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Chung Kao

Publications and source records attributed to Chung Kao.

At least 55 records · Page 3Linked to original sources

Phenomenology of a String-Inspired Supersymmetric Model with Inverted Scalar Mass Hierarchy

Supersymmetric (SUSY) models with heavy sfermions ($m_{\tilde{f}} \sim 10$ TeV) in the first two generations and the third generation sfermion masses below 1 TeV can solve the SUSY flavor and the CP problems as well as satisfy naturalness constraints. We study the phenomenology of a string-inspired scenario and compare it with the minimal supergravity unified model (mSUGRA). The SUSY trilepton signature at the upgraded Tevatron, the $b\to sγ$ branching fraction and the neutralino dark matter relic density in this model can differ significantly from the mSUGRA model.

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Parity Violating Asymmetries in Top Pair Production at Hadron Colliders

We study loop-induced parity violating asymmetries in the strong production of polarized top quark pairs at $pp$ and $p \bar p$ colliders. The electroweak ${\cal O}(α)$ corrections to the helicity amplitudes of $q \bar q \to t \bar t$ and $gg \to t \bar t$ are evaluated in a two Higgs doublet model (2HDM) and the minimal supersymmetric standard model (MSSM). While observables in top quark pair production receive little contribution from standard electroweak interactions, it is possible that they can be significantly enhanced in a 2HDM and the MSSM. We find that the one-loop MSSM electroweak corrections can generate parity violating asymmetries in the total production rate of left- and right-handed top quark pairs up to about 1.7% at the upgraded Tevatron ($\sqrt{S}=2$ TeV) and 3% at the LHC ($\sqrt{S}=14$ TeV).

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Trilepton Signature of Minimal Supergravity at the Upgraded Tevatron

The prospects for detecting trilepton events ($\ell = e$ or $μ$) from chargino-neutralino ($χ^\pm_1 χ^0_2$) associated production are investigated for the upgraded Fermilab Tevatron Collider in the context of the minimal supergravity model (mSUGRA). In some regions of parameter space, $χ^\pm_1$ and $χ^0_2$ decay dominantly into final states with $τ$ leptons and the contributions from $τ-$leptonic decays enhance the trilepton signal substantially when soft cuts on lepton transverse momenta are used. Additional sources of the mSUGRA trilepton signal and dominant irreducible backgrounds are discussed. The dilepton ($\ell^+\ell^-$) invariant mass distribution near the endpoint is considered as a test of mSUGRA mass relations. Discovery contours for $p\bar{p} \to 3\ell +X$ at 2 TeV with an integrated luminosity of 2 fb$^{-1}$ to 30 fb$^{-1}$ are presented in the mSUGRA parameter space of $(m_0,m_{1/2})$ for several choices of $\tanβ$.

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Production of Z Boson Pairs via Gluon Fusion in the Minimal Supersymmetric Model

We present the full one-loop calculation for $gg \to ZZ$ in the Minimal Supersymmetric Model (MSSM) including nonresonant contributions from the squark loop diagrams and provide analytical expressions for the helicity amplitudes. The one-loop process $gg \to ZZ$ via quark loops has been calculated in the Standard Model. In supersymmetric models, additional contributions arise from squark loops. In some regions of the MSSM parameter space, the top and bottom squark loops can make important contributions to the diagrams involving Higgs bosons. The heavy Higgs scalar (H) might be detected at the Large Hadron Collider via $gg \to H \to ZZ$ for $\tanβ\alt 5$.

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gamma nu -> gamma gamma nu and crossed processes at energies below m_W

The cross sections for the processes $γν\to γγν$, $γγ\toγν\barν$ and $ν\barν\toγγγ$ are calculated for a range of center of mass energies from below $m_e$ to considerably above $m_e$, but much less than $m_W$. This enables us to treat the neutrino--electron coupling as a four--Fermi interaction and results in amplitudes which are electron box diagrams with three real photons and one virtual photon at their vertices. These calculations extend our previous low--energy effective interaction results to higher energies and enable us to determine where the effective theory is reliable.

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Trilepton Signal of Minimal Supergravity at the Tevatron Including $τ$-lepton Contributions

The trilepton signal with missing transverse energy ($3\ell+\notE_T$, with $\ell = e$ or $μ$) from chargino-neutralino ($χ^\pm_1 χ^0_2$) associated production and decays is studied for the upgraded Fermilab Tevatron Collider with 2 TeV center of mass energy and integrated luminosity of 2 fb$^{-1}$ (MI) to 20 fb$^{-1}$ (TeV33). In some regions of parameter space in the minimal supergravity model, $χ^\pm_1$ and $χ^0_2$ decay dominantly into final states with $τ$ leptons via real or virtual $\tildeτ_1$ sleptons. The contributions from $τ-$leptonic decays increase the trilepton signal from $χ^\pm_1 χ^0_2$ by at least a factor of two when soft but realistic cuts on lepton transverse momenta are used. With the Main Injector, a trilepton signal can be detected at $\tanβ\equiv v_2/v_1 \sim 3$ for universal masses $m_{1/2}, m_0 \alt$ 200 GeV.

