A low-energy positron beam is used to study the behavior of positrons in low-density (≈ ${10}^{-3\mathrm{}}$ Torr) ${\mathrm{H}}_2$ and He. Positronium is formed in flight and the collision-free Doppler-broadened singlet annihilation linewidth in ${\mathrm{H}}_2$ is found to be 6.4±0.1 keV in agreement with the calculations of Bussard, Ramaty, and Drachman. The measured fraction of positrons surviving below the positronium threshold is larger than predicted. We conclude that the neutral interstellar environment cannot be ruled out as a galactic annihilation site.

The dynamics of a time-dependent quantum system can be qualitatively different from that of its classical counterpart when the latter is chaotic. It is shown that small noise can strongly alter this situation.

We propose the transformation of the Langevin equation into an eigenvalue problem as the framework of the stochastic simulation of quantum systems. The potential of the method is illustrated by numerical results for the low-lying energy eigenvalues of one-particle systems. In the case of many degrees of freedom, the critical dynamics of a classical $d$-dimensional system is mapped onto the statics of an associated quantum system and of its classical ($d+1\mathrm{}$)-dimensional counterpart. These mappings yield an equality relating the dynamical critical exponent $\Delta$ and the static susceptibility exponents of three models.

The estimation problem of the mean-free-path parameter in the case of limited statistics and censored exponential distributions is outlined. A new estimator is suggested which, although biased, has desirable properties and could preferably be used for analyzing nuclear emulsion data as well as data from CR-39 etched track detectors in the search for anomalous behavior of heavy-ion fragments.

Correlations in rapidity space are presented for identified ${\pi }^{\pm }$ and $K^{\pm }$ in $e^{+}{e}^{-}$ annihilation at 29-GeV c.m. energy. Short-range $\mathrm{KK}$ correlations indicate local flavor compensation in the hadronization process. Long-range $\mathrm{KK}$ and $\pi \pi$ correlations prove that the initial partons carry flavor. In addition, we observe significant $K\pi$ correlations as a result of heavy-quark decays.

The possibility is discussed that the lowest-energy state for certain quantum numbers involves a Higgs field polarized into a skyrmion-type configuration. In some models a new type of vacuum instability arises. Phenomenological consequences are indicated schematically.

Using the QCD sum-rule approach, we study the two-body decays into ordinary mesons of $J^{\mathrm{PC}}=1^{-+}$ quark-antiquark-gluon hybrid mesons containing $u$ and $d$ quarks. It is found that the $I=0\mathrm{}$ state is stable for these decays, while the $I=1\mathrm{}$ states decay predominantly into $\pi \rho$ with a characteristic width of 10 to 100 MeV.

We derive an effective Lagrangian for lattice gauge theories at finite temperature in the strong-coupling limit. From this we compute the chiral phase transition and make predictions for the order of the phase transition.

We interpret the $\zeta \left(8.3\mathrm{} \mathrm{GeV}\right)$ as the $0^{++}$ ground state of a scalar-quark ($\sigma \bar{\sigma }$) bound system where $\sigma$ is a color triplet and has electric charge $\mathrm{}\left|Q\right|=\frac{1}{3}$. With the use of the quarkonium potential model, the energy of the third ($1^{--}$) $P$ state is calculated to be within 50 MeV of the $\Upsilon$ state. If it falls within the machine energy spread (20 MeV at DORIS) of the $\Upsilon$, all existing data can be understood.

It is shown that, in gauge theories of chiral Weyl fermions, quantum effects can induce breakdowns of relativistic space-time symmetries in four dimensions. These so-called "anomalies" are caused by U(1) gauge fields, and in combination with gravitational field; non-Abelian gauge fields have no effect. In the standard Glashow-Weinberg-Salam SU(2) ⊗ U(1) model, the U(1) charges of the chiral fermions are such that, within each family, these relativistic anomalies cancel out for leptons and quarks separately.

The total reaction cross sections for intermediate-energy proton scattering on ${}_{}{}^{40}{}_{}{}^{}\mathrm{Ca}$ and ${}_{}{}^{208}{}_{}{}^{}\mathrm{Pb}$ are calculated within the Dirac-eikonal formalism. Comparison with data indicates that the recently proposed impulse-approximation Dirac optical potential for nucleon-nucleon scattering is quite adequate, and accounts for 90% of the measured total reaction cross section.

