We identify a new class of potentially large inclusive cross sections for the production of hadrons when high-energy photons interact with protons. Their origin is the abundant gluon content of very-high-energy photons. They can enhance the hadron (and also muon) content of high-energy electromagnetic showers as indicated by cosmic-ray observations and most dramatically illustrated by our failure to understand the muon content of photon showers from point sources. Experiments at the DESY $\mathrm{ep}$ collider HERA are in an ideal position to identify these contributions to photoproduction.

An approach to Witten string field theory based on the discretization of the world sheet is adopted. We use it to calculate tree amplitudes with the formulation of the theory based on string functionals. The results are evaluated numerically and turn out to be very accurate, giving, for a string approximated by 600 points, values within 0.02% of the prediction of the dual model. The method opens a way to calculate amplitudes in string field theory using nonflat backgrounds as well as compactified dimensions.

The chiral Schwinger model is studied on the lattice with use of Wilson fermions. The arbitrary mass term for the gauge boson is shown to originate from the arbitrariness of the Wilson parameter, which is required to avoid the doubling phenomenon on the lattice. The necessity for such a term is thus demonstrated in contrast to the mere admissibility as indicated by previous continuum calculations.

We calculate the rate of the inclusive loop-induced process $b\to d+\cdots$ for three and four families using the constraint of $B-\overline{B}$ mixing on the quark masses and mixings. We find that the level of background which this process provides to the tree-level $b\to u+\cdots$ decay may reach 10% if $\left|\frac{V_{\mathrm{ub}}}{V_{\mathrm{cb}}}\right|$ and may grow to about 40% if $\left|\frac{V_{\mathrm{ub}}}{V_{\mathrm{cb}}}\right|=0.07\mathrm{}$. The branching ratio of $b\to d+\cdots$ may reach its highest level of 0.5% if $\left|\frac{V_{\mathrm{ub}}}{V_{\mathrm{cb}}}\right|=0.2\mathrm{} \mathrm{and} 0.1\%$ if $V_{\mathrm{ub}}=0\mathrm{}$. In the latter case $b\to d+\cdots$ cannot account by itself for the charmless baryonic modes observed recently by the ARGUS collaboration.

Complex fragments of nuclear charge $Z$ from 4 to 13 were measured at five laboratory angles by use of surface-barrier-detector telescopes. Coincident $\gamma$ rays were measured in a $4\pi$ geometry using 64 NaI detectors. Kinematical analysis of the fragments revealed that those of $Z$ between 4 and 9 are consistent with the decay of a compound nucleus formed in a complete fusion reaction. $\gamma$ rays of excited states of ${}_{}{}^{12}{}_{}{}^{}\mathrm{C}$, ${}_{}{}^{15}{}_{}{}^{}\mathrm{N}$, and ${}_{}{}^{16}{}_{}{}^{}\mathrm{O}$ were measured and effective temperatures of about 2 MeV were extracted. Hauser-Feshbach calculations reproduce the measured cross sections and the effective temperatures very well.

The effect of quantal and thermal quadrupole shape fluctuations in the giant dipole response function of hot nuclei at high spin is studied within the Landau theory of phase transitions. The effects are found to be important in the relation of the nuclear shape to the experimental findings, and in the identification of shape phase transitions.

We report the first observation of photodissociation of ${\mathrm{H}}_3$. Predissociation of the optically prepared ${3s}^2{A}_1^{\prime }$ and ${3d}^2{E}^{\prime \prime }$ states by the ${\tilde{X}}^2{E}^{\prime }$ ground state is detected by monitoring of the production of rovibrationally excited ${\mathrm{H}}_2$ molecules and H atoms. Product excitation is found to be highly dependent on ${\mathrm{H}}_3$ electronic and nuclear configuration.

Classical-trajectory Monte Carlo calculations have been used to study ionizing collisions between charged particles ($p,\overline{p},e^{+},e^{-}$) and He atoms at an incident velocity of 2.83 a.u. While differences in the total single-ionization cross sections for these projectiles are small, our calculations reveal large effects at all angles in the ionized electron spectra, and provide qualitative evidence for a Barkas effect in $p$ and $\overline{p}$ collisions. Experimental data agree well with our fully classical calculations, including cross sections involving ejected electrons of long wavelength.