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The Search for Higgs Bosons of Minimal Supersymmetry at the LHC

The prospects for discovering neutral Higgs bosons in the minimal supersymmetric model (MSSM) and in the minimal supergravity model at the LHC are investigated. Two special discovery channels are discussed: (i) the decay mode of the MSSM CP-odd Higgs boson into photon pairs, and (ii) the decays of neutral Higgs bosons into muon pairs in the MSSM as well as in the minimal supergravity model.

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Effective Lagrangians and low energy photon-photon scattering

We use the behavior of the photon-photon scattering for photon energies $ω$ less than the electron mass, $m_e$, to examine the implications of treating the Euler-Heisenberg Lagrangian as an effective field theory. Specifically, we determine the $ω^2/m_e^2$ behavior of the scattering amplitude predicted by including one-loop corrections to the Euler-Heisenberg effective Lagrangian together with the counterterms required by renomalizability. This behavior is compared with the energy dependence obtained by expanding the exact QED photon-photon scattering amplitude. If the introduction of counterterms in the effective field theory is restricted to those determined by renormalizability, the $ω^2/m_e^2$ dependences of the two expansions differ.

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Relic Density of Neutralino Dark Matter in Supergravity Models

We calculate the relic density ($Ω_{χ^0_1} h^2$) of neutralino dark matter in supergravity models with radiative electroweak symmetry breaking, for values of the Higgs sector parameter $\tanβ\equiv v_2/v_1$ between the two infrared fixed points of the top quark Yukawa coupling, $\tanβ\simeq 1.8$ and $\tanβ\simeq 56$. For $\tanβ\alt 10$, the CP-odd Higgs pseudoscalar ($A^0$) and the heavier CP-even Higgs scalar ($H^0$) are much heavier than the lightest neutralino ($χ^0_1$), and the annihilation cross section of $χ^0_1$ pairs is significantly enhanced only in the vicinity of the lighter CP-even Higgs boson ($h^0$) and the $Z$ boson poles. For $\tanβ\agt 40$, $m_A$ and $m_H$ become comparable to $2m_{χ^0_1}$, and the neutralino annihilation cross section can be significantly enhanced by the poles of the $H^0$ and the $A^0$ as well. For a cosmologically interesting relic density, $0.1 \alt Ω_{χ^0_1} h^2 \alt 0.5$, we find that the supergravity parameter space is tightly constrained. The mass spectra for supersymmetric particles are presented for unification mass parameters in the cosmologically interesting region.

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Detecting the Higgs Bosons of Minimal Supergravity with Muon Pairs

The prospects at the CERN LHC are investigated for the discovery via decays into muon pairs of neutral Higgs bosons in the minimal supergravity model. Promising results are found for the CP-odd pseudoscalar ($A^0$) and the heavier CP-even scalar ($H^0$) Higgs bosons at large $\tanβ\equiv v_2/v_1$. For $\tanβ\agt 10$ and 100 GeV $\alt m_A,m_H \alt$ 550 GeV, the $A^0$ and the $H^0$ masses could be precisely reconstructed from the dimuon invariant mass.

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The Muon Electric Dipole Moment

The electric dipole moment of the muon ($d_μ$) is evaluated in a two Higgs doublet model with a softly broken discrete symmetry. For $\tanβ\equiv |v_2|/|v_1| \sim 1$, contributions from two loop diagrams involving the $t$ quark and the $W$ boson dominate; while for $\tanβ\gsim 10$, contributions from two loop diagrams involving the $b$ quark and the $τ$ lepton are dominant. For $8 \gsim \tanβ\gsim 4$, significant cancellation occurs among the contributions from two loop diagrams and the one loop contribution dominates for $\tanβ\sim 7$. For $\tanβ\gsim 15$, the calculated $d_μ$ can be close to the reach of a recently proposed experiment at the Brookhaven National Laboratory.

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The Electric Dipole Moment of the Muon in a Two Higgs Doublet Model

The electric dipole moment of the muon ($d_μ$) is evaluated in a two Higgs doublet model with a softly broken discrete symmetry. The leading contributions from both one loop and two loop diagrams are considered. For $\tanβ\equiv |v_2|/|v_1|$ close to one, contributions from two loop diagrams involving the $t$ quark and the $W$ boson dominate; while for $\tanβ\agt 10$, contributions from two loop diagrams involving the $b$ quark and the $τ$ lepton are dominant. In these two regions, $d_μ\approx (m_μ/m_e) d_e$. For $8 \agt \tanβ\agt 4$, significant cancellation occurs among the contributions from two loop diagrams and the one loop contribution dominates for $\tanβ\sim 7$. For $\tanβ\agt 15$, the calculated $d_μ$ can be close to the reach of a recently proposed experiment at the Brookhaven National Laboratory.

