arXiv subjects

Jeonghyeon Song

Publications and source records attributed to Jeonghyeon Song.

At least 37 records · Page 2Linked to original sources

arXiv2022

Comprehensive study of the light charged Higgs boson in the type-I two-Higgs-doublet model

In the type-I two-Higgs-doublet model, existing theoretical and experimental constraints still permit the light charged Higgs boson with a mass below the top quark mass. We present a complete roadmap for the light charged Higgs boson at the LHC through the comprehensive phenomenology study, focusing on the normal scenario where the lighter \textit{CP}-even Higgs boson is the observed Higgs boson. In type-I, it is challenging to simultaneously accommodate the light mass of the charged Higgs boson and the constraints from theory, electroweak precision data, Higgs data, $b\to s γ$, and direct search bounds. Consequently, the parameter space is extremely curtailed, which predicts somewhat definite phenomenological signatures. We find that the mass of the pseudoscalar Higgs boson, $M_A$, is the most crucial factor in the phenomenology of the charged Higgs boson. If $M_A$ is light, the charged Higgs boson decays mainly into $A W^\pm$. When $M_A$ is above the $A W^\pm$ threshold, the dominant decay mode is into $ τ^\pm ν$. Over the whole viable parameter space, we study all the possible production and decay modes of charged Higgs bosons at the LHC, and suggest three efficient channels: (i) $pp \to H^+ H^-\to [τν] [τν]$; (ii) $pp \to HA/HH/AA \to H^\pm W^\mp H^\pm W^\mp \to [τν] [τν] W W$; (iii) $pp \to H^+ H^-\to [b \bar b W ] [b \bar b W ] $. Based on the sophisticated signal-background analyses including detector simulation, we showed that the significance of the first final state is large, that of the second one is marginal around three, but the third one suffers from huge $t\bar t$ related backgrounds.

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arXiv2021

Type-X two Higgs doublet model in light of the muon $\mathbf{g-2}$: confronting Higgs and collider data

The recent Fermilab measurement of the muon anomalous magnetic moment yields $4.2 σ$ deviations from the SM prediction when combined with the BNL E821 experiment results. In the Type-X two Higgs doublet model, we study the consequence of imposing the observed muon $g-2$, along with the constraints from theoretical stabilities, electroweak oblique parameters, Higgs precision data, and direct searches. For a comprehensive study, we scan the whole parameter space in two scenarios, the normal scenario where $h_{\rm SM} = h$ and the inverted scenario where $h_{\rm SM}=H$, where $h$ ($H$) is the light (heavy) CP-even Higgs boson. We found that large $\tanβ$ (above 100) and light pseudoscalar mass $M_A$ are required to explain the muon $g-2$ anomaly. This breaks the theoretical stability unless the scalar masses satisfy $M_A^2 \simeq M_{H^\pm}^2 \simeq m_{12}^2 \tanβ\approx M_{H/h}^2$. The direct search bounds at the LEP and LHC exclude the light $A$ window with $M_A \lesssim 62.5~$GeV. We also show that the observed electron anomalous magnetic moment is consistent with the model prediction, but the lepton flavor universality data in the $τ$ and $Z$ decays are not. For a separate exploration of the model, we propose the golden mode $pp \to A h/AH \to 4 τ$ at the HL-LHC.

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arXiv2021

Studies of nonresonant Higgs pair production at electron-proton colliders

The measurement of the Higgs quartic coupling modifier between a Higgs boson pair and a vector boson pair, $κ_{2V}$, is expected to be achieved from vector-boson fusion (VBF) production of a Higgs boson pair. However, this process involves another unmeasured parameter, the trilinear Higgs self-coupling modifier $κ_{λ}$. A sensitivity analysis should target both parameters. Since the LHC cannot avoid the gluon fusion pollution, which becomes severe for non-SM $κ_{λ}$, an electron-proton collider is more appropriate for the comprehensive measurement. In this regard, we study the VBF production of a Higgs boson pair in the $b\bar{b}b\bar{b}$ final state at the LHeC and FCC-he. Performing detailed analysis using the simulated dataset, we devise the search strategy specialized at the LHeC and FCC-he and give a prediction for the sensitivity to both $κ_{2V}$ and $κ_{λ}$. We find that the two electron-proton colliders have high potential: the LHeC has similar exclusion prospects as the HL-LHC; the FCC-he is extremely efficient, excluding the parameter space outside $κ_{2V} \in [0.8, 1.2]$ and $κ_{λ}\in [1, 2.5]$ at 95% C.L. for the total luminosity of $10$ ab$^{-1}$ and 10% uncertainty on the background yields.

