arXiv subjects

Jeonghyeon Song

Publications and source records attributed to Jeonghyeon Song.

At least 73 records · Page 4Linked to original sources

arXiv2007

Role of h --> eta eta in Intermediate-Mass Higgs Boson Searches at the Large Hadron Collider

The dominance of $h\to ηη$ decay mode for the intermediate mass Higgs boson is highly motivated to solve the little hierarchy problem and to ease the tension with the precision data. However, the discovery modes for $m_h \alt 150$ GeV, $h \to γγ$ and $W/Z h \to (\ellν/\ell \bar \ell) (b\bar b)$, will be substantially affected. In this Letter, we show that $h \to ηη\to 4b$ is complementary and we can use this decay mode to detect the intermediate Higgs boson at the LHC, via $Wh$ and $Zh$ production. Requiring at least one charged lepton and 4 $B$-tags in the final state, we can identify a clean Higgs boson signal for $m_h \alt 150$ GeV with a high significance and with a full Higgs mass reconstruction. We use the next-to-minimal supersymmetric standard model and the simplest little Higgs model for illustration.

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arXiv2007

Constraint of B_{d,s}-bar{B}_{d,s} mixing on warped extra-dimension model

Recent CDF measurement of the Bs-Bs oscillation frequency, at the Tevatron imposes significant constraint on various models for new physics. A warped extra-dimension model with custodial isospin symmetry accommodates the Bd-Bd mixing at tree level mainly through the Kaluza-Klein gluons. This is due to the misalignment between the bulk gauge eigenstates and the localized Yukawa eigenstates of the bulk fermions. We adopt the universal 5D Yukawa coupling model where all Yukawa couplings are of order one. The SM fermion mass spectra and mixings are controlled by the bulk Dirac mass parameters. With two versions of the hadronic parameter values, we investigate the implication of the observed BB mixings on this model. The CP-violating effects on the Bd system is shown to provide very strong constraint: The first Kaluza-Klein mass of a gluon has its lower bound about 3.7 TeV with 1 sigma uncertainty.

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arXiv2007

Light pseudoscalar eta and H->eta eta decay in the simplest little Higgs mode

The SU(3) simplest little Higgs model in its original framework without the so-called mu term inevitably involves a massless pseudoscalar boson eta, which is problematic for b-physics and cosmological axion limit. With the mu term introduced by hand, the eta boson acquires mass m_eta ~ mu, which can be lighter than half the Higgs boson mass in a large portion of the parameter space. In addition, the introduced mu term generates sizable coupling of H-eta-eta. The Higgs boson can dominantly decay into a pair of eta's especially when mH below the WW threshold. Another new decay channel of H->Z+eta can be dominant or compatible with H -> WW for mH above the Z+eta threshold. We show that the LEP bound on the Higgs boson mass is loosened to some extent due to this new H->eta eta decay channel as well as the reduced coupling of H-Z-Z. The Higgs boson mass bound falls to about 110 GeV for f=3-4 TeV. Since the eta boson decays mainly into a bb pair, H-> eta eta -> 4b and H-> Z eta -> Z bb open up other interesting search channels in the pursuit of the Higgs boson in the future experiments. We discuss on these issues.

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arXiv2006

Associated production of a single heavy T-quark in the littlest and simplest little Higgs models

The colored SU(2)-singlet heavy T-quark is one of the most crucial ingredients in little Higgs models, which is introduced to cancel the largest contribution of the SM top quark to the Higgs boson mass at one-loop level. In two representative little Higgs models, the littlest Higgs model and the SU(3) simplest Higgs model, we comprehensively study the single heavy T-quark production at Large Hadron Collider (LHC). After presenting the possibility of relatively light (~500 GeV) T-quark in the simplest little Higgs model, we consider all the relevant processes, the 2->2 process of qb->q'T, the 2->3 process of qg->q'Tb, the s-channel process of q bar(q)'->T bar{b}, and the gluon fusion process of gg->T bar{t}. We found that the 2->3 process can be quite important, as its cross section is about 30% of the 2->2 one and it is dominant in high p_T distributions. The s-channel and the gluon fusion processes also show distinctive features in spite of their suppressed cross sections. In the gluon fusion process of the simplest little Higgs model, for example, the pseudo-scalar contribution is rather dominant over the Higgs contribution for relatively light M_T.

