The reptation model for the dynamics of an entangled polymer is considered for the case where the diffusion is driven by a weak field. If the distance between entanglements is greater than the persistence length, the mobility of long chains is predicted to be independent of their size and proportional to the field strength. This difference from previous predictions is due to the fact that fluctuations alter the field dependence of the tube orientation. Implications for the separation of DNA using gel electrophoresis are discussed.

Landau level mixing by external potentials is considered in a Born-Oppenheimer expansion which separates the dynamics of the fast velocities of cyclotron rotation and the slow coordinates of the guiding centers. Level mixing is present already in the extreme adiabatic limit and in transitions at avoided crossings. Nonvanishing Berry’s phase adds effective magnetic field depending on external potential. Explicit results are given for weakly inhomogeneous potentials. The projection on a single Landau level appears as an extreme limit of the formalism.

We report the first quantitative measurements of spontaneous temperature fluctuations in a physical system well modeled by a canonical ensemble. Using superconducting magnetometers and a carefully controlled thermal environment, we have measured the noise spectra of paramagnetic salt thermometers that were coupled to thermal reservoirs at 2 K. The noise spectra were found to be in very good agreement with the predictions of the fluctuation-dissipation theorem. Our observations are at variance with some interpretations of fluctuations in the canonical ensemble.

According to string–five-brane duality, the Green-Schwarz factorization of the D=10 spacetime anomaly polynomial ${\mathit{I}}_{12}$ into ${\mathit{X}}_4$${\mathit{X}}_8$ means that just as ${\mathit{X}}_4$ is the anomaly polynomial of the d=2 string world sheet, so ${\mathit{X}}_8$ should be the anomaly polynomial of the d=6 five-brane world volume. To test this idea we perform a five-brane calculation of ${\mathit{X}}_8$ and find perfect agreement with the string one-loop result.

We discuss gravitationally induced masses and mass splittings of Majorana, Zeldovich-Konopinski-Mahmoud, and Dirac neutrinos. Among other implications, these effects can provide a solution of the solar neutrino puzzle. In particular, we show how this may work in the 17 keV neutrino picture.

I present an extension to arbitrary N of a previously proposed field theoretic model, in which unitary amplitudes for 1→8 processes were obtained. The Born amplitude in this extension has the behavior ${\mathit{A}}_{1\to \mathit{N}}^{\mathrm{tree}}$=${\mathit{g}}^{\mathit{N}\mathrm{-}1}$N! expected in a bosonic field theory. Unitarity is violated when ‖${\mathit{A}}_{1\to \mathit{N}}$‖>1, or when N>${\mathit{N}}_{\mathrm{crit}}$≃e/g. Numerical solutions of the coupled Schrödinger equations show that for weak coupling and a large range of N>${\mathit{N}}_{\mathrm{crit}}$, the exact unitary amplitude is reasonably fitted by a factorized expression ‖${\mathit{A}}_{1\mathrm{-}>\mathit{N}}$${\mathrm{\Vert }}_{\max }$≃(0.73/N)exp(-0.025/${\mathit{g}}^2$). The very small size of the coefficient 1/${\mathit{g}}^2$, indicative of a very weak exponential suppression, is not in accord with standard discussions based on saddle-point analysis, which give a coefficient of ∼1. The weak dependence on N could have experimental implications in theories where the exponential suppression is weak (as in this model). Nonperturbative contributions to few-point correlation functions in this theory would arise at order K≃[(0.05/${\mathit{g}}^2$)+2 lnN]/ln(1/${\mathit{g}}^2$) in an expansion in powers of ${\mathit{g}}^2$.

The mass of the τ lepton has been measured at the Beijing Electron-Positron Collider using the Beijing Spectrometer. A search near threshold for ${\mathit{e}}^{+}$${\mathit{e}}^{\mathrm{-}}$→${\mathrm{\tau }}^{+}$${\mathrm{\tau }}^{\mathrm{-}}$ was performed. Candidate events were identified by requiring that one τ decay via τ→eνν¯, and the other via τ→μνν¯. The mass value, obtained from a fit to the energy dependence of the ${\mathrm{\tau }}^{+}$${\mathrm{\tau }}^{\mathrm{-}}$ cross section, is ${\mathit{m}}_{\mathrm{\tau }}$=1776.$9_{\mathrm{-}0.5}^{+0.4\mathrm{}}$±0.2 MeV.

We present the perturbative QCD results for heavy quark event multiplicities in ${\mathit{e}}^{+}$${\mathit{e}}^{\mathrm{-}}$ annihilation, which provide that the difference ${\mathrm{\delta }}_{\mathit{Q}\mathit{l}}$ between heavy and light quark event multiplicities at the same c.m.-systems (c.m.s.) energy should be a calculable constant, independent of c.m.s. energy. Published data on heavy quark event multiplicities are presented in this light, and are consistent with the energy independence of the multiplicity difference. Averaging over c.m.s. energy, we find that ${\mathrm{\delta }}_{\mathit{b}\mathit{l}}$=4.3±0.9 and ${\mathrm{\delta }}_{\mathit{c}\mathit{l}}$=2.3±1.0 tracks, while the perturbative QCD expectations are 5.5±0.8 and 1.7±0.5 tracks, respectively.

The transmission spectrum of a probe interacting with one-dimensional cesium optical molasses made from two circularly cross-polarized cooling beams (corkscrew molasses) displays a narrow (≊30–70 kHz) Rayleigh resonance. From the width of this resonance we deduce the friction coefficient of the cooling force to be α=(5.8±0.4)ħ${\mathit{k}}^2$ for a detuning from resonance equal to 3Γ. This value is in quantitative agreement with theoretical predictions. We also show that the shape of the Rayleigh resonance depends dramatically on the efficiency of the sub-Doppler cooling mechanism.