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arXiv2020

Phenomenology of the Inert Doublet Model with a global U(1) symmetry

The inert doublet model is a minimal dark matter model with strong theoretical motivations, where the stability of dark matter is usually achieved by imposing a $Z_2$ parity. We promote the $Z_2$ parity into a global $U(1)$ symmetry and study its phenomenological implications. There are two characteristic features of the model: both the CP-even and CP-odd neutral inert scalars, $h_1$ and $h_2$, become DM candidates; the number of model parameters is one less than that with $Z_2$ parity. We first analyze the constraints from LEP experiments, electroweak precision tests, theoretical stability, Higgs precision data, dark matter relic density, and direct detection experiments. It is found that if the model is required to explain at least 10% of the observed relic density, the theory is extremely limited such that the dark matter mass is about $70\;{\rm GeV}$ and the charged Higgs boson is not very heavy. Focusing on this narrow parameter space, we calculate the production cross sections of almost all the possible mono-$X$ and mono-$XX'$ processes at the LHC. The mono-$Wγ$ process is shown to have high discovery potential with the help of the decay of the intermediate-mass charged Higgs boson into $W^\pm h_{1,2}$. A search strategy is designed to increase the potential discovery of the model for the mono-$Wγ$ signal at both the HL-LHC and the FCC-hh. The optimal cut on $E_T^{\rm miss}/\sqrt{H_T}$ is suggested to maximize the signal significance, being about $0.76$ at the HL-LHC and about $7.5$ at the FCC-hh.

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arXiv2020

Disentangling new physics effects on non-resonant Higgs boson pair production from gluon fusion

There are two kinds of new physics effects on non-resonant di-Higgs process from gluon fusion, non-SM Higgs trilinear self-coupling $λ_{hhh}$ or new colored particles running in the loop. With the aim of disentangling different new physics contributions, we study their characteristics in the kinematic distributions. Assuming that the total cross section is observed to be about three times as large as the SM expectation, we consider the cases of $λ_{hhh}/λ_{hhh}^{\rm SM} =-0.5, 5.5$ as well as a new physics model with heavy vectorlike quarks in a type-II two Higgs double model, called the VLQ-2HDM. A reasonable benchmark point is suggested in the exact wrong-sign limit, where the opposite sign between the up-type VLQ and down-type VLQ couplings to the Higgs boson causes the cancellation of their contributions to the single-Higgs production from gluon fusion. Because of the threshold effects from the heavy VLQs in the loop, the VLQ-2HDM accommodates the bumps in the distributions of the invariant mass of the Higgs boson pair ($M_{hh}$) and the transverse momentum of a Higgs boson ($p_T^h$). The positions of two bumps are closely related as $ M_{hh} \simeq 2 M_{\rm VLQ}$ and $p_T^h \simeq M_{\rm VLQ}$. In addition, the bumps located at the heavy VLQ mass naturally lift up the $M_{hh}$ and $p_T^h$ distributions into high-mass and high-$p_T^h$ regions. On the other hand, the non-SM Higgs trilinear coupling cases have the distributions shift into low $ M_{hh}$ and $p_T^h$ regions. Therefore, the kinematic region with high $M_{hh}$ and high $p_T^h$ will be a smoking-gun signal for the VLQ-2HDM. Full HL-LHC simulations for the di-Higgs signals are also performed, confirming that the $b \bar{b} b \bar{b}$ final state can distinguish the VLQ-2HDM.

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arXiv2019

The $W γ$ decay of the elusive charged Higgs boson in the two-Higgs-doublet model with vectorlike fermions

The LHC search for the charged Higgs boson in the intermediate-mass range ($M_{H^\pm} \sim m_t$) is actively being performed after the next-to-leading order calculation of the total production cross section of $p p \to H^\pm W^\mp b \bar{b}$. In the decay part, only the $τν$ mode is mainly concerned because of the experimental difficulty in the $t b$ mode. In the framework of a two-Higgs-doublet model, we suggest that the $W γ$ channel can be helpful in probing this charged Higgs boson, if introducing vectorlike fermions. In type-I-II model where the SM fermions are assigned in type-I while the vectorlike fermions are in type-II, the branching ratio is greatly enhanced up to $\sim \mathcal{O}(0.01)$ in a large portion of the parameter space allowed by the Higgs precision data, the electroweak oblique parameters, and the direct search bounds at the LHC. Two kinds of production channels, $gg\to H^\pm W^\mp b \bar{b}$ and $g g \to H/A \to H^\pm W^\mp$, are also studied. We find that the signal rate $σ\times \mathcal{B}(W γ)$ is quite sizable, more than 10 fb in some parameter space.

