We report the results of a search for the decay ${\mathit{K}}_{\mathit{L}}$→${\mathrm{\pi }}^0$${\mathit{e}}^{+}$${\mathit{e}}^{\mathrm{-}}$ conducted by E799 at Fermilab. The largest contribution to this decay mode is expected to be CP violating. Observations of this decay could either confirm the standard model mechanisms for CP violation or point to physics beyond the standard model. No such events were seen in our search. The 90% confidence level upper limit is determined to be B(${\mathit{K}}_{\mathit{L}}$→${\mathrm{\pi }}^0$${\mathit{e}}^{+}$${\mathit{e}}^{\mathrm{-}}$)<4.3×${10}^{\mathrm{-}9}$.

We report results from a search for the decays ${\mathit{B}}^0$→${\mathrm{\pi }}^{+}$${\mathrm{\pi }}^{\mathrm{-}}$, ${\mathit{B}}^0$→${\mathit{K}}^{+}$${\mathrm{\pi }}^{\mathrm{-}}$, and ${\mathit{B}}^0$→${\mathit{K}}^{+}$${\mathit{K}}^{\mathrm{-}}$. We find 90% confidence level upper limits on the branching fractions, ${\mathit{B}}_{\mathrm{\pi }\mathrm{\pi }}$<2.9×${10}^{\mathrm{-}5}$, ${\mathit{B}}_{\mathit{K}\mathrm{\pi }}$<2.6×${10}^{\mathrm{-}5}$, and ${\mathit{B}}_{\mathit{K}\mathit{K}}$<0.7×${10}^{\mathrm{-}5}$. While there is no statistically significant signal in the individual modes, the sum of ${\mathit{B}}_{\mathrm{\pi }\mathrm{\pi }}$ and ${\mathit{B}}_{\mathit{K}\mathrm{\pi }}$ exceeds zero with a significance of more than 4 standard deviations, indicating that we have observed charmless hadronic B decays.

The cross section for exclusive ${\mathrm{\pi }}^{+}$ electroproduction on the proton has been measured near threshold for the first time at two different values of the virtual photon polarization (ɛ∼0.2 and ɛ∼0.7). Using the low energy theorem for this reaction we deduce the axial and pseudoscalar weak form factors ${\mathit{G}}_{\mathit{A}}$ and ${\mathit{G}}_{\mathit{P}}$ at ‖t‖=0.073, 0.139, and 0.179 (GeV/c${)}^2$. The slope of ${\mathit{G}}_{\mathit{A}}$ agrees with the value obtained in neutrino experiments. ${\mathit{G}}_{\mathit{P}}$ satisfies the pion pole dominance hypothesis, which is thus verified for the first time in this range of transfer.

Cross sections for the ${}_{}{}^8{}_{}{}^{}\mathrm{Li}$(d,n${)}^9$Be (ground state) and ${}_{}{}^8{}_{}{}^{}\mathrm{Li}$(d,t${)}^7$Li reactions, both important to primordial nucleosynthesis in the inhomogeneous models, have been measured using a radioactive beam technique. The cross section for the former reaction is found to be small, so it is important only for synthesis of ${}_{}{}^9{}_{}{}^{}\mathrm{Be}$. The cross section for the latter reaction, however, is found to be large enough to destroy significant quantities of ${}_{}{}^8{}_{}{}^{}\mathrm{Li}$, and thus could affect predictions of primordial nucleosynthesis yields.

Multifragment production was measured for the 60 MeV/nucleon ${}_{}{}^{197}{}_{}{}^{}\mathrm{Au}$${+}^{27}$Al, ${}_{}{}^{51}{}_{}{}^{}\mathrm{V}$, and ${}_{}{}^{\mathrm{nat}}{}_{}{}^{}\mathrm{Cu}$ reactions. The branching ratios for binary, ternary, quaternary, and quinary decays were determined as a function of the excitation energy E and are independent of the target. The logarithms of these branching ratios when plotted vs ${\mathit{E}}^{\mathrm{-}1/2}$ show a linear dependence that strongly suggests a statistical competition between the various multifragmentation channels. This behavior seems to relegate the role of dynamics to the formation of the sources, which then proceed to decay in an apparently statistical manner.

The first Doppler-shift-free crystal-spectrometer measurement of stationary highly stripped uranium ions from a high-energy electron beam ion trap is presented. Thirteen 2${\mathit{s}}_{1/2\mathrm{-}}$2${\mathit{p}}_{3/2}$ transitions in eight ionization states bteween Li-like ${\mathrm{U}}^{89+}$ and Ne-like ${\mathrm{U}}^{82+}$ are identified and measured with an accuracy as high as 37 ppm, providing benchmarks for testing relativistic correlation and quantum electrodynamic effects in highly charged multielectron ions. A value of 47.39±0.35 eV is found for the 2${\mathit{s}}_{1/2}$ Lamb shift in Li-like ${\mathrm{U}}^{89+}$, in excellent agreement with theory.

The recently proposed R-matrix-Floquet theory of multiphoton processes has been used to calculate multiphoton ionization rates for the two-electron systems ${\mathit{H}}^{\mathrm{-}}$ and He in intense laser fields. The theory is nonperturbative and includes electron-electron correlations. Results are presented for total and partial multiphoton ionization rates and novel nonperturbative correlation effects are discussed.

We present theoretical and experimental results of the leading order angular correlations of second harmonic light generated inside random media, for both the transmission and reflection geometries. We find the striking result that correlations in reflection of the second harmonic light scales with the sample thickness L, in sharp contrast to the corresponding short-range correlation function in reflection of the fundamental light in the linear scattering regime, which scales with mean free path ${\mathit{l}}^{\mathrm{*}}$.