The screening and antiscreening contributions to electron loss are separated experimentally through the measurement of the recoil ions and the emergent charge states in the collisions ${\mathrm{He}}^{+}$+(${\mathrm{H}}_2$,He) →${\mathrm{He}}^{2+}$+(${\mathrm{H}}_2^{+}$,${\mathrm{He}}^{+}$). The experimental data are in excellent agreement with calculations which include second-order contributions for the simultaneous ionization of the projectile and the target.

Up to now, all triple-differential cross-section measurements were performed with unpolarized electrons and atoms and, consequently, averaged over direct and exchange scattering. In our crossed-beam experiment, spin-polarized lithium atoms were ionized by spin-polarized electrons of 54.4 eV energy. In co-planar, asymmetric coincidence detection of scattered and ejected electrons, combined with electron-energy analysis, we measured a spin asymmetry which gives new information about the ionization process. Results are compared with recent calculations based on the distorted-wave Born approximation.

Interference has been observed in the mixing of signal photons produced by spontaneous down-conversion in two nonlinear crystals, when the optical path difference through the interferometer greatly exceeds the coherence length of the light. The interference shows up as a periodic modulation of the spectrum. We demonstrate that the interference is nonclassical and connected with the entangled state of the signal and idler photons, by transferring the long optical delay from the path of the signal photons to the path of the idler photons. Even though the idlers are not interfering directly, the effect is found to be unchanged, but the interference is lost when the idler photons are blocked. We discuss the extent to which one photon carries the modulated spectrum.

It is shown that semiclassical path representations provide a well-suited framework for the description of the diffraction of Rydberg atoms by laser light. They allow one to relate the momentum exchange between the atomic center-of-mass motion and the laser field directly to elementary laser-assisted collisions between the excited Rydberg electron and the ionic core. Thus besides a convenient calculational tool they provide also direct physical insight into the underlying internal dynamics of the Rydberg atoms.

Stimulated Raman scattering in the transient limit is an integrable system. In contrast, however, to the usual behavior in integrable systems, solitons are transient and the behavior of the system at long distances is dominated by self-similar solutions which may be found by symmetry reduction. It is shown for fairly general initial conditions precisely which self-similar solution the system tends toward at long distances, and the system evolution is studied numerically. It is argued that this behavior in which self-similar solutions dominate the long-distance evolution should often appear in nonlinear optical systems with memory. A possible experiment is proposed.

The absorption of subpicosecond (≊100 fs) high-intensity (I≊${10}^{20}$–${10}^{21}$ W/${\mathrm{cm}}^2$) laser pulses normally incident on solid density targets is examined by particle simulations, which include electron scattering. For targets which are overdense by factors exceeding 100, electrons accelerated by the ponderomotive force of the laser couple their energy electrostatically to the ions and the absorption efficiency into the ions scales as ${\mathit{I}}^{1/2}$. For example, absorption in a carbon target is only a few percent, but a large fraction of the absorbed energy is transferred to the ions.

A pure electron plasma, confined within an azimuthally symmetric boundary by a coaxial magnetic field, has an equilibrium shape which is cylindrical. We apply perturbations which break the azimuthal symmetry and deform the plasma into a noncircular shape that is stationary in the laboratory frame. These asymmetric equilibria form a broad new class of stable equilibria. A theoretical model correctly predicts the plasma shapes and explains their stability.

Toroidal field ripple loss of neutral-beam-injected fast ions is measured from the heat load on the first wall in JT-60U. The heat load is localized in both toroidal and poloidal directions and increases with both ripple size and safety factor. The overall measured ripple loss is in good agreement with an orbit-following Monte Carlo calculation. Nevertheless, there is a small difference between the experimental and the calculated location of the maximum heat load, which suggests that the radial electric field shifts the impinging points of ripple-trapped ions on the first wall.

A new x-ray structural technique, diffraction anomalous fine structure (DAFS), whcih combines the long-range order sensitivity of diffraction techniques with the short-range order sensitivity of absorption techniques, is described. We demonstrate that synchrotron DAFS measurements for the Cu(111) and Cu(222) Bragg reflections provide the same local atomic structural information as x-ray absorption fine structure and describe how DAFS can be used to provide enhanced site and spatial sensitivities for polyatomic and/or spatially modulated structures.

The structure and composition of the GaAs(001) surface was studied with high-resolution medium-energy ion scattering, from As-rich to Ga-rich reconstructions. In contrast to commonly accepted models, we find that first and second layers in the surface may contain both Ga and As atoms. The surfaces are more Ga-rich than previously believed, with Ga atoms occupying As sites. Such mixed compositions are explained by consideration of charge neutrality, as well as Coulomb repulsion between surface electrons. Implications for heteroepitaxial growth on GaAs(001) are discussed.

The application of pressure to solutions of polyethylene oxide in water causes phase separation through spinodal decomposition similar to that observed when the temperature is raised above the lower critical solution temperature. This phase separation pressure is dependent on molecular weight but independent of concentration. Viscosity measurements indicate a continuous decrease in the radius of gyration from good to theta solvent conditions as pressure is increased. This pressure-induced loss of solvent quality is a result of the loss of the ability of water to form hydrogen bonds at high pressure and the resulting changes in hydrophobic interactions.