Melt fracture of entangled polymers*

We construct a model for a slippage plane in a sheared melt, based on a balance between reptation bridging and shear debonding. The resulting state could show up at rather low shear rates and be locally stable. But it is not easy to nucleate: the conventional entangled state is also locally stable. We propose that slippage occurs on solid walls: either at the container surface, or on dust particles floating in the melt. Slippage at solid/melt interfaces was studied (experimentally and theoretically) long ago. There is a critical stress for slippage: our estimate (for strong adsorption of melt chains on the solid) gives (plateau modulus) for typical cases. Thus, melt fracture is expected at moderate stresses, in agreement with observations by S.Q. Wang and coworkers.     

Observation of Bs production at the Y(5S) resonance

Using the CLEO detector at the Cornell Electron Storage Ring, we have observed the Bs meson in e+e- annihilation at the Y(5S) resonance. We find 14 candidates consistent with Bs decays into final states with a J/psi or a Ds(*)- . The probability that we have observed a background fluctuation is less than 8 x 10(-10) . We have established that at the energy of the Y(5S) resonance Bs production proceeds predominantly through the creation of Bs*Bs* pairs. We find sigma(e+e- --> Bs*Bs*) = [0.11(-0.03))(+0.04)(stat) +/- 0.02(syst)]nb , and set the following limits: sigma(e+e- --> BsBs)/ sigma(e+ e- --> Bs*Bs*) <0.16 and [sigma(e+e- --> BsBs*) + sigma(e+e- --> Bs*Bs)]/sigma(e+e- -->Bs*Bs*) < 0.16 (90% C.L.). The mass of the Bs* meson is measured to be M(Bs*) = [5.414+/- 0.001(stat) +/- 0.003(syst)] GeV/c2 .     

X(3872) and Bound State Problem of D~0(D)*~0((D)~0D*~0)

We have performed a dynamical calculation of the bound state problem of D~0(D)~(*0) by considering the pion and sigma meson exchange potential.Our preliminary analysis disfavors the molecular interpretation of X(3872) if we use the experimental D~* Dπ coupling constant g=0.59 and a reasonable cutoff around 1 GeV,which is the typical hadronic scale.In contrast,there probably exists a loosely bound S-wave B (-B)~* molecular state.Such a molecular state would be rather stable since its dominant decay mode is the radiative decay through B~*→Bγ.     

Cross-correlation tomography: measuring dark energy evolution with weak lensing

A cross-correlation technique of lensing tomography is developed to probe dark energy in the Universe. The variation of weak shear with redshift around foreground galaxies depends only on the angular distances and is robust to the dominant systematic error in lensing. We estimate the margin-alized accuracies that deep lensing surveys with photometric redshifts can provide on the dark energy density Omega(de), the equation of state parameter w, and its evolution w('): sigma(w) approximately equal 0.01f(-1/2)(sky) and sigma(w(')) approximately equal 0.03f(-1/2)(sky), where a prior of sigma(Omega(de))=0.03 is assumed in the marginalization.     

O 1s --> sigma* resonance in O2: inadequacy of only two exchange-split components

The O 1s-->sigma* transition below the O K-edge in O2 has been investigated by absorption, constant ionic state (CIS) experiments, and extensive configuration interaction calculations. CIS scans of the three lowest-lying final states reached in resonant Auger decay provide a wealth of information on energy range, symmetry, and spin multiplicity of the intermediate states with sigma* character. We conclude that the identification of only two exchange-split components is inadequate because a complex manifold of states with sigma* character exists with no unique energy difference between related states.     

Critical scaling of shear viscosity at the jamming transition

We carry out numerical simulations to study transport behavior about the jamming transition of a model granular material in two dimensions at zero temperature. Shear viscosity eta is computed as a function of particle volume density rho and applied shear stress sigma, for diffusively moving particles with a soft core interaction. We find an excellent scaling collapse of our data as a function of the scaling variable sigma/|rho(c)-rho|(Delta), where rho(c) is the critical density at sigma=0 ("point J"), and Delta is the crossover scaling critical exponent. We define a correlation length xi from velocity correlations in the driven steady state and show that it diverges at point J. Our results support the assertion that jamming is a true second-order critical phenomenon.     

Nonlocal effects in flows of wormlike micellar solutions

The flow curve of wormlike micelles usually exhibits a stress plateau sigma* separating high and low viscosity branches, leading to shear-banded flows. We study the flow of semidilute wormlike micellar systems in a confined geometry: a straight microchannel. We characterize their local rheology thanks to particle image velocimetry. We show that flow curves cannot be described by a simple constitutive equation linking the local shear stress to the local shear rate. We demonstrate the existence of nonlocal effects in the flow of wormlike micellar systems and make use of a theoretical framework allowing the measurement of correlation lengths.     

