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T. Han

Publications and source records attributed to T. Han.

At least 55 records · Page 3Linked to original sources
arXiv2001

Top-quark spin correlation at Linear Colliders with anomalous couplings

We investigate the feasibility of probing anomalous top-quark couplings of $Wtb$, $Z t \bar{t}$, and $γt \bar{t}$ in terms of an effective Lagrangian with dimension-six operators at future $e^+e^-$ linear colliders with a c. m. energy $\sqrt s \sim 500-800$ GeV. We first examine the constraints on these anomalous couplings from the $Z\to b \bar{b}$ data at LEP I and from unitarity considerations. We then consider in detail the effects of anomalous couplings on $t \bar{t}$ spin correlations in the top-pair production and decay with three spin bases: the helicity, beamline and off-diagonal bases. Our results show that the polarized beams are more suitable for exploring the effects of different new operators. For polarized beams, the helicity basis yields the best sensitivity.

hep-ph↗
arXiv2001

Top-quark rare decay $t\to c h$ in R-parity-violating SUSY

The flavor-changing top-quark decay $t\to c h$, where $h$ is the lightest CP-even Higgs boson in the minimal supersymmetric standard model, is examined in the R-parity-violating supersymmetric model. Within the existing bounds on the relevant R-parity-violating couplings, the branching fraction for $t\to c h$ can be as large as about $10^{-5}$ in some part of the parameter space.

hep-ph↗
arXiv2001

CP-Violating Phases in SUSY, Electric Dipole Moments, and Linear Colliders

We reexamine large CP-violating phases in the general Minimal Supersymmetric Standard Model, as well as more restricted models. We perform a detailed scan over parameter space to find solutions which satisfy the current experimental limits on the electric dipole moments of the electron, neutron and $^{199}$Hg atom, exploring the allowed configurations of phases and masses, and we attempt to quantify the level of tuning of the parameters necessary to populate the regions of cancellations. We then consider the measurement of CP-violating phases at a future linear collider. We find that measurements of chargino and neutralino masses and production cross-sections allow for a determination of $ϕ_1$(the phase of $M_1$) to a precision of $π/30$, while the EDM constraints require that $θ_μ$ be too small to be measured. Using the EDM constraints we find that the CP-even model parameters and the phase $ϕ_1$ can be determined at a Linear Collider with $400 \gev$ c.m. energy. As long as some information on the size of $|μ|$ is included in the observables, a measurement of $ϕ_1$ is guaranteed for $ϕ_1 > π/10$. To unambiguously identify CP violation, we construct CP-odd kinematical variables at a linear collider. However, the CP asymmetries are rather small, typically about $0.1-1.5%$, and it will be challenging to experimentally observe the predicted asymmetries.

hep-ph↗
arXiv2000

CP-violating ZZh Coupling at e+e- Linear Colliders

We study the general Higgs-weak boson coupling with CP-violation via the process e+e- -> f \bar f h. We categorize the signal channels by sub-processes Zh production and ZZ fusion and construct four CP asymmetries by exploiting polarized e+e- beams. We find complementarity among the sub-processes and the asymmetries to probe the real and imaginary parts of the CP-violating form factor. Certain asymmetries with unpolarized beams can retain significant sensitivity to the coupling. We conclude that at a linear collider with high luminosity, the CP-odd ZZh coupling may be sensitively probed via measurements of the asymmetries.

hep-ph↗
arXiv2000

Cosmology and Hierarchy in Stabilized Warped Brane Models

We examine the cosmology and hierarchy of scales in models with branes immersed in a five-dimensional curved spacetime subject to radion stabilization. When the radion field is time-independent and the inter-brane spacing is stabilized, the universe can naturally find itself in the radiation-dominated epoch. This feature is independent of the form of the stabilizing potential. We recover the standard Friedmann equations without assuming a specific form for the bulk energy-momentum tensor. In the models considered, if the observable brane has positive tension, a solution to the hierarchy problem requires the presence of a negative tension brane somewhere in the bulk. We find that the string scale can be as low as the electroweak scale. In the situation of self-tuning branes where the bulk cosmological constant is set to zero, the brane tensions have hierarchical values. In the case of a polynomial stabilizing potential no new hierarchy is created.