The reaction 1080-MeV ${}_{}{}^{40}{}_{}{}^{}\mathrm{Ar}$+${}_{}{}^{238}{}_{}{}^{}\mathrm{U}$ has been studied via correlated fission fragments and He particles. Between $\frac{1}{3}$ and ½ of the reaction cross section goes into fusionlike events to form composite nuclei excited to 500-800 MeV. The probability for such high-energy containment drops dramatically for ${}_{}{}^{40}{}_{}{}^{}\mathrm{Ar}$ projectiles between 27 and 44 MeV/u. Evaporative He particles for fusionlike events (multiplicity ≈ 1.5) have been found to arise predominantly from the massively excited composite nuclei.

Frequency-resolved laser-induced gamma-ray anisotropy has been achieved and used to measure the $D_1$ hyperfine structure of the 1-μs isomer ${}_{}{}^{85}{}_{}{}^{}\mathrm{Rb}_{}^m{}_{}{}^{}$. The magnetic dipole moment obtained is $(6.046\mathrm{}\pm 0.010){\mu }_N$ and the isomer shift relative to ${}_{}{}^{85}{}_{}{}^{}\mathrm{Rb}$ is —52±9 MHz. The experiment was performed with a sealed cell containing radioactive ${}_{}{}^{85}{}_{}{}^{}\mathrm{Kr}$ (3 mCi) and natural Rb(∼ 10 mTorr). This method is simple and convenient and promises to be of wide usefulness.

The results of an experimental laser study of state-selected, doubly excited states $\mathrm{msnl}$ ($m=7-11\mathrm{}$, $n=13-20\mathrm{}$) in barium are reported. Measurements are made of the autoionization rates of the doubly excited states as a function of "core" size principal quantum number $m$. For $\mathrm{msnd}$ states, the autoionization rate increases rapidly with increasing $m$, while for $\mathrm{msns}$ states, the autoionization rate is independent of $m$. This difference in the dependence of the autoionization rate on $m$ is not predicted by calculations using an independent particle framework (e.g., frozen core Hartree-Fock).

We show that a frequently used nonperturbative approximation for atomic ionization rates is canceled out when corrections are taken into account. This explains the strong gauge dependence of previous results. A convergent and gauge-invariant expansion is obtained. Numerical results show that its first term, which may be calculated analytically in many cases, describes very well the time-dependent behavior of the ionization probability for very strong fields.

Electron-cyclotron and neutral-beam heating have been used to produce currentless plasma with $〈\beta 〉\approx \mathrm{}(1-2)\%$ in the Heliotron-$E$ device. For moderately peaked profiles, magnetohydrodynamic activity that peaks at the $t=1\mathrm{}$ surface has been observed. The characteristics of the fluctuations are consistent with theoretical expectations for an $m=1\mathrm{}$ pressure-driven instability. The magnetohydrodynamic activity is associated with particle and energy losses but has not yet been directly linked to global confinement.

A planar texture of a cholesteric liquid crystal is submitted to a uniaxial steplike strain parallel to the helical axis. Using a time-resolved optical Bragg-scattering technique, we measure the average pitch of the texture (and its dispersion) versus time. We observe a nonuniform hydroelastic dilation, which relaxes to the uniform-strain equilibrium state through a permeative flow.

Synchrotron x-ray diffraction studies of the orientational epitaxy of krypton on single-crystal graphite are reported. The system displays a continuous transition from an aligned to a rotated orientation with increasing mean misfit in a manner predicted by a hexagonal-domain-wall model of the weakly incommensurate phase. A hysteresis at the critical misfit is observed. The results are discussed in light of current views of the structure of the incommensurate phase.

We report observations of optical birefringence indicative of spontaneous, long-range, nematic alignment in gels of polydiacetylene 4BCMU in toluene. The results indicate that an order parameter with magnitude of only ∼ ${10}^{-2\mathrm{}}$ is maintained over macroscopic distances in an otherwise random system.

For a translationally invariant model of a chain of classical particles with competing interactions, the existence of tunneling levels is proved. Their density of states, which exhibits a scaling property, is derived for a special type of quenched disorder. Finally it is shown that the low-temperature specific heat behaves like $c\mathrm{}\left(T\right)\mathrm{}\sim T^{\tilde{d}}$ with a fractional exponent $\tilde{d}=\frac{-\left(\ln 2\right)}{\ln \left|\eta \right|}<1\mathrm{}$, where $\eta$ depends on the coupling constants.