Melting of embedded anisotropic particles: PbIn inclusions in Al

Small particles embedded in the solid state can assume facetted shapes that indicate the anisotropy in interfacial free energy between the particle and the embedding phase. Previous in situ transmission electron microscopy (TEM) of Al–In has shown that such facetted nanoparticles with cubic symmetry embedded in a solid can produce a series of complex melt configurations. To determine the dependence of melt configuration on solid–solid facet energies, this work numerically calculates the equilibrium melt trajectory for a family of cuboctahedral particles parameterized by the ratio of matrix–particle interfacial free energies for orientations taken with respect to the crystal axes of the matrix, ??=?γ₁₀₀/γ₁₁₁. The calculations assume that equilibrium occurs with the minimization of the total interfacial free energy to determine the stable internal melt configuration for a fixed volume of melt confined within a member of the cuboctahedral family. At particular melt volume fractions, abrupt transitions in shape occur when a stable configuration reaches a touching or necking instability. In situ TEM of a dilute Al alloy containing embedded nanosized PbIn inclusions that gradually melt as the temperature is increased from 230 to 260°C is consistent with the results calculated for ??=?1.245.     

Observation of two new N* peaks in J/psi-->ppi-n and ppi+n decays

The decay J/psi-->NNpi provides an effective isospin 1/2 filter for the piN system due to isospin conservation. Using 58x10(6) J/psi decays collected with the Beijing Electromagnetic Spectrometer at the Beijing Electron Positron Collider, more than 100 thousand J/psi-->ppi-n+c.c. events are obtained. Besides the two well-known N* peaks at around 1500 MeV/c2 and 1670 MeV/c2, there are two new, clear N* peaks in the ppi invariant mass spectrum around 1360 MeV/c2 and 2030 MeV/c2 with statistical significance of 11sigma and 13sigma, respectively. We identify these as the first direct observation of the N*(1440) peak and a long-sought missing N* peak above 2 GeV/c2 in the piN invariant mass spectrum.     

Evidence for the rare decay B-->K*l+l- and measurement of the B-->Kl+l- branching fraction

We present evidence for the flavor-changing neutral current decay B-->K*l+l- and a measurement of the branching fraction for the related process B-->K l+l-, where l+l- is either an epsilon+epsilon- or a mu+mu- pair. These decays are highly suppressed in the standard model, and they are sensitive to contributions from new particles in the intermediate state. The data sample comprises 123 x 10(6) Upsilon(4S)-->B(-)B decays collected with the BABAR detector at the SLAC PEP-II epsilon+epsilon- storage ring. Averaging over K(*) isospin and lepton flavor, we obtain the branching fractions B(B-->Kl+l-)=(0.65(+0.14)(-0.13)+/-0.04)x10(-6) and B(B-->K*l+l-)=(0.88(+0.33)(-0.29)+/-0.10)x10(-6), where the uncertainties are statistical and systematic, respectively. The significance of the B-->Kl+l- signal is over 8sigma, while for B-->K*l+l- it is 3.3sigma.     

Real-time determination of the slippage length in autophobic polymer dewetting

We investigated the kinetics of autophobic dewetting, caused by differences in entropy, of a polymer melt slipping on a monolayer of end-grafted identical molecules (polymer brush). From simultaneous measures of the dewetted distance and the width of the resulting rim, we obtained in real time and quantitatively the slippage length and the contact angle, a measure of the "incompatibility" between identical molecules. Furthermore, we could directly relate a decrease in the slippage length in the course of the experiments at elevated temperatures to temporal or permanent attachment of polymers from the melt to the brush.     

Viscosity at small scales in polymer melts

Flows around small colloidal particles of diameter b, or in thin films, capillaries, etc., cannot always be described in terms of the macroscopic polymer viscosity. We discuss these features for entangled polymer melts, where two distinct regimes can be found: (a) the thin regime where b is smaller than the coil radius R₀, but larger than the diameter of the Edwards tube; (b) the ultrathin regime, where . We consider (i) non adsorbing particles, where slippage may occur between the melt and the solid surface; (ii) “hairy” particles, which carry some bound polymer chains. We obtain scaling predictions for mobilities of spheres, of needles, and of clusters of particles. We also discuss translational and rotational diffusion of needles. Received 19 April 1999     

Evidence for an excited hyperon state in pp --> pK+Y0*

Indications for the production of a neutral excited hyperon in the reaction pp --> pK+Y0* are observed in an experiment performed with the ANKE spectrometer at COSY-Jülich at p(beam) = 3.65 GeV/c. Two final states were investigated simultaneously, viz. Y0* --> pi+X- and pi-X+, and consistent results were obtained in spite of the quite different experimental conditions. The parameters of the hyperon state are M(Y0*) = (1480 +/-15) MeV/c2 and gamma(Y0*) = (60 +/- 15) MeV/c2. The production cross section for Y0* decaying through these channels is of the order of few hundred nanobarns. Since the isospin of the Y0* has not been determined here, it could either be an observation of the sigma(1480), a one-star resonance of the Particle Data Group tables, or, alternatively, a lambda hyperon. Relativistic quark models for the baryon spectrum do not predict any excited hyperon in this mass range and so the Y0* may be of exotic nature.     

