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Darwin Chang

Publications and source records attributed to Darwin Chang.

At least 55 records · Page 3Linked to original sources
arXiv1998

Shifting $R_b$ with $A^b_{FB}$

Precision measurements at the $Z$ resonance agree well with the standard model. However, there is still a hint of a discrepancy, not so much in $R_b$ by itself (which has received a great deal of attention in the past several years) but in the forward-backward asymmetry $A^b_{FB}$ together with $R_b$. The two are of course correlated. We explore the possibilty that these and other effects are due to the mixing of $b_L$ and $b_R$ with one or more heavy quarks.

hep-ph↗
arXiv1998

PQCD study of the $B_{SL}$ and $n_c$ controversy in inclusive $B$ decays

We calculate the semileptonic branching ratio and the charm counting of inclusive $B$ meson decays using the perturbative QCD formalism. For the nonleptonic decays, we employ the modified factorization theorem, in which Wilson coefficients evolve with the characteristic scales of the decay modes. It is found that the decay rate of the single-charm mode $b\to c{\bar u}d$ is enhanced, and a lower $B_{SL}$ is obtained without increasing $n_c$. We predict a larger $b\to c τ\barν$ branching ratio compared to that from the conventional heavy-quark-effective-theory based operator product expansion. Our result of the $B$ meson lifetime is also consistent with the data.

hep-ph↗
arXiv1998

A Simple Charged Higgs Model of Soft CP Violation without Flavor Changing Neutral Currents

We propose a model of soft CP violation in which the CP violating mechanism naturally lies only in the charged Higgs sector. The charged Higgs mechanism not only accounts for the measured value of the CP-violating parameter $ε$ but also accommodates the current limits on $ε'/ε$. Our model naturally prevents tree-level Flavor-Changing Neutral Currents (FCNCs) of any kind. Unlike the Weinberg-Branco Three-Higgs Doublet Model, the deviation from the Standard Model rate for $b\to sγ$ is small. Furthermore, leading contributions to the electron (neutron) electric dipole moment are non-zero beginning at the three (two) loop level. Surprisingly similar to the Standard Kobayashi-Maskawa Model, our model is of milliweak character but with seemingly superweak phenomenology.

hep-ph↗
arXiv1998

Interplay Between Strong and Weak CP Phases

We discuss the subtle interplay between strong and weak CP phases that has often been ignored in the literature. We also point out the potentially important role that it plays in various models.

hep-ph↗
arXiv1997

Electron Electric Dipole Moment from CP Violation in the Charged Higgs Sector

The leading contributions to the electron (or muon) electric dipole moment due to CP violation in the charged Higgs sector are at the two-loop level. A careful analysis of the model-independent contribution is provided. We also consider specific scenarios to demonstrate how charged Higgs sector CP violation can naturally give rise to large electric dipole moments. Numerical results show that the electron electric dipole moment in such models can lie at the experimentally accessible level.

hep-ph↗
arXiv1996

New Limits on R-Parity Breakings in Supersymmetric Standard Models

New limits on couplings $λ^i_{jk}{''}$, which break both the baryon number and the $R$--parity, are derived by using a new mechanism that contributes to the neutron-anti-neutron oscillation. The constraints due to proton decay and its potential phenomenology are also reexamined.

hep-ph↗
arXiv1996

Triple Pseudoscalar Decay Mode, $Z \to AAA$

We analyze the production of triple pseudoscalar Higgs bosons in the decay channel of $Z\rightarrow AAA$ for light pseudoscalar bosons when the corresponding scalar boson is too heavy to be produced by $Z$ decay. Analytic results are obtained both at the tree level and at the one--loop level. The branching fraction can be as large as $10^{-5}$ which should be detectable at LEP.

hep-ph↗
arXiv1996

CP Violation in Hyperon Decays

The CP properties in hyperon decays are briefly reviewed. We discuss the general phenomenology and define CP odd observables in hyperon decays. With these observables, we discuss the predictions of some models and their observational potential.

