We have measured inclusive distributions for charged particles in hadronic decays of the Z boson. The variables chosen for study were charged-particle multiplicity, scaled momentum, and momenta transverse to the sphericity axes. The distributions have been corrected for detector effects and are compared with data from ${\mathit{e}}^{+}$${\mathit{e}}^{\mathrm{-}}$ annihilation at lower energies and with the predictions of several QCD-based models. The data are in reasonable agreement with expectations.

In high-temperature QCD, an infinite set of higher loop diagrams must be resummed in order to compute the gluon damping rates to leading order in the coupling constant. After resummation, the damping rates, as determined by the positions of the poles in the gluon propagator, are gauge invariant.

The observed shadowing and antishadowing phenomena of quark structure functions in nuclei at small x are interpreted as a consequence of an antiquark-nucleus multiscattering process.

We have measured the variance in fluorescence intensity as a phase diffusing laser is scanned through resonance with a beam of two-level atoms. For laser intensities below saturation, our results are in quantitative agreement with recent theoretical predictions provided Doppler effects are treated. Above saturation, the agreement is somewhat less quantitative primarily due to the sensitivity of the predictions to the laser intensity profile and the experimental uncertainty in measuring it.

Electron-impact ionization cross sections for ${\mathrm{Fe}}^{15+}$ have been calculated using relativitsic distorted-wave methods. We have performed the most comprehensive calculation to date by including more than 10 000 autoionizing levels for resonant-excitation double-autoionization (REDA) processes. The total cross sections exhibit many strong narrow resonances. The REDA process contributes about 30% to the average total cross sections. General good agreement between theory and experiment is attained, but new experimental investigations are needed to search for the predicted resonances.

Electron-polarized ${\mathrm{N}}^{4+}$ ions were produced via electron capture by a beam of ${\mathrm{N}}^{5+}$ ions in an optically pumped polarized sodium target. The polarization of the ions was detected by measuring the circular polarization of the N v 1s${\mathrm{}}^2$2s${\mathrm{}}^2$S–1s${\mathrm{}}^2$2p${\mathrm{}}^2$P (124 nm) transition taking place after the electron capture. These studies point to the feasibility of a general method for the production of polarized multicharged heavy-ion beams.

The noncoplanar symmetric (e,2e> cross section is obtained for sodium atoms pumped by ${\mathrm{\sigma }}^{+}$ laser light tuned to the ${3\mathrm{S}}_{1/2}^2$(F=2)↔${3\mathrm{P}}_{3/2}^2$(F′=3) transition. The excited pumped atoms are in ${\mathrm{them}}_{\mathrm{F}\prime }$=3 state, or in terms of orbital momentum in the ${\mathrm{m}}_{\mathrm{l}}$=+1 state. The coordinate frame is chosen so that the momentum density of the excited state is probed in the ${\mathrm{k}}_{\mathrm{x}}$ direction with ${\mathrm{k}}_{\mathrm{z}}$ and ${\mathrm{k}}_{\mathrm{y}}$ being fixed and essentially zero. The results are in excellent agreement with the momentum-density profile given by the 3p(${\mathrm{m}}_{\mathrm{l}}$=1) Hartree-Fock wave function. © 1990 The American Physical Society

We present a renormalization-group (RG) approach to the nonlinear diffusion process ${\mathrm{\partial }}_{\mathit{t}}$u=D ${\mathrm{\partial }}_{\mathit{x}}^2$u, with D=1/2 for ${\mathrm{\partial }}_{\mathit{x}}^2$u>0 and D=(1+ε)/2 for ${\mathrm{\partial }}_{\mathit{x}}^2$u<0, which describes the pressure during the filtration of an elastic fluid in an elastoplastic porous medium. Our approach recovers Barenblatt’s long-time result that, for a localized initial pressure distribution, u(x,t)∼${\mathit{t}}^{\mathrm{-}(\mathrm{\alpha }+1/2)}$f(x/ √t, ε), where f is a scaling function and α=ε(2πe${)}^{1/2}$+O(${\mathrm{\epsilon }}^2$) is an anomalous dimension, which we compute perturbatively using the RG. This is the first application of the RG to a nonlinear partial differential equation in the absence of noise.

We report on convection in horizontal layers of an ethanol-water mixture in a rectangular and an annular container heated from below. When the tempearature difference ΔT exceeds a critical value, localized regions of traveling-wave convection evolve via a backward bifurcation in both geometries and coexist with the conduction state for a range of ΔT. The size, shape, wave number, and frequency of the localized states are reproducible and geometry independent. The coexistence range stands in contrast to systems with a potential, where droplets of a given phase are not stable for first-order transitions at fixed values of the thermodynamic fields.