Search for leptoquark pairs decaying into nunu+jets in pp collisions at square root[s] = 1.8 TeV

We present the results of a search for leptoquark (LQ) pairs in (85.2+/-3.7) pb(-1) of pp* collider data collected by the D0 experiment at the Fermilab Tevatron. We observe no evidence for leptoquark production and set a limit on sigma(pp*-->LQLQ-->nunu+jets) as a function of the mass of the leptoquark (m(LQ)). Assuming the decay LQ-->nuq, we exclude scalar leptoquarks for m(LQ) < 98 GeV/c(2), and vector leptoquarks for m(LQ) < 200 GeV/c(2) and coupling which produces the minimum cross section, at a 95% confidence level.     

Spreading dynamics of 2D dipolar Langmuir monolayer phases

We study the spreading of a liquid 2D dipolar droplet in a Langmuir monolayer. Interfacial tensions (line tensions) and microscopic contact angles depend on the scale on which they are probed and obey a scaling law. Assuming rapid equilibration of the microscopic contact angle and ideal slippage of the 2D solid/liquid and solid/gas boundary, the driving force of spreading is merely expressed by the shape-dependent long-range interaction integrals. We obtain good agreement between experiment and numerical simulations using this theory.Received: 15 April 2004, Published online: 27 July 2004PACS: 68.08.Bc Wetting - 83.50.Lh Slip boundary effects (interfacial and free surface flows) - 82.45.-h Electrochemistry and electrophoresis     

Solute segregation at the curved surface of a growing crystal (steady state)

We consider a cylindrical crystal growing from the melt at a constant speed with a convex interface. Impurities from the melt segregate at the interface, and their approximate distribution in the solid is calculated in a dimensionless system with the following assumptions: a) the process occurs in the steady state; b) the impurities do not diffuse within the solid; c) the segregation at the interface is large. The determining parameter is the product of growth velocity and average curvature of the interface. When this parameter is large, the impurity concentration near the cylinder mantle is many orders of magnitude higher than near its axis. This gives rise to aradial refining process, which can be applied when diffusion in the liquid is low and convection has to be prevented. The problem is discussed for different shapes of the interface.     

Light gluino,light bottom squark scenario,and LEP predictions

The scenario of light gluinos and light sbottoms was advocated to explain the discrepancy between the measured and theoretical production of b quarks at the Tevatron. This scenario will have model-independent predictions for Z-->qqgg at the Z0 pole, and e(+)e(-)-->qqgg at LEPII. We show that the data for Z-->qqg*-->qqbb; at LEPI cannot constrain the scenario, because the ratio Gamma(Z-->qqgg)/Gamma(Z-->qqg*-->qqbb)=0.15-0.04 for m(g)=12-16 GeV is smaller than the uncertainty of the data. However, at LEPII the ratio sigma(e(+)e(-)-->qqgg)/sigma(e(+)e(-)-->qqg*-->qqbb) approximately 0.4-0.2 for m(g)=12-16 GeV, which may give an observable excess in qqbb events; especially, the 4b events.     

介质势垒放电(DBD)等离子体中NO荧光发射谱研究

利用介质阻挡放电 (DBD)等离子体技术对大气污染物NO分子进行了光谱研究 ,得到了低气压条件下放电等离子体在 2 1 0～ 2 80nm光谱范围内的荧光发射谱。该谱明显的表现为双峰结构 ,谱线均成对出现 ,强度分布符合Frank Condon原理 ,且最大峰值位置出现在 2 36nm处 ,将该组谱线归属为NO分子的A2 ΣA →X2 Π1 / 2 ,2 / 3 跃迁。荧光产生过程为 :基态NO分子与高能电子发生非弹性碰撞被激发至激发态A2 Σ 后自发跃迁回基态同时辐射出荧光。通过测量等离子体中NO分子和N2 分子 337nm谱线强度随时间的变化关系 ,初步证实了放电等离子体中存在的NO分子的分解机制为 :e NO→N O e,N NO→N2 O ,O NO→NO2 hν。     

Measurement of forward-backward asymmetry and Wilson coefficients in B --> K*l+l-

We report the first measurement of the forward-backward asymmetry and the ratios of Wilson coefficients and in A9/A7 and A10/A7 in B --> K*l+l-, where l represents an electron or a muon. We find evidence for the forward-backward asymmetry with a significance of 3.4sigma. The results are obtained from a data sample containing 386 x 10(6) BB pairs that were collected on the Y(4S) resonance with the Belle detector at the KEKB asymmetric-energy e+e- collider.