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arXiv2018

Confronting the fourth generation two Higgs doublet model with the phenomenology of heavy Higgs bosons

A sequential fourth generation is known to be excluded because the non-decoupling contribution to $κ_g$, the Higgs coupling modifier with a gluon pair, is unacceptably large. Recently a new way to save the model was suggested in the Type-II two Higgs doublet model: if the Yukawa couplings of down-type fermions have wrong-sign, the contributions from $t'$ and $b'$ to $κ_g$ are cancelled. We study the theoretical and experimental constraints on this model, focusing on the heavy Higgs bosons. Two constraining features are pointed out. First the exact wrong-sign limit does not allow the alignment, which makes the perturbative unitarity for the scalar-scalar scattering put the upper bounds on the heavy Higgs boson masses like $M_H, M_A \lesssim 920$ GeV and $M_{H^\pm} \lesssim 620$ GeV. Secondly, the Yukawa couplings of the fourth generation fermions to the heavy Higgs bosons are generically large as being proportional to the heavy fermion mass and, for the down-type fermions, to $\tanβ$ as well. The gluon fusion productions of $H$ and $A$ through the fourth generation quark loops become significant. We found that the current LHC data on $pp \to Z Z$ for $H$ along with the theoretical and indirect constraints exclude the model at leading order.

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arXiv2018

Constraints from Heavy Higgs boson masses in the two Higgs doublet model

Upon the absence of signals of new physics at the LHC, a reasonable strategy is to assume that new particles are very heavy and the other model parameters are unknown yet. In the aligned two Higgs doublet model, however, heavy Higgs boson masses above 500 GeV enhance some couplings in the scalar potential, which causes a breakdown of the perturbative unitariry in general. Some tuning among model parameters is required. We find that one information on the heavy Higgs boson mass, say $M_H$, has significant theoretical implications: (i) the other heavy Higgs bosons should have similar masses to $M_H$ within $\pm \mathcal{O}(10)\%$; (ii) the inequalities from the theoretical constraints are practically reduced to an equation such that $m_{12}^2 \tanβ$ is constant, where $m_{12}^2$ is the soft $Z_2$ breaking parameter and $\tanβ$ is the ratio of two vacuum expectation values; (iii) the triple Higgs coupling $λ_{HHh}$ is constant over $\tanβ$ while $λ_{HHH}$ and $λ_{AAH}$ are linearly proportional to $\tanβ$. The double Higgs-strahlung process of $e^+ e^- \to ZHH$ is also studied, of which the total cross section is almost constant with the given $M_H$.

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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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arXiv2016

Higgs potential and hidden light Higgs scenario in two Higgs doublet models

In two Higgs doublet models (2HDM), there exists an interesting possibility, the hidden light Higgs scenario, that the discovered SM-like Higgs boson is the heavier \textit{CP}-even Higgs boson $H^0$ and the lighter \textit{CP}-even $h^0$ has not been observed yet in any experiment. We study the current status of this scenario in Types I, II, X, and Y, through the scans of the 2HDM parameters with all relevant theoretical and experimental constraints. We employ not only the most up-to-date Higgs signal strength measurements with the feed-down effects, but also all the available LHC exclusion limits from heavy Higgs searches. Adjusting the heavier $H^0$ to the 125 GeV state while hiding the lighter $h^0$ from the LEP Higgs search prohibits the extreme decoupling limit: there exist upper bounds on the masses of the pseudoscalar $A^0$ and the charged Higgs $H^\pm$ below about 600 GeV. In addition, the $Z_2$ symmetry is shown to be a good approximate symmetry since the soft $Z_2$ symmetry breaking parameter $m_{12}^2$ should be less than about (45 GeV)$^2$. Most interestingly, a few parameters in the Higgs potential and the related Higgs triple and quartic couplings are shown to be meaningfully constrained by the current data. The double Higgs-strahlung process at an $e^+ e^-$ collider is also studied.