Experimental evidence is reported for the presence of oppositely rotating plasma cells during spheromak formation. The toroidal rotations are caused by j×B forces and cause a twisting of axial (i.e., poloidal) magnetic field into the toroidal direction. Calculations of the magnetic helicity input are consistent with measured helicity values. This method of flux conversion can be interpreted as a mechanical source of magnetic helicity injection possibly useful for magnetic confinement of plasmas.

A plateau-shaped electron distribution function is observed when an electrostatic electron-trapping potential is formed by the electron cyclotron heating (ECH) in the plug region of the GAMMA 10 tandem mirror. Also, a remarkable thermal isolation effect due to a kV-range thermal barrier is observed along with the difference between distribution functions in thermally separated regions. These new findings as well as the relation between ion-confining potentials and thermal-barrier potentials in the kV range consistently support the validity of Cohen’s strong ECH theory.

An elliptically polarized cw laser beam propagating in a nematic liquid-crystal film can induce molecular reorientation and lead the system through various dynamic regimes: torsional oscillations, precession, nutation with precession, and others.

We report observations of cellular growth at the smectic-B–smectic-A interface of the liquid crystal 40.8. We link the existence of an angular discontinuity at the cell tips to the missing orientations in the equilibrium shape. The cell drift that we observe can be explained as resulting from the diffusion anisotropy in the smectic-A phase. Finally, we describe how facets destabilize.

We have been investigating the behavior of thin core inviscid vortices of constant circulation in an effort to model quantized vortices in helium II. We have found that at T=0 K, Kelvin waves (standing cosine waves on vortex cores) are unstable to the buildup of sidebands. This instability is recurrent in the sense of the Fermi-Pasta-Ulam phenomenon.

Using neutron spin-echo spectroscopy and rheometry, we investigated the melt dynamics of poly(ethylene-propylene) alternating copolymers. The experiments show that beyond a certain length scale the relaxation of density fluctuations within a given chain is strongly reduced. The presence of this intermediate dynamic length scale implies the existence of a well-defined entanglement distance confirming the essential assumption of the reptation concept. Its value is found to be in excellent agreement with the entanglement distance obtained from the rheological measurement.

We have directly observed the effects of roton-roton scattering by crossing two beams of ballistic rotons in liquid ${}_{}{}^4{}_{}{}^{}\mathrm{He}$ at a temperature of ∼0.1 K. One beam is weak and is used as the probe. Its attenuation is measured as it passes through the stronger scattering beam which is predominantly perpendicular to the probe beam. The beams are pulsed and the dispersion of the roton velocities enable the momenta of the rotons to be resolved by time-of-flight measurements. It appears that the scattering is strongly wave-vector dependent.

We present results from computer simulations of anharmonic vibrations of one- and two-dimensional lattices. For disordered systems we have found that anharmonicity destabilizes the localized modes and that, when energy is fed locally to the lattice, it spreads out beyond the localization length via some kind of anomalous diffusion. Moreover, the diffusion exponent ν seems dependent on the strength of anharmonicity but not on the amount of disorder. In perfect lattices we have observed the (-1/2;1;-1/2) localized mode predicted recently [S. Taken, K. Kisoda, and A. J. Sievers, Prog. Theor. Suppl. 94, 242 (1988)].

Existing calculations of diffusion coefficients in solids have so far relied on empirical potentials and/or dynamical simulations, both of which entail important limitations. We present a practical approach that is based on rate theory and allows the calculation of temperature-dependent diffusion coefficients from static first-principles calculations. Results for hydrogen in silicon are in excellent agreement with recent first-principles dynamical calculations at high temperatures and with experiment. They further elucidate the nature of diffusion pathways and anharmonic effects.

We study in d+1 dimensions a new deposition and evaporation model of a d-dimensional surface which bears a Potts-spin representation. For the pure deposition case, our simulations on systems up to ${11520}^2$ sites in d=2 and 2×${192}^3$ sites in d=3 yield roughness exponents which violate recent conjectures. Including evaporation, we observe a nonequilibrium surface-roughening transition in d=3, but only a smooth crossover behavior in d=2. A logarithmic anomalous scaling form for surface width at the transition is conjectured.

A detailed LEED beam profile analysis of the structural changes induced by the catayltic CO oxidation on Pt(110) has been carried out with a high-resolution instrument (SPALEED). Under conditions where the 1×2⇄1×1 phase transition also takes place one observes spatial pattern formation via a faceting process. Facts of roughly uniform size and orientation build up a regular array characterized by a lateral periodicity of 70 lattice units along [11¯0]. The mechanism is discussed in terms of a nonequilibrium structure arising from kinetic instabilities.