Search Results (89 total)

Using a sample of 58 million J/ψ events collected with the BESII detector at the BEPC, more than 100 000 J/ψ→pp̅ π⁰ events are selected, and a detailed partial wave analysis is performed. The branching fraction is determined to be Br(J/ψ→pp̅ π⁰)=(1.33±0.02±0.11)×10^-3. A long-sought missing N^*, first observed in J/ψ→pn̅ π^-, is observed in this decay too, with mass and width of 2040_-4⁺³±25   MeV/c² and 230_-8⁺⁸±52   MeV/c², respectively. Its spin-parity favors 3 / 2⁺. The masses, widths, and spin parities of other N^* states are obtained as well.

An analytical method of electromagnetic wave interactions with a general radially anisotropic cloak is established. It is able to deal with arbitrary parameters [ε_r(r), μ_r(r), ε_t(r), and μ_t(r)] of a radially anisotropic inhomogeneous shell. The general cloaking condition is proposed from the wave relations, in contrast to the method of transformation optics. Spherical metamaterial cloaks with improved invisibility performance are achieved with optimal nonlinearity in transformation and core-shell ratio.

A solution to a 3-satisfiability (3-SAT) formula can be expanded into a cluster, all other solutions of which are reachable from this one through a sequence of single-spin flips. Some variables in the solution cluster are frozen to the same spin values by one of two different mechanisms: frozen-core formation and long-range frustrations. While frozen cores are identified by a local whitening algorithm, long-range frustrations are very difficult to trace, and they make an entropic belief-propagation (BP) algorithm fail to converge. For the BP algorithm to reach a fixed point, the spin values of a tiny fraction of variables (chosen according to the whitening algorithm) are externally fixed during the iteration. From the calculated entropy values, we infer that, for a large random 3-SAT formula with constraint density close to the satisfiability threshold, the solutions obtained by the survey-propagation or WALKSAT algorithms belong neither to the most dominating clusters of the formula nor to the most abundant clusters. This work indicates that a single-solution cluster of a random 3-SAT formula may have further community structures.

Through a combined scanning tunneling microscopy and angle-resolved photoemission spectroscopy study, we report the observation of two distinct gaps (a small and a large gap) that coexist both in real space and in the antinodal region of momentum space, below the superconducting transition temperature (T_c) of Bi₂Sr_2-xLa_xCuO_6+δ. We show that the small gap is associated with superconductivity. The large-gap persists above T_c, and seems linked to observed charge ordering. We find a strong correlation between the large and small gaps suggesting that they are affected by similar physical processes.

Based on 58×10⁶ J/ψ events collected with the BESII detector at the Beijing Electron-Positron Collider, the baryon pair processes J/ψ→Σ⁺Σ̅ ^- and J/ψ→Ξ⁰Ξ̅ ⁰ are observed for the first time. The branching fractions are measured to be B(J/ψ→Σ⁺Σ̅ ^-)=(1.50±0.10±0.22)×10^-3 and B(J/ψ→Ξ⁰Ξ̅ ⁰)=(1.20±0.12±0.21)×10^-3, where the first errors are statistical and the second ones are systematic.

The design of perfect electromagnetic cloaks is usually based on the coordinate transformation method. However, for cloaks with arbitrary shapes, it is difficult to express analytically such coordinate transformation. Thus, we have to resort to numerical methods to design approximate cloaks. In this paper, we propose a numerical method of designing cloaks with arbitrary shapes. According to this method, spatial compressions are treated as homeomorphous mappings and described in numerical forms. The corresponding coordinate transformations are therefore expressed numerically. As an application of this method, we deduced the discrete constitutive parameters of a two-dimensional polygon cloak, and verified the results by numerical simulations. This method can be also used in cases with acoustic waves and matter waves.

We observe an obvious anomalous line shape of the e⁺e^-→ hadrons total cross sections in the energy region between 3.700 and 3.872 GeV. It is inconsistent with the explanation for only one simple ψ(3770) resonance with a statistical significance of 7σ. The anomalous line shape may be explained by two possible enhancements of the inclusive hadron production near the center-of-mass energies of 3.764 and 3.779 GeV, indicating that either there is likely a new structure in addition to the ψ(3770) resonance around 3.773 GeV, or there are some physics effects reflecting the DD̅ production dynamics.

A general Monte Carlo simulation method of calculating the elastic constants of a polydisperse hard-sphere colloidal crystal is developed. Elastic constants of a size-polydisperse hard-sphere fcc crystal are calculated. The pressure and three elastic constants (C₁₁, C₁₂, and C₄₄) increase significantly with the polydispersity. It was also found from extrapolation that there is a mechanical terminal polydispersity above which a fcc crystal will be mechanically unstable.

We designed chiral phononic crystals and studied their properties by using the layer multiple scattering method. The transmittance curves and the corresponding band structures show that this kind of structure possesses significant polarization gaps. The chiral structures break the symmetry so that the degenerate transverse modes split into a pair of right-hand polarized mode and left-hand polarized mode. The polarization splitting in the low-frequency range is enhanced in systems of large filling ratios. We also demonstrate that chiral structures containing strongly resonant units can induce negative group velocity in elastic waves, and stronger resonance brings a wider band of negative group velocity.

