Irreversibility stabilizes certain locally interacting discrete systems against the nucleation and growth of a most-stable phase, thereby enabling them to behave in a computationally complex and nonergodic manner over a set of positive measure in the parameter space of their local transition probabilities, unlike analogous reversible systems.

For a closed system of two coupled nonlinear oscillators, chaotic orbits are punctuated by holes associated with stable periodic orbits. For the corresponding class of Hamiltonian maps we demonstrate that the combined area for all holes of size $\epsilon$ or greater scales as a power law with exponent $\beta$ and asymptotic area $0<\mathrm{}\mu <1\mathrm{}$. In contrast to previous results, this is a global scaling property, valid for a set of positive Lebesgue measure. It suggests that these chaotic orbits are fat fractals, i.e., Cantor-set-like objects of positive area. We numerically compute lower bounds on their area.

We use the reaction $e^{+}{e}^{-}\to {\mu }^{+}{\mu }^{-}$, in the Mark J detector at the DESY high-energy $e^{+}{e}^{-}$ collider PETRA, to test the standard electroweak theory and find good agreement. We also set limits on the parameters of several extended gauge theories.

The spectrum of a supersymmetric quantum mechanical theory that leads to a generalized superalgebra is computed exactly. It has the feature that both the ground and first excited levels have unequal numbers of Bose and Fermi states.

We propose an SO(18) grand-unified model that predicts three light generations, if the horizontal symmetry below the grand-unification scale is chosen to be $\mathrm{Sp}\mathrm{}\left(4\right)\mathrm{}\otimes \mathrm{SU}{\mathrm{}\left(2\right)\mathrm{}}_d$. A novel feature of this model is the existence of an unbroken SU(2) gauge symmetry, under which the light generations are singlets. This raises some interesting phenomenological possibilities.

A global $\mathrm{U}{\mathrm{}\left(1\right)\mathrm{}}_A$ symmetry is used to solve both the fermion-mass-hierarchy problem and the strong $\mathrm{CP}$ problem. A domain-wall-free invisible-axion model requires heavy fermions at the axion scale. The dominant contribution to the off-diagonal elements of the fermion mass matrix comes from the small vacuum expectation value of superheavy scalar doublets, which is guaranteed by the Peccei-Quinn symmetry.

With use of a recently developed particle-hole theory for pion production from coherent isobar formation and decay, calculations of total cross sections and kinetic-energy distributions for neutral subthreshold pions produced in carbon-carbon collisions at incident energies below 100 MeV/nucleon are made and compared with recent experimental data.

Differential cross sections for ${}_{}{}^{14}{}_{}{}^{}\mathrm{N}(\gamma , {\pi }^{+}){}_{}{}^{14}{}_{}{}^{}\mathrm{C}(\mathrm{g}.\mathrm{s}.\mathrm{})$ ($0^{+}$1) have been measured for an incident photon energy $E_{\gamma }=200\mathrm{}$ MeV at laboratory angles of 45°, 60°, 75°, 90°, 120°, and 140°. Momentum-space distorted-wave-impulse-approximation calculations, using a complete treatment of a one-body pion-photoproduction operator, are found to be in excellent agreement with the experimental data. The effect of the momentum-dependent terms in the nuclear photoproduction operator is observed. The reaction is found to be sensitive to input nuclear structure elements not observable in the ($e, e^{\prime }$) form-factor data.

Measurements are presented of spin asymmetries for superelastic scattering of 10-eV spin-polarized electrons from the excited $\mathrm{Na}\left(3\mathrm{}{P}_{\frac{3}{2}}\right)$ state created by linearly polarized laser optical pumping. Asymmetries as large as 16% are observed in scattering from a state which is not spin polarized. Results are shown both as a function of scattering angle with fixed laser polarization direction, and as a function of the laser polarization direction at a fixed scattering angle.

${\mathrm{He}}^{-}$ photodetachment cross sections near the $\mathrm{He}\left(2\mathrm{}{}_{}{}^3{}_{}{}^{}P\right)$ threshold have been measured and analyzed. The data were found to deviate rapidly from the Wigner threshold law because of the strong influence of the nearby ${}_{}{}^4{}_{}{}^{}P_{}^e{}_{}{}^{}$ shape resonance. A modified formula has been derived, which accounts for this effect. It successfully fits the data not only in the threshold region but over the resonance itself. The threshold data yield an electron affinity for $\mathrm{He}\left(2\mathrm{}{}_{}{}^3{}_{}{}^{}S\right)$ of 77.5±0.8 meV, in agreement with the accurate calculations of Bunge and Bunge.

