Classical transport coefficients in a $d$-dimensional medium with a potential $V\left(\mathrm{x}\right)$ and/or conductivity $a\left(\mathrm{x}\right)$ are found to vary discontinuously as functions of the "wavelengths" of the inhomogeneities. For example, with a potential depending on one direction only, say $V\left(\mathrm{x}\right)=cos2\pi x_1+cos2\pi k{x}_1$, the effective diffusion coefficient $D\left(k\right)\mathrm{}$ has the same value $D^{*}$ for all irrational $k$, but differs from $D^{*}$ and depends on $k$ for $k$ rational. Thus $D\left(k\right)\mathrm{}$ is discontinuous at rational $k$. Moreover, $D\left(k\right)\mathrm{}$ is continuous at irrational $k$. This pathology is reflected in the time scales on which the diffusion approaches its limiting behavior.

A new treatment is proposed for the rare kaon decays $K^{+}\to {\pi }^{+}{e}^{+}{e}^{-}$ and $K^{+}\to {\pi }^{+}{\mu }^{+}{\mu }^{-}$. Although the individual rates seem impossible to predict accurately at present because of large cancellations of different amplitudes, it is pointed out that a measurement of certain ratios involving these decays could provide interesting information on the weak nonleptonic interaction.

The ${\mathrm{E}}_8$ ⊗ E${\prime }_8$ superstring models in Calabi-Yau internal space have the cosomological axionic domain-wall problem, $N_{\mathrm{DW}}>~2$, if two non-Abelian gauge groups $\mathrm{SU}{\mathrm{}\left(3\right)\mathrm{}}_c\otimes G$ survive. Breaking $G$ down to its Abelian subgroup removes the domain-wall problem. In general, if $\mathrm{SU}{\mathrm{}\left(3\right)\mathrm{}}_c$ is the only surviving non-Abelian group of the superstring models, the domain-wall problem is not present, because of the model-independent invisible axion. We also point out an elegant method of solving the axionic domain-wall problem with many would-be axions.

We present a high-statistics computation of finite-temperature QCD: The effects of dynamical quarks are included by the pseudofermion method. We concentrate only on one value of the bare quark mass and try to determine the order of the transition. The metastability observed in a previous preliminary study of ours is confirmed also by this new set of data.

Constraints are surveyed for massive neutrinos which decay without emitting radiation. We point out that the delayed neutrino flux signal at the time of a type-II supernova and high-energy neutrinos from Cygnus X-3 will give information on the scale parameter that causes neutrino decays far beyond the reach of laboratory experiments. The possibility is also shown that for a particular parameter value the decaying neutrinos from a supernova may increase the background events is underground detectors.

The production of ${\mu }^{+}{\mu }^{-}$ pairs with large longitudinal momentum has been studied in ${\pi }^{-}N$ collisions. The data confirm an earlier result that most of the pair production takes place via longitudinally polarized virtual photons as $x_{\mathrm{F}}\to 1$. The transverse-momentum distribution of the muon pairs shows a marked narrowing in the same limit. The distribution of valence quarks in the pion is extracted, and indicates a nonzero probability of having a single quark carry all available longitudinal momentum. These results are compared with a QCD model of pion structure which includes the effect of higher-twist corrections.

In the standard model there is practically no observable $\mathrm{CP}$-nonconserving effect in $B_s$ decays because of very delicate cancellations. Presence of new physics will disturb these cancellations and change the result dramatically. This is illustrated by consideration of a supersymmetric extension of the standard model. A key ingredient to the calculation is an observable phase which has been overlooked in all previous analyses.

Expansion of supersymmetric string theory suggests that the leading quadratic curvature correction to the Einstein action is the Gauss-Bonnet invariant. We show that this model has both flat and anti-de Sitter space as solutions, but that the cosmological branch is unstable, because the graviton becomes a ghost there: The theory solves its own cosmological problem. The general static spherically symmetric solution is exhibited; it is asymptotically Schwarzschild. The sign of the Gauss-Bonnet coefficient determines whether there is a normal event horizon (for the string-generated sign) or a naked singularity. We discuss the effects of higher-curvature corrections and of an explicit cosmological term on stability.

The production of kaons at subthreshold energies from heavy-ion collisions is sensitive to the nuclear equation of state. In the Boltzmann-Uehling-Uhlenbeck model, the number of produced kaons from central collisions between heavy nuclei at incident energies around 700 MeV/nucleon can vary by a factor of ∼ 3, depending on the equation of state.

A precise study of ${}_{}{}^{148}{}_{}{}^{}\mathrm{Dy}$ Gamow-Teller ${\beta }^{+}$ decay gives 97% direct feeding to the $\left(\pi h_{\frac{11}{2}}\nu h_{\frac{9}{2}}\right)1^{+}$ state in the daughter nucleus ${}_{}{}^{148}{}_{}{}^{}\mathrm{Tb}$, corresponding to $logft=3.95\mathrm{}\left(3\right)\mathrm{}$. The transition strength is only 15% of the value calculated with the quasiparticle random-phase approximation, which takes into account pairing effects and ground-state correlation. The strength reduction is significantly larger than found in other $\pi j_{>}\to \nu j_{<}$ Gamow-Teller ${\beta }^{+}$ decays.