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arXiv2016

Top-phobic heavy Higgs boson as the 750 GeV diphoton resonance

A hint of a new resonance at a mass of 750\,GeV has been observed in the diphoton channel of LHC Run 2 at $\sqrt{s}=13$ TeV. The signal rate is too large to interpret it as a new Higgs boson in the context of weakly-coupled renormalizable models. One way is to reduce its total decay rate, which is possible if the CP-even heavy Higgs boson $H^0$ in the aligned two Higgs doublet model becomes top-phobic. To ensure sufficient gluon fusion production, we introduce vector-like quarks (VLQ). The Higgs precision data as well as the exclusion limits from no excesses in other 8 TeV LHC searches of $Zγ$, $b\bar{b}$, $τ^+τ^-$, and $jj$ channels are simultaneously included. In Type I, top-phobic $H^0$ cannot explain the 750\,GeV diphoton signal and the Higgs precision data simultaneously since the universal Yukawa couplings of the up-type and down-type VLQs always make more contribution to $h^0$ than to $H^0$. In Type II, small Yukawa coupling of the up-type VLQ but sizable Yukawa coupling for the down-type VLQ is shown to explain the signal while satisfying other LHC exclusion limits.

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arXiv2016

Interference effect on heavy Higgs resonance signal in γγand ZZ channels

The resonance-continuum interference is usually neglected when the width of a resonance is small compared to the resonance mass. We re-examine this standard by studying the interference effects in high-resolution decay channels, $γγ$ and $ZZ$, of the heavy Higgs boson $H^0$ in nearly aligned two-Higgs-doublet models. For the $H^0$ with a sub-percent width-to-mass ratio, we find that, in the parameter space where the LHC 14 TeV $ZZ$ resonance search can be sensitive, the interference effects can modify the $ZZ$ signal rate by ${\cal O}(10)\%$ and the exclusion reach by ${\cal O}(10)$ GeV. In other parameter space where the $ZZ$ or $γγ$ signal rate is smaller, the LHC 14 TeV reach is absent, but a resonance shape can be much more dramatically changed. In particular, the $γγ$ signal rate can change by ${\cal O}(100)\%$. Relevant to such parameter space, we suggest variables that can characterize a general resonance shape. We also illustrate the relevance of the width on the interference by adding non-standard decay modes of the heavy Higgs boson.

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arXiv2015

How Resonance-Continuum Interference Changes 750 GeV Diphoton Excess: Signal Enhancement and Peak Shift

The new scalar resonance contribution to the 750 GeV diphoton excess observed at the LHC 13 TeV necessarily interferes with the continuum background in the $gg\to γγ$. The interference has two considerable effects: (1) enhancing or suppressing diphoton signal rate due to the imaginary-part interference and (2) distorting resonance shape due to the real-part interference. From the best-fit study of two benchmark models (two Higgs doublets with $\sim$50 GeV widths and a singlet scalar with 5 GeV width, both extended with vector-like fermions), we find that the resonance contribution to the 750 GeV excess can be enhanced by a factor of 2(1.6) for 3(6) fb signal rate and the 68%(95%) CL best-fit mass range can shift by 1-4 (any ${\cal O}$(1)) GeV. If the best-fit excess rate decreases with future data, the interference effects will become more significant. The inevitable interferences can also provide a consistency check of a resonance hypothesis, whether or not future precision shape measurements confirm a Breit-Wigner shape or discover interesting deviations.

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arXiv2015

Dip or nothingness of a Higgs resonance from the interference with a complex phase

We show that new resonance shapes -- a pure dip, nothingness and an enhanced pure peak -- can be produced from the interference between resonance and continuum with a relative phase. Production conditions of those new shapes are derived based on a general parameterization of the interference. The narrow width approximation is modified to work with the non-zero imaginary part of interference, and the correction factor can characterize the resonance shape. We demonstrate that the new resonance shapes of heavy Higgs bosons, $H^0$ and $A^0$ in the Type II aligned two Higgs doublet model, generally show up in $gg \to H^0/A^0 \to t\bar{t}$ as well as $b\bar{b}$ and $γγ$ channels. The pure $A^0$ resonance dip in the $t\bar{t}$ channel is a particularly interesting signal as it can be probed well by the current search techniques that do not even take into account interferences; the high-luminosity LHC 14 TeV can perhaps probe a large part of its parameter space.

