In the standard model a right-handed neutrino can appear only by mixing with the left-handed neutrinos. Precise electroweak data constrain this mixing. We consider models in which the mixing is not proportional to neutrino masses. We have analyzed data and constraints on ${\mathit{m}}_{\mathit{t}}$,${\mathit{m}}_{\mathit{H}}$ with and without mixing (sinε) of neutrinos with new inert fermions. For universal ν mixing, sinε=0.09; if only ${\nu }_{\mathrm{\tau }}$ mixes, sin${\mathrm{\epsilon }}_{\mathrm{\tau }}$=0.22. Thus ${\nu }_{\mathrm{\tau }}$ data, Γτ and ${\mathrm{\Gamma }}_{\mathit{Z}}^{\mathrm{inv}}$, if interpreted without caution, perhaps favor a nonzero ${\nu }_{\mathrm{\tau }}$ mixing and the existence of heavy neutral fermions.

We present a model accommodating a 17-keV neutrino with 1% mixing with the electron neutrino and bounds on the electron-neutrino mass and neutrinoless double-β decay. However, in contrast to previous models, there is only a single state with mass 17 keV. This model is consistent with cosmological and supernova-cooling constraints, and incorporates the Mikheyev-Smirnov-Wolfenstein explanation of the low solar-neutrino counts. Possible signatures of our model include an excess of muon neutrinos from a supernova explosion, spread over a period of 10–1000 sec, and a Higgs-boson decay signature of leptons plus missing energy.

Using the Collider Detector at Fermilab, the W-boson differential cross section dσ/${\mathit{dP}}_{\mathit{T}}$ is measured using W→eν events in proton-antiproton collisions at √s =1.8 TeV. A next-to-leading-order theoretical calculation agrees well with the data. The cross section (σ) for ${\mathit{P}}_{\mathit{T}}$>50 GeV/c is measured to be 423±58(stat)±108(syst) pb.

We have measured the distribution of photoion charge states resulting from vacancy cascades and electron shakeoff following K-shell photoionization of ${\mathrm{Ar}}^{2+}$ ions. The ${\mathrm{Ar}}^{2+}$ ions were produced by electron-impact ionization of Ar, stored in a Penning ion trap, and then photoionized using broadband synchrotron radiation. Time-reversed integration of coupled differential equations was used to account for contributions from electron capture with residual gas. The results are in reasonable agreement with a simple theoretical prediction.

Calculations of the screened electron self-energy are carried out in local central potentials that approximate a Hartree-Fock potential. The resulting energy corrections are combined with screened vacuum-polarization calculations and used to predict the Lamb shift for n=2 states of lithiumlike uranium, n=3 states of sodiumlike platinum, and n=4 states of copperlike gold. The theoretical values of the screened Lamb shift are found to be in good agreement with values inferred from experiments.

We have carried out a molecular-dynamics simulation of hundreds of ions confined in a Paul trap. The simulation includes the trapped particles’ micromotion and interparticle Coulomb interactions. We have implemented a random walk in velocity to bring the secular motion to a given temperature which we have numerically measured. When the coupling Γ is large the ions form concentric shells which undergo a quadrupole oscillation at the rf frequency, while the ions within a shell form a 2D hexagonal lattice. Ion clouds at 5 mK show no rf heating for ${\mathit{q}}_{\mathit{z}}$≲0.6, whereas rapid heating is seen for ${\mathit{q}}_{\mathit{z}}$=0.8.

Exchange collisions of electrons with Na and Hg atoms and ${\mathrm{O}}_2$ and NO molecules have been studied by means of polarized electrons for energies between 4 and 15 eV and scattering angles ranging from 0° to 110°. While significant exchange effects have been observed for collisions of electrons with Na and Hg atoms, in agreement with theoretical predictions, differential spin-exchange cross sections for elastic collisions from the open-shell molecules ${\mathrm{O}}_2$ and NO are much smaller. This was not anticipated before. A satisfactory explanation of this different behavior is still missing.

The electron-ion complex formed in a collision is treated by eigenchannel R matrix and multichannel quantum defect methods. Convergence of the results to the limit of peripheral collisions where the Born approximation holds is displayed. Cascade effects are included quantitatively. Extensive results of electron collisions with alkaline earth ions are illustrated here by the prototype e+${\mathrm{Mg}}^{+}$.

We propose an algebraic model of n coupled one-dimensional anharmonic oscillators and apply it to the study of the stretching modes of ${\mathit{XY}}_6$ octahedral molecules. We derive a new result on the theory of discrete groups and use it within the framework of the algebraic model to provide a four-parameter fit to the published vibrational energies of ${\mathrm{SF}}_6$, ${\mathrm{WF}}_6$, and ${\mathrm{UF}}_6$ accurate within 0.9 ${\mathrm{cm}}^{\mathrm{-}1}$.

