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Darwin Chang

Publications and source records attributed to Darwin Chang.

At least 37 records · Page 2Linked to original sources
arXiv2000

The effects of spin fluctuations on the tunneling spectroscopy in High-T_c Superconductors

We investigate the effects of spin fluctuations on the tunneling spectra of the NS junction. In the high junction resistance limit, the dip/hump structure observed in ARPES data for the high Tc superconductors is reproduced in a RPA treatment of the t-t'-J model. It is shown that the dip/hump structure weakens as doping increases as reflected in the data. In the other limit, we predict that the zero bias Andreev peak can coexist with the dip/hump structure. Furthermore, the c-axis tunneling spectra is found to be very similar to a recent STM data once these fluctuations are included.

cond-mat.supr-con↗
arXiv2000

Challenges in Hyperon Decays

We give an personal overview of some of the unsolved problems related to hyperon decays. We cover nonleptonic decays, radiative decays and magnetic moments. Some of the theoretical issues are also touched upon.

hep-ph↗
arXiv2000

Ruling out the Weinberg Model of Spontaneous CP Violation

There have been many declarations of the death of the Weinberg model of spontaneous CP violation. Previous studies, before the recent measurements of $ε'/ε$, indicated that the model could not accommodate the experimental values on $ε$ in $K^0-\bar K^0$ mixing, the neutron electric dipole moment (EDM), the branching ratio of $b\to s γ$ and the upper limit on $ε'/ε$. We point out that these studies were based on optimistic estimates of the uncertainties in the calculations and that when more realistic estimates of these errors are used the Weinberg model cannot be conclusively ruled out from these considerations alone. Here we use these realistic error estimates to analyze the present situation of the Weinberg model. The latest results from Belle and BaBar on $\sin 2 β$ allow the small values of this parameter which occur naturally in the Weinberg model. However, in this model, the recently measured value of $\Re(ε'/ε) = (1.92\pm 0.25)\times 10^{-3}$ cannot be made compatible with the branching ratio $B(b\to s γ) = (3.15\pm0.54)\times 10^{-4}$. As a result we conclude that the Weinberg model is now confidently and conservatively ruled out.

hep-ph↗
arXiv2000

The Neutron Electric Dipole Moment and CP-violating Couplings in the Supersymmetric Standard Model without R-parity

We analyze the neutron electric dipole moment (EDM) in the Minimal Supersymmetric Model with explicit R-parity violating terms. The leading contribution to the EDM occurs at the 2-loop level and is dominated by the chromoelectric dipole moments of quarks, assuming there is no tree-level mixings between sleptons and Higgs bosons or between leptons and gauginos. Based on the experimental constraint on the neutron EDM, we set limits on the imaginary parts of complex couplings ${λ'}_{ijk}$ and $λ_{ijk}$ due to the virtual b-loop or tau-loop.

hep-ph↗
arXiv2000

Vector Quark Model and B to X_s gamma Decay

We study the $B$ meson radiative decay $B \to X_s γ$ in the vector quark model. Deviation from the Standard Model arises from the non-unitarity of the charged current KM matrix and related new FCNC interactions. We establish the relation between the non-unitarity of charged current mixing matrix and the mixing among the vector quark and the ordinary quarks. We also make explicitly the close connection between this non-unitarity and the flavor changing neutral currents. The complete calculation including leading logarithmic QCD correction is carefully carried out. Using the most updated data and the NLO theoretical calculation, the branching fraction of the observed $B$ meson radiative decay places a limit on the mixing angles as stringent as that from the process $B\to X μ\barμ$.

hep-ph↗
arXiv2000

Pseudo-Dirac Neutrinos

We propose a scheme in which a pseudo-Dirac structure for three family of light neutrinos is generated naturally. An extended Higgs sector with a majoron is used for the generation of the leptonic number violating neutrino Majorana mass. The resultant neutrino mass matrix could easily fit all available experimental data. We discuss relevant constraints on the scales involved for the model to be phenomenologically viable.

hep-ph↗
arXiv1999

Radiatively Induced Neutrino Majorana Masses and Oscillation

We review and remark on models of radiatively induced neutrino Majorana masses and oscillations. It is pointed out that while the models are capable of accounting for the observed solar and atmospheric neutrino oscillation, some of them can also induce neutrinoless double beta decay and $μ^-$ -- $e^+$ conversion in nuclei large enough to be potentially observable in the near future.

hep-ph↗
arXiv1999

Additional two-loop contributions to electric dipole moments in supersymmetric theories

We calculate the two-loop contributions to the electric dipole moments of the electron and the neutron mediated by charged Higgs in a generic supersymmetric theories. The new contributions are originated from the potential CP violation in the trilinear couplings of the charged Higgs bosons to the scalar-top or the scalar-bottom quarks. These couplings did not receive stringent constraints directly. We find observable effects for a sizeable portion of the parameter space related to the third generation scalar-quarks in the minimal supersymmetric standard model.