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arXiv2014

Gigantic diphoton rate of heavy Higgs bosons in the aligned two Higgs doublet models with small $\tanβ$

We study the implications of the LHC heavy neutral Higgs search data on the aligned two Higgs doublet model with softly broken $Z_2$ symmetry. When $\tanβ$ is small, the gluon fusion production of the heavy CP-even scalar $H^0$ or the CP-odd scalar $A^0$ becomes large enough to constrain the model by the current $γγ$, $τ^+τ^-$, and $t\bar{t}$ data. By reinvestigating this small $\tanβ$ region, we find that the indirect constraints like $Δρ$, $b\to s γ$, $ΔM_{B_d}$, $R_b$, $ε_K$, and the perturbativity of running top Yukawa coupling are rather weak to be $\tanβ> 0.5$. The current heavy Higgs search results are shown to put more significant bounds. If $m_H \simeq m_A$, the $t\bar{t}$ mode excludes $\tanβ< 1.5$ for $m_{H,A}=500-600\,$GeV in all four types, and the $γγ$ and $τ^+τ^-$ modes exclude $\tanβ< 1-3$ ($\tanβ< 3-10$) for $m_{H,A}=120-340\,$GeV in Type I, II, and Y (Type X).

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arXiv2014

Determining the Dark Matter Particle Mass through Antler Topology Processes at Lepton Colliders

We study the kinematic cusps and endpoints of processes with the "antler topology" as a way to measure the masses of the parity-odd missing particle and the intermediate parent at a high energy lepton collider. The fixed center of mass energy at a lepton collider makes many new physics processes suitable for the study of the antler decay topology. It also provides new kinematic observables with cusp structures, optimal for the missing mass determination. We also study realistic effects on these observables, including initial state radiation, beamstrahlung, acceptance cuts, and detector resolution. We find that the new observables, such as the reconstructed invariant mass of invisible particles and the summed energy of the observable final state particles, appear to be more stable than the commonly considered energy endpoints against realistic factors and are very efficient at measuring the missing particle mass. For the sake of illustration, we study smuon pair production and chargino pair production within the framework of the minimal supersymmetric standard model. We adopt the log-likelihood method to optimize the analysis. We find that at the 500 GeV ILC, a precision of approximately 0.5 GeV can be achieved in the case of smuon production with a leptonic final state, and approximately 2 GeV in the case of chargino production with a hadronic final state.

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arXiv2014

Two Higgs doublet models for the LHC Higgs boson data at $\sqrt{s}=$ 7 and 8 TeV

Updated LHC data on the new 126 GeV boson during the 7 and 8 TeV runnings strengthen the standard model Higgs boson interpretation further. Through the global $χ^2$ analysis, we investigate whether the new particle could be one of the scalar particles in two Higgs doublet models. Four types (Type I, II, X and Y) are comprehensively studied. Taking the recent analysis on the spin-parity of the new boson, we consider two scenarios: the new boson is either the light CP-even one ($h^0$) or the heavy CP-even one ($H^0$). It is found that both scenarios are consistent with the new data, not only in the parameter regions near the decoupling limit but also in other regions far from the decoupling limit. In addition, the current data are compatible with the possibility that the light Higgs boson $h^0$ is hidden in the mass window of 90-100 GeV. The diphoton or $ττ$ channel can provide a probe of this possibility by the enhanced signal rates.

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arXiv2013

Using the Higgs boson to probe the littlest Higgs model with T-parity through $Z_H W_H$ production at the LHC

In the littlest Higgs model with T-parity, the production cross section of the T-odd heavy gauge boson pair $Z_H W_H$ is quite sizable at the LHC. In addition, both the $W_H$ and $Z_H$ bosons have almost exclusively one decay channel into $W A_H$ and $H A_H$ respectively, where the dark matter candidate $A_H$ yields a large missing energy signal. Upon the discovery of the Higgs boson at 125 GeV, we study the discovery sensitivity of the final state $ pp \to b\bar{b} l+\rlap{\,/}{E}_T$ to probe the model at the LHC. We find that the standard model backgrounds are manageable by applying suitable kinematic cuts. The LHC running at $\sqrt{s}=14$ TeV with a 100/fb total luminosity is sensitive to the model if the symmetry breaking scale $f$ is below about 850 GeV.

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