The B_c→B_q^(*)P, B_qV decays are studied with the QCD factorization approach (where P and V denote pseudoscalar and vector mesons, respectively; q=u, d, and s). Considering the contributions of both current-current and penguin operators, the amplitudes of branching ratios are estimated at the leading approximation. We find that the contributions of the penguin operators are very small due to the serious suppression by the Cabibbo-Kobayashi-Maskawa elements. The most promising decay modes are B_c→B_s^(*)π, B_sρ, which might be easily detected at hadron colliders.

Using ψ(2S)→π⁺π^-J/ψ events in a sample of 14.0×10⁶ ψ(2S) decays collected with the BES-II detector, a search for the decay of the J/ψ to invisible final states is performed. No signal is found, and an upper limit at the 90% confidence level is determined to be 1.2×10^-2 for the ratio B(J/ψ→invisible) / B(J/ψ→μ⁺μ^-). This is the first search for J/ψ decays to invisible final states.

The decays of J/ψ→ηϕf₀(980)[η→γγ,ϕ→K⁺K^-,f₀(980)→π⁺π^-] are analyzed using a sample of 5.8×10⁷ J/ψ events collected with the BESII detector at the Beijing Electron-Positron Collider. A structure at around 2.18  GeV/c² with about 5σ significance is observed in the ϕf₀(980) invariant mass spectrum. A fit with a Breit-Wigner function gives the peak mass and width of m=2.186±0.010(stat)±0.006(syst)  GeV/c² and Γ=0.065±0.023(stat)±0.017(syst)  GeV/c², respectively, which are consistent with those of Y(2175), observed by the BABAR Collaboration in the initial-state radiation process e⁺e^-→γ_ISRϕf₀(980). The production branching ratio is determined to be Br(J/ψ→ηY(2175))Br(Y(2175)→ϕf₀(980))Br(f₀(980)→π⁺π^-)=[3.23±0.75(stat)±0.73(syst)]×  10^-4, assuming that the Y(2175) is a 1^-- state.

The decays of J/ψ→ωKK̅ π and J/ψ→ϕKK̅ π are studied using 5.8×10⁷ J/ψ events collected with the Beijing Spectrometer (BESII) at the Beijing Electron-Positron Collider (BEPC). The K_S⁰K^±π^∓ and K⁺K^-π⁰ systems, produced in J/ψ→ωKK̅ π, have enhancements in the invariant mass distributions at around 1.44  GeV/c². However, there is no evidence for mass enhancements in the KK̅ π system in J/ψ→ϕKK̅ π. The branching fractions of J/ψ→ωK_S⁰K^±π^∓, ϕK_S⁰K^±π^∓, ωK^*K̅ +c.c., and ϕK^*K̅ +c.c. are obtained, and the J/ψ→ηK_S⁰K^±π^∓ branching fraction is measured for the first time.

Using coordinate transformation stated earlier by Pendry [Science 312, 1780 (2006)], we deduced the material parameter equation for elliptic cylinder cloaks and reported the numerical simulations as well. The theoretical and numerical results indicate that, an elliptic cylinder cloak, which is constructed with metamaterials possessing the permittivity and permeability ruled by our equation, can exclude the electromagnetic fields from the interior region without perturbing the exterior fields and therefore the objects in it can be effectively invisible to the outer region.

Using 58×10⁶ J/ψ events collected with the Beijing Spectrometer (BESII) at the Beijing Electron-Positron Collider, the decays J/ψ→γϕρ and J/ψ→γωρ are searched for, and upper limits on their branching fractions are reported at the 90% C.L. No clear structures are observed in the γρ, γϕ, or ρϕ mass spectra for J/ψ→γϕρ nor in the γρ, γω, or ρω mass spectra for J/ψ→γωρ.

By analyzing the data collected at the center-of-mass energy E_cm=3.773  GeV and below the DD̅ meson pair production threshold with the BES-II detector at the BEPC Collider, we directly measured the observed non-DD̅ cross section of ψ(3770) decay to be σ_{ψ(3770)→non-DD̅} ^obs=(0.95±0.35±0.29)  nb at E_cm=3.773  GeV, and the branching fraction BF[ψ(3770)→non-DD̅ ]=(13.4±5.0±3.6)% for inclusive non-DD̅ decay of ψ(3770). We also determined the cross section for DD̅ meson pair production to be σ_DD̅ ^obs=(6.12±0.37±0.23)  nb at E_cm=3.773  GeV.

Using the idea of a transformation medium, a cloak can be designed to make a domain invisible to one target frequency. We examine the possibility of extending the bandwidth of such a cloak. We find that causality requirements impose severe constraints on the system parameters of the transformation medium, and we show that a specific form of “reduction” can help us to create a cloak that offers a reduced cross section in a finite frequency range. We also give a simple inequality that limits the bandwidth of operation.