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Parity Violation in Bottom Quark Pair Production at Polarized Hadron Colliders

Parity violation induced by the chromo-anapole form factor of the bottom quark, generated from weak corrections, is studied in polarized hadron collisions. The forward-backward asymmetry in the bottom quark pair production at polarized $pp$ and $p\bar{p}$ colliders is evaluated in the Standard Model and in a Two Higgs Doublet Model to examine the effects of parity violation. In the models studied, promising results are found for polarized $p\bar{p}$ colliders.

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Simultaneous Search for Two Higgs Bosons of Minimal Supersymmetry at the LHC

The prospects of detecting the CP-odd Higgs pseudoscalar ($A$) in the minimal supersymmetric model via its decay into a $Z$ boson and the lighter CP-even Higgs scalar ($h$) at the CERN Large Hadron Collider are investigated. The final state of $Z \to l^+l^-$ and $h \to b\bar{b}$, may provide a promising way to simultaneously detect the $A$ and the $h$. The compact muon solenoid detector performance is adopted for a realistic study of observability. In this discovery channel, the masses of the $h$ and the $A$ can be reconstructed. The impact of supersymmetric decay modes is considered.

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A Two Higgs Doublet Model for the Top Quark

A two Higgs doublet model with special Yukawa interactions for the top quark and a softly broken discrete symmetry in the Higgs potential is proposed. In this model, the top quark is much heavier than the other quarks and leptons because it couples to a Higgs doublet with a much larger vacuum expectation value. The electric dipole moment (EDM) of the electron is evaluated with loop diagrams of the third generation fermions as well as the charm quark. The electron EDM is significantly enhanced for a naturally large $\tanβ\equiv |v_2|/|v_1|$.

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SUPERSYMMETRY REACH OF AN UPGRADED TEVATRON COLLIDER

We examine the capability of a $\sqrt{s}=2$ TeV Tevatron $p\bar p$ collider to discover supersymmetry, given a luminosity upgrade to amass $25\ fb^{-1}$ of data. We compare with the corresponding reach of the Tevatron Main Injector ($1\ fb^{-1}$ of data). Working within the framework of minimal supergravity with gauge coupling unification and radiative electroweak symmetry breaking, we first calculate the regions of parameter space accessible via the clean trilepton signal from $\tw_1\tz_2\to 3\ell +\eslt$ production, with detailed event generation of both signal and major physics backgrounds. The trilepton signal can allow equivalent gluino masses of up to $m_{\tg}\sim 600-700$ GeV to be probed if $m_0$ is small. If $m_0$ is large, then $m_{\tg}\sim 500$ GeV can be probed for $μ<0$; however, for $μ>0$ and large values of $m_0$, the rate for $\tz_2\to\tz_1\ell\bar{\ell}$ is suppressed by interference effects, and there is {\it no} reach in this channel. We also examine regions where the signal from $\tw_1\overline{\tw_1}\to \ell\bar{\ell}+\eslt$ is detectable. Although this signal is background limited, it is observable in some regions where the clean trilepton signal is too small. Finally, the signal $\tw_1\tz_2\to jets+\ell\bar{\ell} +\eslt$ can confirm the clean trilepton signal in a substantial subset of the parameter space where the trilepton signal can be seen. We note that although the clean trilepton signal may allow Tevatron experiments to identify signals in regions of parameter space beyond the reach of LEP II, the dilepton channels generally probe much the same region as LEP II.

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DISCOVERING THE HIGGS BOSONS OF MINIMAL SUPERSYMMETRY WITH MUONS

The prospects of detecting neutral Higgs bosons in the minimal supersymmetric model via their decays into muon pairs at the LHC are investigated. The CMS detector performance is adopted for a realistic study of observability. It is found that the muon pair decay mode might provide a very promising channel to search for the neutral Higgs bosons of minimal supersymmetry. This channel will allow precise mass reconstruction for neutral Higgs bosons.

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Parity Violation in Top Quark Pair Production at the Fermilab Tevatron Collider

The leading weak corrections to the production of top quark pairs via $q\bar{q} \to t\bar{t}$ in $p\bar{p}$ collisions are evaluated. The chromo-anapole form factor of the top quark and effects of parity violation are studied in the Standard Model (SM) and the Minimal Supersymmetric Model (MSSM). The parity violation effect in $q\bar{q} \to g \to t\bar{t}$ from the SM weak corrections is found to be very small. In the MSSM, the effects of parity violation can be enhanced for $\tanβ\equiv v_2/v_1 > \sqrt{m_t/m_b}$.

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