The breakup reaction $\mathrm{dp}\to \mathrm{ppn}$ was studied at ten momenta in the incident-deuteron-momentum range 2.0-3.7 GeV/c. The magnitudes of the differential cross sections for quasielastic scattering of protons and neutrons are lower by 10%-20% than those of the free $\mathrm{pp}$ and $\mathrm{np}$ elastic cross sections. However, the shapes of the quasielastic and free elastic differential cross sections are quite similar except possibly in the very forward region.

The tensor polarization $t_{20}$ in ${\pi }^{+}d$ elastic scattering has been measured as a function of energy at 15° (laboratory) over the energy range $T_{\pi }=118\mathrm{} \mathrm{to} 148$ MeV. The angular distributions at 135 and 142 MeV between 15° and 35° were also measured. The $t_{20}$ values are negative for all energies and angles investigated and are consistent with the data of Holt et al. and Ungricht et al., resolving the previous discrepancy with the results of Ulbricht et al., Grüebler et al., and Konig et al. As there are no significant rapid variations in $t_{20}$, our data do not lend support to the existence of dibaryon resonance effects at these kinematic ranges.

Interaction cross sections (${\sigma }_I$) for all known Li isotopes (${}_{}{}^6{}_{}{}^{}\mathrm{Li}$-${}_{}{}^{11}{}_{}{}^{}\mathrm{Li}$) and ${}_{}{}^7{}_{}{}^{}\mathrm{Be}$, ${}_{}{}^9{}_{}{}^{}\mathrm{Be}$, and ${}_{}{}^{10}{}_{}{}^{}\mathrm{Be}$ on targets Be, C, and Al have been measured at 790 MeV/nucleon. Root mean square radii of these isotopes as well as He isotopes have been deduced from the ${\sigma }_I$ by a Glauber-type calculation. Appreciable differences of radii among isobars (${}_{}{}^6{}_{}{}^{}\mathrm{He}$-${}_{}{}^6{}_{}{}^{}\mathrm{Li}$, ${}_{}{}^8{}_{}{}^{}\mathrm{He}$-${}_{}{}^8{}_{}{}^{}\mathrm{Li}$, and ${}_{}{}^9{}_{}{}^{}\mathrm{Li}$-${}_{}{}^9{}_{}{}^{}\mathrm{Be}$) have been observed for the first time. The nucleus ${}_{}{}^{11}{}_{}{}^{}\mathrm{Li}$ showed a remarkably large radius suggesting a large deformation or a long tail in the matter distribution.

A new measurement of parity nonconservation in cesium is reported. The experimental technique involves measurement of the $6S\to 7S$ transition rate by use of crossed atomic and laser beams in a region of perpendicular electric and magnetic fields. Our results are $\frac{\mathrm{Im}{\mathcal{E}}_{\mathrm{PNC}}}{\beta }=-1.65\mathrm{}\pm 0.13$ mV/cm and $C_{2p}=-2\mathrm{}\pm 2$. These results are in agreement with previous measurements in cesium and the predictions of the electroweak standard model. This experimental technique will allow future measurements of significantly higher precision.

A new method for investigating fluctuations in macroscopic coherences is suggested and demonstrated by measurement of radiator reorientation kernels in Stark-tunable infrared transitions of ${}_{}{}^{13}{}_{}{}^{}\mathrm{C}$${\mathrm{H}}_3$F. The dominant ($\Delta M=1\mathrm{}$) kernel is obtained by a scale transformation from the shape of a curve of two-pulse photon-echo intensity versus Stark voltage, at fixed large echo time delay.

Gas displacing a high-viscosity fluid in a two-dimensional porous disk intrudes in the form of ramified fingers similar to the structures obtained in diffusion-limited aggregation. We find that the resulting finger structures are described by a fractal dimension $D=1.62\mathrm{}\pm 0.04$ consistent with $D$ for diffusion-limited aggregation clusters. This result confirms the analogy between diffusion-limited aggregation and two-fluid displacement in porous media introduced by Paterson.

We report the observation of weak localization of light in a random medium. In a sample consisting of a highly concentrated suspension of polystyrene particles in water, polarization-dependent enhanced backscattering of laser light was found within a cone of approximately 0.2° (half angle) for the highest concentration.

Coherent backscattering of waves by a disordered scattering medium is responsible for weak localization. We have directly observed this effect for the first time using visible light and concentrated aqueous suspensions of submicron-size polystyrene spheres. The scattered intensity is found enhanced by up to 75% within a narrow cone centered at the backscattering direction. As predicted by theory, the aperture of the cone is inversely proportional to the light mean-free path; the latter was controlled by the concentration of spheres. The importance of light polarization and particle size is discussed.

We report studies of the lattice dynamics and one-phonon absorption spectrum for the $\pi$-bonded-chain model of Si(111)2×1. The surface conductivity is dominated by a localized mode near 50 meV associated with a longitudinal optical phonon on the surface chains which is coupled to charge fluctuations in the electronic surface states. The dipole activity of this feature is anomalously strong and, interestingly, polarized parallel to the surface, along the chain direction. The dipole activity polarized normal to the chains is also presented.

We have observed highly reproducible isolated dissipation events in the flow of pure superfluid ${}_{}{}^4{}_{}{}^{}\mathrm{He}$ through an aperture of submicronic size at 10 mK, when the flow velocity just exceeds a well-defined critical threshold in agreement with Feynman's criterion for vortex emission. These events, which we interpret as phase slippages by $2\pi$ in the sense of Anderson, occur at a rate given by the Josephson frequency relation.