arXiv subjects
Publications and source records attributed to Da-Xin Zhang.
We make a detailed analysis on the proton decay in a supersymmetric SO(10) model proposed by K.Babu, I.Gogoladze, P.Nath, and R. Syed. We introduce quark mixing, and find that this model can generate fermion mass without breaking the experimental bound on proton decay. We also predict large CKM unitarity violations.
We analyze the inclusive decay mode $B\to X_s+{\rm Missing Energy}$ in the unparticle model, where an unparticle can also serve as the missing energy. We use the Heavy Quark Effective Theory in the calculation. The analytical result of the decay width in the free quark limit and that of the differential decay rate to the order of $1/m_b^2$ are presented. Numerical results of the inclusive mode show interesting differences from those of the exclusive modes. Near the lower endpoint region, the $d_{\mathcal U}<2$ unparticle has very different behavior from the Standard Model particles.
We analyze the branching ratio and spectrum for the decay mode $K^+ \toπ^++{\not}E$(missing energy) in the unparticle model, where an unparticle can also serve as the missing energy. A vector unparticle can even mediate the $K^+ \to π^+ +ν\barν$, resulting complicated interference with the Standard Model.
We study the novel features in a model with Gauge Mediated Supersymmetry Breaking. If the messenger fields have positive R-parity, there will be new sources of flavor violations. We show that the dimension-5 operators will be quite important. When dressing these operators by wino-loops, the constraints on them by the present data are given.
The rare decay $b\to ss\bar{d}$ is studied in a vector quark model by adding the contributions from exotic vector-like quarks. We find that the contribution from box diagrams amounts to $10^{-9}$ in the branching ratio, while the $Z$-mediated tree level contribution is negligible.
The rare decay b -> s s bar{d} is studied in the supersymmetric standard model by considering the contribution from the chargino box diagrams. We find that this contribution amounts to 10^{-9} in the branching ratio.
A class of $|Δs|=1$ transition is analyzed in the $D_s^+-D^+$ and $\bar B_s^0-\bar B^0$ systems. Short distance Wilson coefficients are calculated within HQET. Novel features of the transitions are discussed. We find that these transitions are unobservable in the standard model.
Assuming one nonzero product of two $λ'$-type couplings and working in two different bases for the left-handed quark superfields, the neutral meson mixings are used to bound these products. We emphasize the strong basis dependence of the bounds: in one basis many products contribute to neutral meson mixings at tree level, while in the other these products except one contribute at 1-loop level only and the GIM mechanism takes place. Correspondingly, these bounds differ between bases by orders of magnitudes.
In the Standard Model, the box-diagram mediated decay $b\to s s \bar d$ is predicted to occur with an extremely small branching ratio of below $10^{-11}$, thus providing a safe testing ground for exposing new physics. We study this process in several Two Higgs Doublet Models and explore their parameter space, indicating where this process becomes observable.
For any new physics possibly emerging in the future B experiments, the problem is how to extract the signals from the SM background. We consider the decay $b\to s s\bar d$ which is very small in the SM. In the MSSM this decay is possibly accessible in the future experiments. In the supersymmetric models with R-parity violating couplings, this channel is not strictly constrained, thus being useful in obtaining bounds on the lepton-number violating couplings. A typical candidate for the suggested search is the $B^-\to K^-K^-π^+$ mode.
We discuss the issue of the local quark-hadron duality at high energies in two- and four-dimensional QCD. A mechanism of the dynamical realization of the quark-hadron duality in two- dimensional QCD in the limit of large number of colors, N_c -> \infty, (the 't Hooft model) is considered. A similar mechanism of dynamical smearing may be relevant in four-dimensional QCD. Although particular details of our results are model-dependent, the general features of the duality implementation conjectured previously get further support.
The exclusive decay modes $B \to K γγ$ and $B_s \to η(η') γγ$ are shown to have significant branching ratios of approximately $0.5\times 10^{-7}$. This first calculation of these modes employs a model based on a cascade transition $B\to Vγ\to Pγγ$ for estimating the long-distance contribution and the process $b\to sγγ$ for the short distance one.
Using the PQCD method we calculate the W-exchange and the W-annihilation processes of B mesons, which in general involve a charm quark or anti-quark in the final state. The nonvanishing amplitudes of these processes are found to be suppressed by a factor of $m_c/m_b$ compared to the tree or the time-like penguin processes, but some of them are within the reach of observation at the future B-factories, and $\bar B_d^0 \to D^+_s K^-$ whose branching ratio is found to be $6.6 \times 10^{-6}$ can be found even before the B-factory era. Comparisons with the results based on the BSW model are also given.
We analyze the effects of the space-like penguin diagrams in two body B decays $B^-\to K^-{\bar K}^0$ and $B^-\to π^-{\bar K}^0$. Special attention is paid to the operator $Q_6$, whose contribution is large in the BSW model, but vanishes in the PQCD method in the approximation of neglecting the masses in the final states. Suppressions of the space-like penguin diagrams are found in the PQCD method, which implies that the contributions of the space-like penguins are small compared to those of the time-like penguins.
We re-analyse the long distance contributions to the process $b \to s l^+ l^-$. Full $q^2$-behavior of the vector meson dominance amplitude is used together with the effect of Terasaki suppression, and comparisons with the previous results are given. We show that the interference between short- and long- distance contributions makes it difficult to extract the short distance information from the dominant long distance background, either in the dilepton invariant mass distribution or in the single lepton energy spectrum.
We calculate the long distance contribution to $D^{+,0}\toπ^{+,0}l^+l^-$ decays by the use of a vector meson dominance model, in which the $ϕ$-meson plays the central role. The branching ratios obtained are $10^{-6}$ and a few times $10^{-7}$ for the resonance and non-resonance regions respectively. The analysis includes a calculation of $D^+\toπ^+ϕ$, consistent with the experimental value.
The effective Hamiltonian for the R-parity violating couplings induced nonleptonic decay $B^-\to K^-K^0$ is calculated including leading log QCD corrections running from $m_{\tilde q}$, the mass of the up-type squark, to $m_b$. Experimental upper limit of this decay is used to give the bound on the R-parity violating couplings $λ''$.
We investigate the radiative dileptonic decays $B_s(B_d)\toγl^+l^-$ within the standard model. Using the constituent quark model, the branching ratios turn out to be around $5\times 10^{-9}$ for $B_s\to γμ^+μ^- $ and around $6\times 10^{-10}$ for $B_d \to γμ^+μ^- $, with slightly larger values for $B_s(B_d) \to γe^+e^- $. The differential rate as a function of the dilepton invariant mass is given. The possibility of using these processes to determine the decay constants of $B_s$ or $B_d$ is discussed.