hep-ph↗
arXiv2000

$\tanβ$ Determination From Heavy Higgs Boson Production at Linear Colliders

We study the production at future $e^+e^-$ linear colliders of the heavy neutral Higgs bosons $H$ and $A$ of the minimal supersymmetric standard model in association with top and bottom quarks. The cross sections have a strong dependence on the parameter $\tanβ$, and thus provide a good way to determine it. At a linear collider with $\sqrt s = 0.5-1$ TeV and expected integrated luminosities, we find significant sensitivities for determining $\tanβ$. In the Supergravity scenario, the sensitivity is particularly strong for $\tanβ> 10$, reaching a 15% or better measurement. In the general MSSM scenario, the interplay between the $4b$ and $4t$ channels results in a good determination for $\tanβ< 10$, while the sensitivity is weakened for higher values of $\tanβ$.

hep-ph↗
arXiv2000

Top-quark couplings to TeV resonances at future lepton colliders

We study the processes $W_L W_L \to t \bar t$ and $W_L Z_L \to t\bar b (\bar t b)$ at future lepton colliders as probes of the couplings of the top quark to resonances at the TeV scale. We consider the cases in which the dominant low energy feature of a strongly interacting electroweak symmetry breaking sector is either a scalar or a vector resonance with mass near 1 TeV. We find that future lepton colliders with high energy and high luminosity have great potential to sensitively probe these physics scenarios. In particular, at a 1.5 TeV linear collider with an integrated luminosity of 200 fb$^{-1}$, we expect about 120 events for either a scalar or a vector to decay to $t\bar t, t \bar b$. Their leading partial decay widths, which characterize the coupling strengths, can be statistically determined to about 10% level.

hep-ph↗
arXiv2000

Higgs Boson Decays to tau-pairs in the s-channel at a Muon Collider

We study the observability of the $\tautau$ decay mode of a Higgs boson produced in the $s$-channel at a muon collider. We find that the spin correlations of the $\tautau$ in $τ\to πν_τ, ρν_τ$ decays are discriminative between the Higgs boson signal and the Standard Model background. Observation of the predicted distinctive distribution can confirm the spin-0 nature of the Higgs resonance. The relative coupling strength of the Higgs boson to $b$ and $τ$ can also be experimentally determined.

hep-ph↗
arXiv2000

Physics Potential of a Tevatron Tripler

We explore the capabilities for new physics discovery in proton-antiproton collisions at 5.4 TeV center-of-mass energy with luminosity $10^{33} cm^{-2} s^{-1}$ at a Tripler upgrade of the Tevatron collider. The prospects are robust for the usual Higgs boson and supersymmetry benchmarks. With an integrated luminosity of 40 fb$^{-1}$, discoveries at 5$σ$ could be made for a standard Higgs boson of mass $\alt 680$ GeV (600 GeV for 10 fb$^{-1}$), a lighter chargino of mass $\alt 380$ GeV, and an extra $Z$ boson of mass $\alt 2.6$ TeV; four-fermion contact interactions from new physics with scale $\alt 74$ TeV could be excluded at the 95% confidence level.

hep-ph↗
arXiv2000

Top-quark decay via R-parity violating interactions at the Tevatron

We consider the top-quark decay $t\to \tilde τb$ and $t\to τb \tildeχ^0_1$ via explicit R-parity violating interactions in SUSY theories. We discuss the observability of those channels at the Fermilab Tevatron collider. The existing Tevatron data indicate a 95% confidence level upper bound on the coupling to be less than 0.94(0.63) for a long-lived (short-lived) $\tildeχ^0_1$ with $m_{\tildeτ}=70$ GeV. At Tevatron Run II with an integrated luminosity of 2 (10) fb$^{-1}$, one can obtain a 2$σ$ constraint as 0.38 (0.24) for a long-lived $\tildeχ^0_1$ and 0.29(0.19) for a short-lived $\tildeχ^0_1$, beyond the current indirect limit.

hep-ph↗
arXiv2000

Measuring CP Violating Phases at a Future Linear Collider

At a future Linear Collider one will be able to determine the masses of charginos and neutralinos and their pair production cross sections to high accuracies. We show how systematically including the cross sections into the analysis improves the measurement of the underlying mass parameters, including potential CP violating phases. In addition, we investigate how experimental errors will affect the determination of these parameters. We present a first estimate on the lower limit of observable small phases and on the accuracy in determining large phases.

