A measurement of the τ polarization at hadron colliders offers an excellent signal for the detection of charged Higgs bosons, via the decay ${\mathit{H}}^{\pm }$→${\mathrm{\tau }}^{\pm }$ν. We show how such a determination can distinguish these events from the ${\mathit{W}}^{\pm }$→${\mathrm{\tau }}^{\pm }$ν background, for each of the τ→πν, ρν, ${\mathit{a}}_1$ν, and lνν¯ decay channels.

Asymmetric fission of small doubly-charged sodium clusters, investigated via a local-spin-density molecular-dynamics method, is shown to occur predominantly via ${\mathrm{Na}}_{\mathit{n}}^{+2\mathrm{}}$→${\mathrm{Na}}_{\mathit{n}\mathrm{-}3}^{+}$+${\mathrm{Na}}_3^{+}$, for 4≤n≤12. For the smallest cluster (n≤6), fission is barrierless, while fission of larger ones involves barriers. For n=8, 10, and 12 the barriers are 0.16, 0.71, and 0.29 eV, respectively, reflecting the closed-shell stability of the parent with n=10. The fission barriers for n>8 exhibit a double-hump shape, and the dynamics of fission involves a precursor mechanism.

We report the observation of electromagnetically induced transparency on the collisionally broadened resonance line of Pb vapor. By applying a 1064-nm laser beam, the transmission at 283 nm is increaesd by at least a factor of exp(10), with nearly all of the Pb atoms remaining in the ground state.

We present a general method for calculating two-photon processes in real atoms by a variant of the usual multichannel quantum-defect theory (MQDT). Most notably, this method enables us to describe processes in which a single photon has enough energy to ionize the atom (above-threshold processes). All effects of intermediate Rydberg series, continua, or autoionizing series are treated on an equal footing and with the same ease; the final state is also treated properly by standard MQDT. This method should also work for many three- or four-photon processes.

Flux-velocity contour maps were obtained for the inelastic collision process Ba${(}^1$${\mathit{P}}_1$)+${\mathrm{O}}_2$${\mathrm{N}}_2$→Ba${(}^3$${\mathit{P}}_2$)+${\mathrm{O}}_2$${\mathrm{N}}_2$ from Doppler scans of scattered Ba${(}^3$${\mathit{P}}_2$) taken over a range of probe laser directions in a crossed-beam experiment. Collision with ${\mathrm{O}}_2$ resulted in sharply forward scattered Ba${(}^3$${\mathit{P}}_2$), with efficient conversion of inital electronic energy into ${\mathrm{O}}_2$ internal energy and little momentum transfer. Collision with ${\mathrm{N}}_2$ was dominated by wide-angle scattering with most of the available electronic energy appearing in product translation. The results suggest the importance of large-impact-parameter collisions and a near-resonant energy transfer in the case of ${\mathrm{O}}_2$, while for ${\mathrm{N}}_2$ close collisions dominate despite the presence of an analogous near-resonant channel. The results represent the first direct experimental demonstration of a near-resonant quenching process.

As a test case for solving the quantum Monte Carlo sign problem, we propose an exact and stable algorithm for few-electron systems. The energies of an excited state of He and of the ground states of Li and Be are determined. We use equal numbers of positive and negative walkers, sampling new positions from a distribution that couples all walkers using a Green’s function of known form. For long-term stability, the importance functions are different for positive and negative walkers. A novel transformation of the potential facilitates the evaluation of the energy.

We present experimental studies of the transitions between conduction, hexagons, and rolls in non-Boussinesq convection of gaseous ${\mathrm{CO}}_2$ in a cylindrical cell of radius-to-height ratio 86. Except for the transition from conduction to hexagons, transitions occur when the two states involved have nearly the same value of a generalized potential rather than at the stability limits. Conduction gives way to hexagons via the propagation of a front connecting the two states, while the transitions between hexagons and rolls are facilitated at the cell walls which appear to nucleate the minority state.

We report on a new regime of free-electron-laser operation with reversed axial guide magnetic field, in which the rotational motion of the electrons in the helical wiggler field is opposed by the presence of the uniform guide field. The 33.3-GHz free-electron-laser amplifier is driven by a mildly relativistic electron beam (750 kV, 300 A, 30 ns) and generates 61 MW of radiation with a 27% conversion efficiency. Measurements with the conventional orientation of the axial field show a considerable loss of power and efficiency.

We study the physics of a free-electron laser based on a ‘‘two-frequency’’ undulator (TFU) which induces large nonlinear effects, especially on the spectral dynamics. These effects are analyzed in an extended formalism where the spontaneous emission, the low-gain regime, and the strong-field saturation regime are studied. Numerical simulations show that an optimized TFU generates a laser field having both a large extraction efficiency and a narrow spectrum.