Hyper-Rayleigh scattering in solution is used for the determination of the hyperpolarizability of nonlinear optical molecules. Obtaining the first hyperpolarizability β without having to independently determine the dipole moment μ and the second hyperpolarizability γ is an advantage over the electric-field-induced second-harmonic generation technique. Values of the first hyperpolarizability β of 23×${10}^{\mathrm{-}30}$ esu for para-nitroaniline, 105×${10}^{\mathrm{-}30}$ esu for 4-methoxy-4’-nitrostilbene, and 95×${10}^{\mathrm{-}30}$ esu for 4-hydroxy-4’-nitrostilbene dissolved in chloroform have been obtained. The equivalent-internal-field model is shown to hold for small saturated molecules.

A simple phenomenological model for a passive scalar subject to a mean gradient and undergoing random advection plus molecular mixing yields an exponential distribution as a consequence of general analytic properties of the steady-state solution. The result persists in the presence of a coherent mean flow. Experimental realizations of our model are proposed.

First observations of an evaporative instability in absorptive dye (Nigrosin) doped water droplets heated by a pulsed Nd:YAlG laser (532 nm) are reported. Measurement of the instability thresholds at different dye concentrations shows an approximate inverse linear dependence on droplet absorption index, suggesting that the instability is caused by superheating of the droplet. The instability may trigger subsequent droplet deformation or or disintegration; however, below a well-defined fluence threshold, complete recovery of the droplet occurs following laser excitation. A qualitative model based on Landau instability theory is used to describe the instability.

The dynamic relaxation of various structural properties of a single freely jointed N-mer (N=6,9,30) in explicit solvent at relatively high dilution is investigated by molecular dynamics. In the large-k region, the normalized intermediate scattering function I(k,t) appears to be a universal function of ${\mathit{tk}}^{\mathit{x}}$ with x=2.9±0.1, in good agreement with experiments and close to the prediction of the Zimm model (x=3).

The dynamics of a single polymer chain immersed in a large number of solvent particles is studied by molecular dynamics. This is the first simulation where chain length (30, 40, and 60 monomers) and statistical accuracy are sufficient to test the predictions of the Zimm model as a result of the particle-particle interactions: The short-time diffusion constant is in good agreement with the Kirkwood prediction, and the monomer motions exhibit the expected dynamic scaling. The long-range hydrodynamic interaction requires a data analysis that explicitly includes the periodic images via Ewald sums.

The influence of wetting forces exerted by an interface on the coarsening mechanism of two coexisting liquid phases is studied. Experiments show that in a polymer mixture undergoing spinodal decomposition domains of characteristic length l coarsen with time t according to the power law l∼${\mathit{t}}^{3/2}$ in the direction parallel to a quartz interface. This is in contrast to the bulk behavior, where l∼t. Although a preliminary argument suggests that wetting effects driven by van der Walls forces are responsible for the growth exponent, a proper hydrodynamic theory is not yet available.

Experiments show that charge-stabilized colloidal suspensions form crystals which can melt by applied shear stress. We present a molecular-dynamics simulation study of shear melting in colloids. The nonequilibrium phase diagram is calculated for a volume fraction which has an equilibrium fcc structure. The mechanism for flow in the sheared solid is found to be planes sliding over planes. We find that weak equilibrium solids (high added salt) shear melt, but strong equilibrium solids do not. Instead, shear produces a crossover to a new solid structure. This appears in the phase diagram as a reentrant solid phase. Results for shear stress and phase diagram are consistent with existing experiments.

We study the d=4 O(4) symmetric nonlinear sigma model at the pseudocritical points for $8^4$–${28}^4$ lattices. The Fortuin-Kasteleyn-Coniglio-Klein clusters are shown to have fractal dimension ${\mathit{d}}_{\mathit{f}}$≃3—in accordance with the conjectured scaling relation involving the odd critical exponent δ. For the one cluster algorithm introduced recently by Wolff the dynamical critical exponent z comes out to be compatible with zero in this model.

A fractal model of radon emanation from solids was developed, based on α recoil from the α decay of radium. Range straggling of the recoiling radon atoms in the solid state was included and the fractal geometry was employed to describe the roughness of the emanating surface. This allowed derivation for the first time of the relation between the radon emanating power and the specific surface area as measured by the gas adsorption. It is suggested that emanating-power measurements can be used to determine the fractal dimensions of surfaces on the scale from tens to hundreds of nanometers.

Random strain fields have been introduced into samples of KCN by pressing pellets from powder and by thermal cycling. X-ray diffractograms show that the low-temperature structure depends strongly on the sample history. In some cycles the noncubic low-temperature phases have been suppressed in mesoscopic surface regions of the samples and the quadrupolar-glass state formed instead.

A two-dimensional (2D) and a novel ‘‘reduced four-dimensional (4D)’’ exchange NMR experiment are applied to investigate slow dynamics in poly(vinyl acetate) 20 K above its ${\mathit{T}}_{\mathit{g}}$. The 2D ${}_{}{}^{13}{}_{}{}^{}\mathrm{C}$ NMR spectra indicate a diffusive process with strongly nonexponential loss of correlation. In the ‘‘reduced 4D’’ experiment, the underlying distribution of correlation times is found to be heterogeneous on the time scale of the average correlation time, becoming homogeneous, however, at later times due to fluctuations which are 2 orders of magnitude slower.