Using 14.0×10⁶ψ(2S) events collected with the BES-II detector, the C-parity violating process J/ψ→γγ via ψ(2S)→π⁺π^-J/ψ is studied. We determine a new upper limit for the J/ψ→γγ branching ratio of B(J/ψ→γγ)<2.2×10^-5 at the 90% C.L., which is about 20 times lower than the previous measurement.

Using 58×10⁶ J/ψ and 14×10⁶ ψ(2S) events collected by the BESII detector at the BEPC, branching fractions or upper limits for the decays J/ψ and ψ(2S)→ΛΛ̅ π⁰ and ΛΛ̅ η are measured. For the isospin violating decays, the upper limits are determined to be B(J/ψ→ΛΛ̅ π⁰)<6.4×10^-5 and B[ψ(2S)→ΛΛ̅ π⁰]<4.9×10^-5 at the 90% confidence level. The isospin conserving process J/ψ→ΛΛ̅ η is observed for the first time, and its branching fraction is measured to be B(J/ψ→ΛΛ̅ η)=(2.62±0.60±0.44)×10^-4, where the first error is statistical and the second one is systematic. No ΛΛ̅ η signal is observed in ψ(2S) decays, and B[ψ(2S)→ΛΛ̅ η]<1.2×10^-4 is set at the 90% confidence level. Branching fractions of J/ψ decays into Σ⁺π^-Λ̅ and Σ̅ ^-π⁺Λ are also reported, and the sum of these branching fractions is determined to be B(J/ψ→Σ⁺π^-Λ̅ +c.c.)=(1.52±0.08±0.16)×10^-3.

Using 14×10⁶ ψ(2S) events accumulated at the BESII detector, we report first measurements of branching fractions or upper limits for ψ(2S) decays into γpp̅ , γ2(π⁺π^-), γK_S⁰K⁺π^-+c.c., γK⁺K^-π⁺π^-, γK^*0K^-π⁺+c.c., γK^*0K̅ ^*0, γπ⁺π^-pp̅ , γ2(K⁺K^-), γ3(π⁺π^-), and γ2(π⁺π^-)K⁺K^- with the invariant mass of hadrons below 2.9  GeV/c². We also report branching fractions of ψ(2S) decays into 2(π⁺π^-)π⁰, ωπ⁺π^-, ωf₂(1270), b₁^±π^∓, and π⁰2(π⁺π^-)K⁺K^-.

We report scanning-tunneling-microscopy observations and first-principles calculations for the formation and evolution of self-organized Ge nanostructures on Si(111)-(7×7) surfaces for Ge coverages up to 0.5 ML. We show that individual Ge atoms initially form a triangular lattice. At higher coverages, Ge nanoparticles 1 nm in diameter gradually form in both the faulted and unfaulted half unit cells with an initial preference in the faulted halves, ultimately driving ordered hexagonal arrays. The underlying 7×7 surface periodicity, the triangular single-Ge lattice, and the nanoparticle hexagonal superstructures coexist. Charge transfer from Si adatoms to Ge nanoparticles is shown to play a key role in the self-organization.

Scanning tunneling microscopy observations and first-principles quantum mechanical calculations were employed to investigate the nanoscale structures formed on Si(111) surfaces upon germanium deposition at a coverage of ∼0.3 monolayer. At room temperature, Ge atoms form nanoclusters with sizes of 1.5–6 nanometers in width. After annealing, the nanoclusters become two-dimensional islands with typical size of ∼10 nanometers in width. We propose that the annealing or high-T deposition results in a partial transformation of (7×7) reconstructed unit cells to unreconstructed Si(111) configurations on which the Ge adatoms reside at the T₄ sites and form a (sqrt[3]×sqrt[3])R30° reconstruction.

Based on the formalism developed by Psarobas [Phys. Rev. B 62, 278 (2000)], using the multiple-scattering theory to calculate properties of simple phononic crystals, we propose a very simple method to study NaCl-type phononic crystals. The NaCl-type phononic crystal consists of two kinds of nonoverlapping elastic spheres with different mass densities, Láme coefficients, and radius following the same periodicity of the ions in real NaCl crystals. We focus on the (001) surface and view the crystal as a sequence of planes of spheres, each plane of spheres having identical two-dimenssional periodicity. We obtained the complex band structure of the infinite crystal associated with this plane and also calculated the transmission, reflection, and absorption coefficients for an elastic wave (longitudinal or transverse) incident, at any angle, on a slab of the crystal of finite thickness.

Based on the generating function of Laguerre polynomials, we propose a Laguerre polynomial expansion scheme in the calculation of the evolution of the time-dependent Schrödinger equation. Theoretical analysis and numerical tests show that the method is equally as good as the Chebyshev polynomial expansion method in efficiency and accuracy, with the additional merits that no scaling to the Hamiltonian is needed and it has wider suitability.

We report measurements of the continuum R_uds near the center-of-mass energy of 3.70 GeV, the R_{uds(c)+ψ(3770)}(s) and the R_had(s) values in e⁺e⁻ annihilation at 68 energy points in the energy region between 3.650 and 3.872 GeV with the BES-II detector at the BEPC Collider. We obtain the R_uds for the continuum light hadron (containing u, d, and s quarks) production near the DD̅ threshold to be R_uds=2.141±0.025±0.085.