leptons and the taus decay into an electron-muon pair, i.e. $bg \to bϕ^0 \to bτ^+τ^- \to be^\pmμ^\mp + E\!\!\!/_T$, $ϕ^0 = h^0, H^0, A^0$. Our study has been done within the framework of the Minimal Supersymmetric Standard Model. We consider the dominant physics backgrounds including the production of Drell-Yan processes ($bτ^+τ^-$ and $jτ^+τ^-, j = q, g$), top quark pair ($t\bar{t}$), $tW$ and $jWW$ with realistic acceptance cuts and efficiencies. We present $5σ$ discovery contours for the neutral Higgs bosons in the ($M_A,\tanβ$) plane as well as the region with a favored light Higgs mass (123 GeV $\le m_h\le$ 129 GeV). Promising results are found for the CP-odd pseudoscalar ($A^0$) and the heavier CP-even scalar ($H^0$) Higgs bosons with masses up to 800 GeV and $\tanβ\simeq 50$ at the LHC with a center of mass energy ($\sqrt{s}$) of 14 TeV and an integrated luminosity ($L$) of 300 fb$^{-1}$. With $\sqrt{s} =$ 14 TeV and $L =$ 3000 fb$^{-1}$, LHC will be able to discover the Higgs pseudoscalar and the heavier Higgs scalar beyond $M_A = 1000$ GeV.

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arXiv2013

Comparison of $H\to\ell\bar{\ell}γ$ and $H\toγ\,Z, Z\to\ell\bar{\ell}$ including the ATLAS cuts

A precise comparison is made between the Dalitz decay $H\to\ell\bar{\ell}γ$ and the two body decay $H\toγZ,Z\to\ell\bar{\ell}$ for electrons and for muons including experimental cuts appropriate for the ATLAS detector. The widths for these two processes differ by 8% for electrons and 3% for muons. Given that there remain QCD radiative corrections of this order that are not included, this suggests that the isolation of the Dalitz decay will be challenging.

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arXiv2013

When the Higgs meets the Top: Search for t --> ch^0 at the LHC

The newly discovered "Higgs" boson h^0, being lighter than the top quark t, opens up new probes for flavor and mass generation. In the general two Higgs doublet model, new ct, cc and tt Yukawa couplings could modify h^0 properties. If t --> ch^0 occurs at the percent level, the observed ZZ^* and γγsignal events may have accompanying cbW activity coming from t\bar{t} feeddown. We suggest that t --> ch^0 can be searched for via h^0 --> ZZ^*, γγ, WW^* and b\bar{b}, perhaps even τ^+τ^- modes in t\bar{t} events. Existing data might be able to reveal some clues for t --> ch^0 signature, or push the branching ratio B(t --> ch^0) down to below the percent level.

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arXiv2012

Confirming the LHC Higgs Discovery with WW

We investigate the prospects of observing a neutral Higgs boson decaying into a pair of $W$ bosons (one real and the other virtual), followed by the $W$ decays into $qq' \ellν$ or $jj\ellν$ at the CERN Large Hadron Collider (LHC). Assuming that the missing transverse energy comes solely from the neutrino in $W$ decay, we can reconstruct the $W$ masses and then the Higgs mass. At the LHC with a center of mass energy ($\sqrt{s}$) of 8 TeV and an integrated luminosity ($L$) of 25 fb$^{-1}$, we can potentially establish a $6σ$ signal. A $5σ$ discovery of $H \to WW^* \to jj\ellν$ for $\sqrt{s} = 14$ TeV can be achieved with $L = $ 6 fb$^{-1}$. The discovery of $H \to WW$ implies that the recently discovered new boson is a CP-even scalar if its spin is zero. In addition, this channel will provide a good opportunity to study the $HWW$ coupling.

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arXiv2012

Top Decays with Flavor Changing Neutral Higgs Interactions at the LHC

We investigate the prospects for discovering a top quark decaying into one light Higgs boson along with a charm quark in top quark pair production at the CERN Large Hadron Collider (LHC). A general two Higgs doublet model is adopted to study the signature of flavor changing neutral Higgs decay $t \to cϕ^0$, %or $\bar{t} \to \bar{c}ϕ^0$ where $ϕ^0$ could be CP-even ($H^0$) or CP-odd ($A^0$). The dominant physics background is evaluated with realistic acceptance cuts as well as tagging and mistagging efficiencies. For a reasonably large top-charm-Higgs coupling ($λ_{tc}/λ_{t} \agt 0.09$), the abundance of signal events and the %that our acceptance cuts reduction in physics background allow us to establish a $5σ$ signal for $M_ϕ\sim 125$ GeV at the LHC with a center of mass energy ($\sqrt{s}$) of 8 TeV and an integrated luminosity of 20 fb$^{-1}$. The discovery potential will be greatly enhanced with the full energy of $\sqrt{s} = 14$ TeV.

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arXiv2011

Prospects for Higgs Searches with the Tri-bottom Channel in Unified SUSY Models

We investigate the prospects for the discovery of a neutral Higgs boson produced in association with a $b$ quark, followed by the Higgs decay into a pair of bottom quarks, $pp \to bϕ^0 \to b b\bar{b} +X$, at the CERN Large Hadron Collider (LHC) within the framework of unified supersymmetric models. The Higgs boson $ϕ^0$ can be a heavy scalar $H^0$ or a pseudoscalar $A^0$. Furthermore, this direct discovery channel is compared with the indirect Higgs searches in the rare decay $B_s \to μ^+μ^-$ at hadron colliders. Promising results are found for the minimal supergravity (mSUGRA) model, the anomaly mediated supersymmetry breaking (AMSB) model, and the gauge mediated supersymmetry breaking (GMSB) model. We find that the indirect search for $B(B_s \to μ^+μ^-) \ge 5\times 10^{-9}$ is complementary to the direct search for $bϕ^0 \to bb\bar{b}$ with $\sqrt{s} = 14$ TeV and an integrated luminosity ($L$) of 300 fb$^{-1}$. In the AMSB and GMSB models, $bϕ^0 \to bb\bar{b}$ with $L = 300$ fb$^{-1}$ covers a larger area in the parameter space than $B(B_s \to μ^+μ^-) \ge 5\times 10^{-9}$. In addition, we present constraints from $b \to sγ$ and muon anomalous dipole moment ($Δa_μ$) on the parameter space.

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