Stochastic motions of parametrically pumped electron oscillators synchronize abruptly when the pump power exceeds a threshold and the oscillators are radiatively cooled in a cylindrical Penning trap. Self-organized collective behavior far from thermal equilibrium is manifested by the coherent, phase-bistable motion of the electrons’ center of mass, which was observed. Synchronized electrons are an ideal probe of the radiation field of a trap cavity, opening the way to a new generation of electron magnetic-moment measurements and to sideband cooling of an electron motion to mK temperatures.

The time evolution of beam divergence induced by electomagnetic instabilities in applied-B ion diodes is examined using the 3D particle-in-cell code quicksilver. The evolution is generally characterized by a high-frequency, low-divergence phase, associated with the diocotron mode, followed by a low-frequency, high-divergence phase, associated with the two-stream-like ion mode. Limiting the extent of the electron density profile evolution allows the diocotron phase to be sustained with a resulting divergence of ≊10 mrad.

Following boronization, tokamak discharges in DIII-D have been obtained with confinement times up to a factor of 3.5 above the ITER89-P L-mode scaling and 1.8 times greater than the DIII-D/JET H-mode scaling relation. Very high confinement phases are characterized by relatively high central density with ${\mathit{n}}_{\mathit{e}}$(0)≊1×${10}^{20}$ ${\mathrm{m}}^{\mathrm{-}3}$, and central ion temperatures up to 13.6 keV at moderate plasma currents (1.6 MA) and heating powers (12.5–15.3 MW). These discharges exhibit a low fraction of radiated power, P≤25%, ${\mathit{Z}}_{\mathit{e}\mathit{f}\mathit{f}}$(0) close to unity, and lower impurity influxes than comparable DIII-D discharges before boronization.

A method is introduced joining together forward-scattering diffraction data taken in a small angular window at different photoelectron energies. This method extends the usable range in phase space for three-dimensional image reconstruction. Examples based on theoretical simulations demonstrate that a spatial resolution of ≤1 Å is achievable. We also show that using a small angular window in the backscattering geometry eliminates splittings in the reconstructed image.

Properly phased combinations of images reconstructed from holograms recorded at several wave numbers suppress interobject multiple scattering, self-interference, and the troublesome holographic twin image. When applied to point-source electron holographies, this solves the long-standing problem of electron multiple scattering confusing the determination of surface structure; when applied to in-line holograms of any kind it solves the classical problem of twin images which dates from the original work of D. Gabor.

The transport of 29-${\mathrm{cm}}^{\mathrm{-}1}$ phonons in single-crystal fibers of ${\mathrm{Al}}_2$${\mathrm{O}}_3$:${\mathrm{Cr}}^{3+}$ (ruby) has been investigated at 2 K. The fiber diameters are of the order of the mean free path, so that transport is dominated by diffuse scattering and by acoustic mismatch at the lateral crystal interfaces. Depending on experimental conditions, we have observed ballistic and diffusive phonon transport. We determine for the first time the probability of frequency nonconserving surface scattering for high-frequency phonons.

We report for the first time the direct determination of x-ray-reflection phases of a large-unit-cell volume, 350 000 Å ${}_{}{}^3{}_{}{}^{}$, macromolecular crystal using three-beam diffraction. The crystal polarity (enantiomorph) is determined unambiguously from the peak-intensity measurement. Quantitative analysis of phase angle, with a scaling scheme for intensities, is presented.

We describe x-ray-scattering studies of the orientational epitaxy displayed by the hexagonally reconstructed Pt(001) surface. At 1685 K, the reconstructed surface layer undergoes a continuous rotational transformation with respect to the underlying bulk and exhibits mean-field behavior. A second, discontinuous rotational transformation is observed at 1580 K. These results are inconsistent with the predictions of current theories and lead us to propose a mean-field description of rotational transformations.

We study the nonequilibrium growth of the liquid-condensed phase of monolayers subject to a pressure-induced phase transition. Because of dipolar electrostatic interactions, the Mullins-Sekerka instability is significantly modified at small wave vectors. We argue that this will have a dramatic effect on possible dendritic growth in these systems, leading to a maximum allowed size for the dendrite tip and a crossover in the velocity versus driving curve at small undercooling.

A model fluid whose molecules have classical degrees of freedom in 2D and two internal quantum states is studied by path-integral Monte Carlo simulation. We obtain a rich phase diagram including phase coexistences and tricritical and triple points with high precision by combining for the first time block-analysis finite-size scaling ideas with quantum Monte Carlo simulations. We find a square lattice solid phase in coexistence with gas and/or liquid phases. Compressibilities and susceptibilities are compared to mean-field predictions and low-density expansions.

The first electronic total-energy calculations are performed on a 6D-3D projection model for the TiMn quasicrystal, as well as two refined models obtained by filling vacant spaces. The total energies correlate almost perfectly with the reduction of the vacant space, and suggest that future modeling efforts should concentrate on increasing the coordination number of Ti sites in the ‘‘glue’’ regions between the large icosahedral clusters which form the backbone of the structure.