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arXiv2006

A minimal supersymmetric scenario with only μat the weak scale

Inspired by split supersymmetry, we study a minimal supersymmetric scenario with only the Higgsino mass parameter $μ$ below the TeV scale. The motivation is to satisfy the gauge coupling unification and dark matter constraints with the minimal particle contents at the electroweak scale in supersymmetric models. With the neutral Higgsino as the lightest supersymmetric particle, we discuss the dark matter signals in both direct and indirect detection. We also discuss collider phenomenology associated with the two lightest neutralinos and the lightest chargino, which are almost degenerate in mass even after taking into account radiative corrections. Unfortunately, the collider signals may be very difficult for identifying such a scenario because the pions or leptons in the final state are too soft.

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arXiv2005

Probing the radion-Higgs mixing at photon colliders

In the Randall-Sundrum model, the radion-Higgs mixing is weakly suppressed by the effective electroweak scale. A novel feature of the gravity-scalar mixing would be a sizable three-point vertex of $\grav$-$h$-$ϕ$. We explore the potential of photon colliders, achieved by the laser backscattering technique, in probing the radion-Higgs mixing via the associated production of the radion with the Higgs boson. The advantage of photon colliders is the capability of adjusting the polarization of the incoming photons such that the signal of the spin-2 graviton exchange can be largely enhanced. The enhancement factor is shown to be about 5, except for small-$ξ$ region. We also study the corresponding backgrounds step-by-step in detail.

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arXiv2005

Hadronic production and decays of Charginos and Neutralinos in Split Supersymmetry

In the scenario recently proposed by Arkani-Hamed and Dimopoulos, the supersymmetric scalar particles are all very heavy, at least of the order of $10^9$ GeV but the gauginos, higgsino, and one of the CP-even neutral Higgs bosons remain light under a TeV. In addition to gluino production, the first SUSY signature would be pair production of neutralinos and/or charginos. Here we study, with explicit CP violation, the production of neutralinos and charginos at hadronic colliders as well as the possible decay modes. We notice that the branching ratio of the neutralino radiative decay can be sizable unlike the case with light sfermions. In particular, the decay of N3->N1+gamma has a branching ratio of the order O(1) percent. In colliders, it would give rise to signatures of an isolated single photon plus missing energy.

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arXiv2005

Kaluza-Klein masses of bulk fields with general boundary conditions in AdS$_5$

Recently bulk Randall-Sundrum theories with the gauge group $SU(2)_L \times SU(2)_R \times U(1)_{B-L}$ have drawn a lot of interest as an alternative to electroweak symmetry breaking mechanism. These models are in better agreement with electroweak precision data since custodial isospin symmetry on the IR brane is protected by the extended bulk gauge symmetry. We comprehensively study, in the $S^1/\ZZ$ orbifold, the bulk gauge and fermion fields with the general boundary conditions as well as the bulk and localized mass terms. Master equations to determine the Kaluza-Klein (KK) mass spectra are derived without any approximation, which is an important basic step for various phenomenologies at high energy colliders. The correspondence between orbifold boundary conditions and localized mass terms is demonstrated not only in the gauge sector but also in the fermion sector. As the localized mass increases, the first KK fermion mass is shown to decrease while the first KK gauge boson mass to increase. The degree of gauge coupling universality violation is computed to be small in most parameter space, and its correlation with the mass difference between the top quark and light quark KK mode is also studied.