hep-ph↗
arXiv1999

Diphoton Signals for Large Extra Dimensions at the Tevatron and CERN LHC

We analyze the potentiality of hadron colliders to search for large extra dimensions via the production of photon pairs. The virtual exchange of Kaluza--Klein gravitons can significantly enhance this processes provided the quantum gravity scale ($M_S$) is in the TeV range. We studied in detail the subprocesses $q \bar{q} \to γγ$ and $g g \to γγ$ taking into account the complete Standard Model and graviton contributions as well as the unitarity constraints. We show that the Tevatron Run II will be able to probe $M_S$ up to 1.5--1.9 TeV at 2$σ$ level, while the LHC can extend this search to 5.3--6.7 TeV, depending on the number of extra dimensions.

hep-ph↗
arXiv1999

Drell-Yan plus missing energy as a signal for extra dimensions

We explore the search sensitivity for signals of large extra dimensions at hadron colliders via the Drell-Yan process pp -> l+ l- + E_T(miss) X (l = e,mu) where the missing transverse energy is the result of escaping Kaluza-Klein gravitons. We find that one is able to place exclusion limits on the gravity scale up to 560 GeV at the Fermilab Tevatron, and to 4.0 (3.3) TeV at the CERN LHC, for n = 3 (4) extra dimensions.

hep-ph↗
arXiv1999

$e^+e^- \to t \bar t H$ with Non-standard Higgs Boson Couplings

We consider a general effective Lagrangian for couplings of a Higgs boson to the top-quark to dimension-six operators including CP violation effects. Constraints on some of them are derived from the $Z\to b\bar b$ data. We study the process $e^+e^- \to t\bar t H$ to probe the non-standard couplings. We find that at a linear collider with a c. m. energy $\sqrt s \sim 0.5-1.5$ TeV and a high luminosity of 10-1000 fb$^{-1}$, these non-standard couplings may be sensitively probed.

hep-ph↗
arXiv1999

Astrophysical Constraints on Large Extra Dimensions

In the Kaluza-Klein (KK) scenario with n large extra dimensions where gravity propagates in the 4+n dimensional bulk of spacetime while gauge and matter fields are confined to a four dimensional subspace, the light graviton KK modes can be produced in the Sun, red giants and supernovae. We study the energy-loss rates through photon-photon annihilation, electron-positron annihilation, gravi-Compton-Primakoff scattering, gravi-bremsstrahlung and nucleon-nucleon bremsstrahlung, and derive lower limits to the string scale M_S. The most stringent lower limit obtained from SN1987A leads to $M_S> 30 - 130$ TeV (2.1-9.2 TeV) for the case of two (three) large extra dimensions.

hep-ph↗
arXiv1998

Slepton Oscillation at e-gamma Colliders

We study the possible signature for lepton-flavor violation processes in the scalar-lepton sector of the minimal supersymmetric model at future electron-photon colliders. We find that an $\egm$ collider can provide a good opportunity to probe the mass differences and flavor mixing angles between the sleptons of the first generation and the other two generations for the right-handed as well as the left-handed sector. The sensitivity reach for lepton-flavor violation is comparable to that obtained at an $e^+e^-$ and $e^-e^-$ collider, and is significantly better than the current bound from low-energy rare processes such as $μ\to e γ$.

hep-ph↗
arXiv1998

Single Top Quark Production via FCNC Couplings at Hadron Colliders

We calculate single top-quark production at hadron colliders via the chromo-magnetic flavor-changing neutral current couplings $\bar tcg$ and $\bar tug$. We find that the strength for the anomalous $\bar tcg$ ($\bar tug$) coupling may be probed to $κ_c / Λ= 0.092 {TeV}^{-1}$ ($κ_u / Λ= 0.026 {TeV}^{-1}$) at the Tevatron with $2 {fb}^{-1}$ of data and $κ_c / Λ= 0.013 {TeV}^{-1}$ ($κ_u / Λ= 0.0061 {TeV}^{-1}$) at the LHC with $10 {fb}^{-1}$ of data. The two couplings may be distinguished by a comparision of the single top signal with the direct top and top decay signals for these couplings.

hep-ph↗