The dispersion of the nonlinear susceptibilities of Ar, ${\mathrm{N}}_2$, and ${\mathrm{O}}_2$ have been measured for wavelengths from 700.0 to 457.9 nm. A comparison is made with phenomenological models used in the literature.

Measured frequency fluctuation spectra $S_{\delta f}$ in ring lasers show that $S_{\delta f}=\frac{h_{-1\mathrm{}}}{\nu }+\frac{h_{-2\mathrm{}}}{{\nu }^2}$ at low Fourier frequencies $\nu$, while white noise prevails at higher frequencies. Experimentally, $S_{\delta f}=\left(\frac{\overline{A}{f}_0^2}{Q^4}\right)\left(\frac{1}{\nu }\right)$ has been found for the dependence of the $\frac{1}{\nu }$ noise (flicker noise) on the quality factor $Q$, with $\overline{A}\approx 4$ ($f_0=\mathrm{laser}\mathrm{frequency}$). The $Q^{-4\mathrm{}}$ dependence is readily explained by loss (or gain) fluctuations, through use of a Van der Pol oscillator.

The light of a quasi-isotropic, single-mode gas laser is linearly polarized for a line $J_a=1\mathrm{}\to J_b=2\mathrm{}$. However, the azimuth of polarization can vary inside the line. Among the many different effects which cause these variations, we describe the role played by the detector optical feedback. We show experimentally that its amplitude and phase can be varied in a controlled manner in order to obtain abrupt switches between two perpendicular polarization states. Polarization bistability and hysteresis effects can then be commanded and studied from the outside of the laser.

Electrons heated by the absorption of laser energy on planar targets generate megagauss magnetic field which, together with the coproduced electric fields, transport the electrons away from the laser spot. We present a computational model showing that these superthermal electrons deposit their energy into plasma thermal energy in an expanding annular region in the target plane centered at the spot. The model provides a quantitative explanation for some complex and heretofore poorly understood lateral transport phenomenology.

The propagation and spatial evolution of a potential hump generated explosively near the cathode were observed in the current-limiting phase of a high-voltage straight discharge with a preexisting hydrogen plasma. Potential profiles along the axis numerically calculated from the real-time $E$ field data demonstrate the evolution of a potential hump into a strong double layer (asymmetric electron hole) with an inverse potential jump ($\frac{e{\phi }_D}{k{T}_e}\lesssim {10}^3$) as moving toward the center of the device with a mean velocity (1.2-1.8) × ${10}^8$ cm/sec.

The signs of two structure factors corresponding to thermally excited forbidden reflections have been determined by use of multiple Bragg scattering. This is the first direct experimental determination of unknown phases in a mosaic crystal.

We have discovered a new form of Fréedericksz transition in which uniform splay distortion is replaced by a complex periodic twist-splay pattern having a lower critical field than that for uniform splay. Experimental evidence is presented along with an exact calculation of both the critical field and the initial form of the distortion as a function of the ratio of the splay and twist elastic constants.

Correlations of fourth order in polarization fluctuations, the quantity whose ordering is frustrated, are observed in a structural glass via strictive coupling to sound. The results were obtained on a ${\mathrm{Rb}}_{1-x}{\left(\mathrm{N}{\mathrm{H}}_4\right)}_x{\mathrm{H}}_2\mathrm{P}{\mathrm{O}}_4$ crystal with Brillouin scattering. The existence of an Edwards-Anderson order parameter much above Vogel-Fulcher freezing is revealed. This suggests that the observed susceptibility slowing-down might, in many glasses, be controlled by a much slower hidden competing network.

The dynamics of He atoms, which were implanted into copper at 10 K, were observed via their trapping at ${}_{}{}^{111}{}_{}{}^{}\mathrm{In}$ atoms, using the perturbed $\gamma \gamma$ angular correlation technique. During annealing in the temperature range 10 to 300 K, two types of In-He complexes were detected with the He atom occupying either a trapped single vacancy or an octahedral interstitial lattice site. The formation of the latter He complex shows that He atoms migrate via interstitial sites in Cu already at 25 K.

We present a model of the rod-to-coil transformation in a single-stranded polymer which includes the effects of long-wavelength degrees of freedom in the straight parts of the chain. The model exhibits a true first-order phase transition in qualitative agreement with the results of an experiment.