hep-ph↗
arXiv1996

How to Observe the Interference Effects of Top quark polarizations at Tevatron

Using a simple analytic expression for $q \bar{q}, g g \rightarrow t \bar{t} \rightarrow b W^+ \bar{b} W^- \rightarrow b \bar{l} ν_l \bar{b} l' \bar{ν_{l'}}$ with the interference effects due to the polarizations of the $t$ and $\bar{t}$, we demonstrate how the effects can be measured at Tevatron at $3σ$ level.

hep-ph↗
arXiv1996

Solutions to the R_b, R_c and alpha_s puzzles by Vector Fermions

We propose two minimal extensions of Standard Model, both of which can easily accommodate the recent puzzling observations about the excess in $R_b$, the deficit in $R_c$ and the discrepancy in the low energy and high energy determinations of $α_{s}$. Each model requires three additional heavy vectorial fermions in order to resolve the puzzles. The current phenomenological constraints and the new potential phenomena are also discussed.

hep-ph↗
arXiv1995

On the Observation of Top Spin Correlation Effect at Tevatron

We propose to observe the top spin correlation effect at Tevatron through the measurement of the correlated asymmetries of the charged lepton momenta using the dilepton decay events of top and anti-top quark pairs. The possibility of reconstructing the events and observing the asymmetries at 3$σ$ level with projected luminosity of the Tevatron for Run II is demonstrated by simulation. The effect can provide the first direct measurement of the spin of a quark.

hep-ph↗
arXiv1995

Transfer Matrix Method in Sandpile Models

We present a transfer matrix method which is particularly useful for solving some classes of sandpile models. The method is then used to solve the deterministic nonabelian sandpile models for N=2 and N=3. The possibility of generalization to arbitrary N is discussed briefly.

cond-mat↗
arXiv1995

Exact Solution of an One Dimensional Deterministic Sandpile Model

Using the transfer matrix method, we give the exact solution of a deterministic sandpile model for arbitrary $N$, where $N$ is the size of a single toppling. The one- and two-point functions are given in term of the eigenvalues of an $N \times N$ transfer matrix. All the n-point functions can be found in the same way. Application of this method to a more general class of models is discussed. We also present a quantitative description of the limit cycle (attractor) as a multifractal.

cond-mat↗
arXiv1995

The Effect of Top Quark Polarization at Hadronic Colliders

We derive a simple analytic expression for q \bar{q}, g g -> t \bar{t} -> b W^+ \bar{b} W^- -> b \bar{l} ν_l \bar{b} l' \bar{ν_{l'}} for on shell intermediate states with the interference effects due to the polarizations of the t and \bar{t}. We then investigate how this effect may be measured at Tevatron or other hadronic colliders.

hep-ph↗
arXiv1994

CP Violation In Hyperon Decays Due To Left-Right Mixing

We consider CP violation due to left-right mixing in a class of Left-Right symmetric models and show that it leads to observable effects in hyperon decays. For S-wave, the contribution to CP violating asymmetry A of the polarization in hyperon and anti-hyperon decays is proportional to $ε'/ε$. While the tree level L-R operators contribution is constrained to be less than $ 10^{-5}$, the gluon penguin operator contribution can be as large as $10^{-4}$. For P-wave, the contribution is not directly related to $ε'/ε$. Its contribution to A can be larger and may reach $6\times 10^{-4}$ in $Λ\rightarrow pπ^-$. This is much larger than the value expected for A in the Standard Model.

hep-ph↗
arXiv1994

A Scheme for radiative CP violation

We present a simple model in which CP symmetry is spontaneously broken only after the radiative corrections are taken into account. The model includes two Higgs-boson doublets and two right-handed singlet neutrinos which induce the necessary non-hermitian interaction. To evade the Georgi-Pais theorem, some fine-tuning of coupling constants is necessary. However, we show that such fine-tuning is natural in the technical sense as it is protected by symmetry. Some phenomenological consequences are also discussed.

hep-ph↗
arXiv1994

CP violation in the cubic coupling of neutral gauge bosons

We investigate the CP violating form factor of the ZZZ$ and $ZZγ$ vertices in the pair production of $Z^0$ bosons. Useful observables in azimuthal distributions are constructed to probe $CP$ nonconservation which may originate from these vertices. A simple Two Higgs Model of $CP$ violation is used as an illustration.

hep-ph↗