hep-ph↗
arXiv1999

Fitting Precision Electroweak Data with Exotic Heavy Quarks

The 1999 precision electroweak data from LEP and SLC persist in showing some slight discrepancies from the assumed standard model, mostly regarding $b$ and $c$ quarks. We show how their mixing with exotic heavy quarks could result in a more consistent fit of all the data, including two unconventional interpretations of the top quark.

hep-ph↗
arXiv1999

New two-loop contribution to electric dipole moment in supersymmetric theories

We calculate a new type of two-loop contributions to the electric dipole moments of the electron and neutron in supersymmetric theories. The new contributions are originated from the potential CP violation in the trilinear couplings of the Higgs bosons to the scalar-top or the scalar-bottom quarks. These couplings were previously very weakly constrained. The electric dipole moments are induced through a mechanism analogous to that due to Barr and Zee. We find observable effects for a sizeable portion of the parameter space related to the third generation scalar-quarks in the minimal supersymmetric standard model which cannot be excluded by earlier considerations.

hep-ph↗
arXiv1998

Preformed Pairs, SU(2) Slave-boson Theory, and High T_c Superconductors

A preformed-pair model was considered. The quantum disordered phase of the d-wave superconducting state was obtained by turning on an on-site repulsion. The low energy effective theory of the quantum disordered phase was derived, which was found to be qualitatively equivalent to the low energy effective theory of the SU(2) slave-boson theory in the staggered-flux phase. Many physical properties of the disordered phase at low energies were obtained, such as the Mott insulator property at zero doping, spin gap and low superfluid density at low doping.

cond-mat.supr-con↗
arXiv1998

On A New Class of Models for Soft CP Violation

We elaborate on a new class of models proposed recently by us and compare with another class proposed by Georgi and Glashow(GG). The models can be roughly classified as the righted-handed (or left-handed) models in our (or GG's) case. Both classes of models use softly broken CP symmetry to suppress tree level KM phase as well as the strong CP phase. The measured value of the CP-violating parameter $ε$ are accounted for by employing a new heavy sector of scalars and vectorial fermions. The models can be milliweak or superweak in nature depending on the scale of the heavy sector. We review the phenomenology of the right-handed models and compare with the left-handed models.

hep-ph↗
arXiv1998

Alternative Interpretation of the Tevatron Top Events

We propose an alternative interpretation of the top events discovered at the Tevatron in 1995. Given that the charge of the $b$ quark jet cannot be measured for the whole sample with certainty, the signal can be due to a quark of charge -4/3 at the reported mass, i.e. 174 GeV, while the top quark is actually heavier, say above 230 GeV. We point out in this paper that such a scenario is actually hinted at by the latest precision electroweak measurements of $Z$ decay. To rule out this possibility in the future, the $b$ quark jet charge analysis has to become definitive, or the single "top" production cross section has to be measured.

hep-ph↗
arXiv1998

CP--odd Correlation in the Decay of Neutral Higgs Boson into $ZZ$, $W^+W^-$, or $t{\bar t}$

We investigate the possibility of detecting CP--odd angular correlations in the various decay modes of the neutral Higgs boson including the modes of a $ZZ$ pair, a $W^+W^-$ pair, or a heavy quark pair. It is a natural way to probe the CP character of the Higgs boson once it is identified. Final state interactions (i.e. the absorptive decay amplitude) is not required in such correlations. As an illustrative example we take the fundamental source of the CP nonconservation to be in the Yukawa couplings of the Higgs boson to the heavy fermions. A similar correlation in the process $e^+e^- \to l^+ l^- H$ is also proposed. Our analysis of these correlations will be useful for experiments in future colliders such as LEP II, SSC, LHC or NLC.

hep-ph↗
arXiv1998

CP Violating Observables in $e^-e^+ \to W^-W^+$

We consider various integrated lepton charge-energy asymmetries and azimuthal asymmetries as tests of CP violation in the process $e^-e^+ \to W^-W^+$. These asymmetries are sensitive to different linear combinations of the CP violating form factors in the three gauge boson $W^-W^+$ production vertex, and can distinguish dispersive and absorptive parts of the form factors. It makes use of purely hadronic and purely leptonic modes of $W$'s decays as well as the mixed modes. The techniques of using the kinematics of jets or missing momentum to construct CP--odd observables are also employed. These CP violating observables are illustrated in the generalized Left-Right Model and the Charged Higgs Model.

hep-ph↗
arXiv1998

CP Violation in top pair production at an e^+e^- collider

We investigate possible CP violating effects in $e^+e^-$ annihilation into top quark pairs. One of the interesting observable effects is the difference in production rates between the two CP conjugate polarized states $t_L\bar t_L$ and $t_R\bar t_R$. The result is an asymmetry in the energy spectra of the lepton and the anti-lepton from the heavy quark decays. Another CP-odd observable is the up-down asymmetry of the leptons with respect to the reaction plane. These two asymmetries measure complementarily the absorptive and dispersive form factors of the electric dipole moment. Finally, as an illustration, we calculate the size of the CP violating form factors in a model where the CP nonconservation originates from the Yukawa couplings of a neutral Higgs boson.

hep-ph↗