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arXiv2003

Probing the Radion-Higgs mixing at hadronic colliders

In the Randall-Sundrum model, the radion-Higgs mixing is weakly suppressed by the effective electroweak scale. One of its novel features would be a sizable three-point vertex of KK graviton-Higgs-radion. We explored the potential of the Fermilab Tevatron and the CERN LHC in probing the radion-Higgs mixing via the associated production of the radion with the Higgs boson. The observation of the rare decay of the KK gravitons into a Higgs and a radion is then the direct and exclusive signal of the radion-Higgs mixing. We also studied all the partial decay widths of the KK gravitons in the presence of the radion-Higgs mixing, and found that if the mixing parameter is of order one, the decay rate into a radion and a Higgs boson becomes as large as that into a Higgs boson pair, with the branching ratio of order 0.001.

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arXiv2003

Muon anomalous magnetic moment and the heavy photon in a little Higgs model

In the Littlest Higgs model, we comprehensively study the phenomenology of the heavy photon A_H which is lightest, in most of the parameter space, among newly introduced heavy gauge bosons and top-like vector quark. Unexpected behavior is that lighter A_H suppresses the corrections to the electroweak precision observables. For the global symmetry breaking scale f~3TeV, the heavy photon can be light enough to be produced at the 500 GeV linear collider. Through the calculation of the one-loop correction to the muon anomalous magnetic moment, we show that even the light A_H with mass around 200 GeV results in the negligible contribution. This is consistent with the current inconclusive status of the theoretical calculation of the (g-2)_mu in the SM. The effects of the Littlest Higgs model on the process e+ e- -> mu+ mu- are also studied, which is one of the most efficient signals to probe the A_H.

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arXiv2003

A probe of the Radion-Higgs mixing in the Randall-Sundrum model at e^+ e^- colliders

In the Randall-Sundrum model, the radion-Higgs mixing is weakly suppressed by the effective electroweak scale. A novel feature of the existence of gravity-scalar mixing would be a sizable three-point vertex among the KK graviton, Higgs and radion. We study this vertex in the process e^+ e^- -> h phi, which is allowed only with a non-zero radion-Higgs mixing. It is shown that the angular distribution is a unique characteristic of the exchange of massive spin-2 gravitons, and the total cross section at the future e^+ e^- collider is big enough to cover a large portion of the parameter space where the LEP/LEP II data cannot constrain.

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arXiv2002

Top quark Kaluza-Klein mode mixing in the Randall-Sundrum bulk Standard Model and B -> X_s gamma

We study top quark Kaluza-Klein (KK) mode mixing in the Randall-Sundrum scenario with all the SM fermions and gauge bosons in the bulk. Even though the simple assumption of universal bulk fermion mass m_psi leads to the same KK mass spectrum for all the SM fermions and thus suppresses new contributions to the flavor changing neutral current and the rho parameter, large Yukawa coupling of the top quark generates the mixing among its KK modes and breaks the degeneracy: Unacceptably large contribution to rho occurs. In order to satisfy the rho constraint, we relax the model by assigning a different bulk fermion mass to SU(2) singlet bottom quark, and demonstrate that there exists some limited parameter space where the $Δρ$ constraint is satisfied. It is also shown that the current measurement of Br(B -> X_s + gamma) can be accommodated in this modified model, and one sigma level precision constrains the effective weak scale k_{EW} such as k_{EW} ~ 3 TeV for m_psi/k=-0.4, where k_{EW} is the warp-suppressed AdS_5 curvature.

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arXiv2002

Kaluza-Klein gravitino production with a single photon at e^+ e^- colliders

In a supersymmetric large extra dimension scenario, the production of Kaluza-Klein gravitinos accompanied by a photino at e^+ e^- colliders is studied. We assume that a bulk supersymmetry is softly broken on our brane such that the low-energy theory resembles the MSSM. Low energy supersymmetry breaking is further assumed as in GMSB, leading to sub-eV mass shift in each KK mode of the gravitino from the corresponding graviton KK mode. Since the photino decays within a detector due to its sufficiently large inclusive decay rate into a photon and a gravitino, the process e^+ e^- -> photino + gravitino yields single photon events with missing energy. Even if the total cross section can be substantial at sqrt(s)=500 GeV, the KK graviton background of e^+ e^- -> photon + graviton is kinematically advantageous and thus much larger. It is shown that the observable, sigma(e^-_L)-sigma(e^-_R), can completely eliminate the KK graviton background but retain most of the KK gravitino signal, which provides a unique and robust method to probe the supersymmetric bulk.