Measurements based on balanced detection demonstrate more than 3 dB photon-number squeezing from a quantum well laser injection locked to a coherent signal. We show that injection locking significantly reduces the photon-number fluctuation by suppressing weak side modes that feature intensity noise far above the standard quantum limit. The measurements also reveal that the photon statistics vary qualitatively with the polarization of the laser field.

Self-phase modulation resulting from radiation influence on material parameters of a medium can, like the Doppler and gravitational redshifts, result in a nearly uniform frequency shift. This is confirmed experimentally by the use of the orientational mechanism of the interaction of a laser beam with a nematic liquid crystal. The self-induced frequency shift is proportional to light intensity and the distance light travels in the medium.

Optical bistability in a two-dimensional nonlinear superlattice, resulting from the index-modulation mechanism, was observed in experiment for the first time. The two-dimensional nonlinear superlattice was constructed by recording a two-dimensional refractive index grating into a photorefractive material, a Fe-doped ${\mathrm{LiNbO}}_3$ single crystal. Optical instability was also observed in this experiment.

The static as well as the dynamic behavior of granular material are determined by dynamic and static friction. There are well known methods to inlcude static friction in molecular dynamics simulations using scarcely understood forces. We propose an ansatz based on the geometrical shape of nonspherical particles which does not involve an explicit expression for static friction. It is shown that the simulations based on this model are close to experimental results.

Direct numerical simulations with up to ${4096}^2$ resolution are performed to address the question of universality of statistical properties of the enstrophy cascade in homogeneous two-dimensional turbulence driven by large-scale Gaussian white-in-time noise. Data with different Reynolds numbers are compared with each other. The energy spectrum is found to be very close to 1/${\mathit{k}}^3$. It is shown that the primary contribution to the enstrophy transfer function comes from wave-number triads with one small leg and two long ones, corresponding to wave numbers in the inertial range.

A rare exact solution to the nonlinear Boltzmann equation describing rheological phenomena far from equilibrium is reconsidered. We show the existence of a critical shear rate beyond which the solution fails to exist. The implication of this exact result is a singular transition from simple uniform shear flow to a more complex state, perhaps involving spatial ordering.

Chaotic transport in a laminar fluid flow in a rotating annulus is studied experimentally by tracking large numbers of tracer particles for long times. Sticking and unsticking of particles to remnants of invariant surfaces (Cantori) around vortices results in superdiffusion: The variance of the displacement grows with time as ${\mathit{t}}_{\mathrm{\delta }}$ with γ=1.65±0.15. Sticking and flight time probability distribution functions exhibit power-law decays with exponents 1.6±0.3 and 2.3±0.2, respectively. The exponents are consistent with theoretical predictions relating Lévy flights and anomalous diffusion.

Particle simulations compare the behavior of nonrelativistic sheet electron beams in uniform static and nonuniform time-harmonic magnetic fields. The time-harmonic fields are equivalent to periodically cusped magnetic (PCM) focusing. While the sheet beam in a uniform field exhibits diocotron instability, the PCM-focused beam is stabilized by ponderomotive forces, in agreement with recent analytic predictions [J. Appl. Phys. 73, 4140 (1993)]. Mismatched Pcm-focused beams exhibit envelope oscillation and initially rapid emittance growth followed by a region of slower increase, in agreement with a recent semianalytic Fokker-Planck model.

Thomson scattering measurements have been used to determine the electron energy distribution in UV optically ionized He and Ne at pressures up to 1/2 bar irradiated at 3×${10}^{17}$ W/${\mathrm{cm}}^2$ in 12 ps. The electrons evolve from a multiphoton ionization spectrum to a thermal distribution with increasing pressure. The low frequency part of the scattered spectrum clearly shows the ‘‘zero-damped’’ ion acoustic mode for high values of ${\mathit{ZT}}_{\mathit{e}}$/${\mathit{T}}_{\mathit{i}}$. Experimental and modeling results indicate that nonlinear inverse bremsstrahlung heating and radial conduction cooling dominate the electron energy balance.

Dynamic coexistence of several phaselike forms, a solid, microcrytalline phase, a homogeneously melted phase, and phases exhibiting a solid core and a melted surface, is found in isothermal molecular dynamics simulations of magic-number rare gas clusters. We present a mean-field model showing that the observed behavior is possible if (a) the core and surface both undergo finite-system analogs of first-order transitions with overlapping coexistence regions and (b) the mode-softening defects of the core exhibit an attractive interaction with the vacancy-floater pairs on the surface.

We report computer simulations of the phase behavior of dipolar (ferro-) fluids. We consider a model in which the dispersive interactions can be varied independently from the dipolar (magnetic) interactions. The simulation results show that a minimum amount of dispersive energy is required to observe liquid-vapor coexistence. If the dispersive energy is below this threshold, as for example in the dipolar hard-sphere fluid, the system forms chains of dipoles aligning nose to tail. Our simulations did not give any evidence that these ‘‘polymerlike’’ systems phase separate into a liquid and vapor phase.

The phase separation in a quenched system with an attractive and a long-range repulsive interaction is studied through Langevin simulations. Systems with both conserved and nonconserved order parameters are considered. After the initial shape transitions, the system forms modulated patterns broken up by topological defects which anneal away as the system orders. The defect collision mechanisms for the hexagonal phase are identified and the ordering process described.