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arXiv2001

Z boson pair production at LHC in a stabilized Randall-Sundrum scenario

We study the Z boson pair production at LHC in the Randall-Sundrum scenario with the Goldberger-Wise stabilization mechanism. It is shown that comprehensive account of the Kaluza-Klein graviton and radion effects is crucial to probe the model: The KK graviton effects enhance the cross section of $g g \to Z Z$ on the whole so that the resonance peak of the radion becomes easy to detect, whereas the RS effects on the $q\bar{q} \to Z Z$ process are rather insignificant. The $p_T$ and invariant-mass distributions are presented to study the dependence of the RS model parameters. The production of longitudinally polarized Z bosons, to which the SM contributions are suppressed, is mainly due to KK gravitons and the radion, providing one of the most robust methods to signal the RS effects. The $1 σ$ sensitivity bounds on $(Λ_π, m_ϕ)$ with $k/M_{\rm Pl} =0.1$ are also obtained such that the effective weak scale $Λ_π$ of order 5 TeV can be experimentally probed.

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arXiv2001

A Phenomenological Study on Lepton Mass Matrix Textures

The three active light neutrinos are used to explain the neutrino oscillations. The inherently bi-large mixing neutrino mass matrix and the Fritzsch type, bi-small mixing charged lepton mass matrix are assumed. By requiring the maximal ν_μ-ν_τmixing for the atmospheric neutrino problem and the mass-squared difference approperiate for the almost maximal mixing solution to the solar neutrino problem, the following quantities are predicted: the ν_e-ν_μmixing, V_{e3}, CP violation in neutrino oscillations, and the effective electron-neutrino mass relevant to neutrinoless double beta decays.

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arXiv2001

Stau LSP and comparison with H^+(-) phenomenology

In supersymmetric models with explicit breaking of R-parity the lightest supersymmetric particle (LSP) may be the lightest stau, $\stau_1$. Such a scenario would provide a clear sign of R-parity violating SUSY, although its phenomenology may resemble that of a charged Higgs boson, $H^\pm$. We discuss various ways of distinguishing a LSP $\stau_1$ from $H^\pm$ at future colliders, and address the case of $\stau_1$ mimicking the signal for $H^\pm$. As an example we suggest that the recent L3 signal for $H^+H^-\to qq'qq'$ and $H^+H^-\to qq'τν_τ$ could be more easily explained by a LSP $\stau_1$.

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arXiv2001

A Model for Neutrino Warm Dark Matter and Neutrino Oscillations

The muon- and tau-neutrinos with the mass in the keV range, which are allowed in a low reheating temperature cosmology, can compose the warm dark matter of the universe. A model of four light neutrinos including the keV scale $ν_μ$ and $ν_τ$ is studied, which combines the seesaw mechanism and the Abelian flavor symmetry. The atmospheric neutrino anomaly is due to the $ν_μ-ν_τ$ oscillation. The solar neutrino problem is answered by the oscillation into the light sterile neutrino, where the SMA, LMA, and LOW-QVO solutions can be accommodated in our scenario.

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arXiv2001

Muon Anomalous Magnetic Moment (g-2)_mu and the Randall-Sundrum Model

We study the effects of the Kaluza-Klein gravitons in the Randall-Sundrum model on the recent BNL measurements of the muon (g-2) deviation from the standard model prediction. By examining the J-partial wave unitarity bounds of the elastic process gamma+gamma->gamma+gamma, the cut-off on the number of massive KK gravitons, n_c, has been introduced. We found that the recently measured (Delta a_mu) can be accommodated in the RS model, within the natural parameter space allowed by the perturbative unitarity. For example, dozens (hundreds) of the n_c for Lambda_pi=1-2 TeV (3 TeV) can explain the reported Delta a_mu.

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