By means of the supersonic expansion technique in conjunction with a high-resolution mass spectrometer, two separate forms of doubly and triply charged water cluster have been observed. The doubly charged clusters have been identified as $\left({\mathrm{H}}_2\mathrm{O}\right)_n^{}{}_{}{}^{2+}$ and ${\left({\mathrm{H}}_2\mathrm{O}\right)}_n\mathrm{H}_2^{}{}_{}{}^{2+}$, and the triply charged as ${\left({\mathrm{H}}_2\mathrm{O}\right)}_n{\mathrm{H}}^{3+}$ and ${\left({\mathrm{H}}_2\mathrm{O}\right)}_n\mathrm{H}_3^{}{}_{}{}^{3+}$. Coulomb explosion imposes a minimum value of $n$ below which each form is unstable, and that value is different for each of the four types of multiply charged cluster.

We have made an improved measurement of the parity-nonconserving electric-dipole transition amplitude between the $6S$ and $7S$ states of atomic cesium. We obtain $\frac{\mathrm{Im}\left(E_{\mathrm{PNC}}\right)}{\beta =-1.576\mathrm{}\left(34\right)\mathrm{}}$ mV/cm, which is in good agreement with the predictions of the standard model and earlier less precise measurements. This places more stringent constraints on alternatives to the standard model. We also see the first evidence of a nuclear-spin-dependent contribution to atomic parity nonconservation. The nuclear-spin dependence observed is in agreement with that predicted to arise from a nuclear anapole moment.

We have measured total charge-exchange cross sections in collisions between mass-selected $\mathrm{Na}_n^{}{}_{}{}^{+}$ cluster ions ($1\le n\le 21$) and atomic cesium. Cross sections for clusters ranging from the monomer to the 21-mer lie between 40 and 10 ${\mathrm{\AA }}^2$, decreasing slowly with increasing mass. Fast neutral products of charge exchange are photoionized and mass analyzed in a secondary time-of-flight spectrometer. Charge-transfer products present a narrow distribution of at most three masses ${\mathrm{Na}}_n$, ${\mathrm{Na}}_{n-1\mathrm{}}$, and ${\mathrm{Na}}_{n-2\mathrm{}}$, with the most probable product being ${\mathrm{Na}}_n$ or ${\mathrm{Na}}_{n-1\mathrm{}}$.

We show experimentally that propagation of light waves in an effectively rotating fiber-optic ring resonator leads to a photon band structure due to interference of elastically scattered waves. The rotation is simulated by means of a Faraday-active element in the ring.

Dispersion formulas are presented for the second hyperpolarizabilities of atoms relevant to the nonlinear optical processes (a) dc Kerr, (b) degenerate four-wave mixing, (c) electric-field-induced second-harmonic generation, and (d) third-harmonic generation. They are more comprehensive than previously published formulas and exact up to the second power in the optical frequency.

We have studied the nonlinear optical response of the quasi-1D semiconductor polydiacetylene-toluene sulfonate under ultrashort optical pulse excitation. The effect of the excitation is to "burn a hole" in the exciton line. The line shape of the hole depends sensitively on the pump frequency, ${\omega }_p$, and its magnitude peaks when ${\omega }_p={\omega }_x-{\omega }_o$, where ${\omega }_o$ and ${\omega }_x$ are the phonon and exciton frequencies. We discuss these results with a model in which optical nonlinearity results from phonon-mediated interactions between virtual excitons.

The steady states of an array of bistable optical elements are analyzed by the use of a nonlinear-dynamics analogy. Spatial chaos is an essential counterpart of effective optical-memory action. Criteria for stability and maximum packing density are obtained.

We report a detailed quantitative comparison between experimental measurements of the structure of a nonwetting fluid invasion front into a porous medium and a gradient percolation model accounting for the influence of gravity. The 2D correlation function of the invaded area in horizontal cuts follows a universal behavior at short distances in the fractal domain in good agreement with numerical predictions. Variations with the cut height of the mean saturation and of the crossover length between the fractal and the Euclidean domains can also be fitted precisely with theoretical gradient percolation curves.

We investigate the effects of quickly creating a plasma around a monochromatic electromagnetic source wave, on time scales on the order of a cycle of the wave. It is found that this results in an upward shifting of the wave frequency, which can be varied by changing of the plasma density. It is also found that a substantial fraction of the magnetic field associated with the initial wave can be sustained in the plasma as a time-independent magnetic field. Computer simulations have been used to study this process in detail, including the effects of finite ionization time. For long ionization times, strong plasma heating results.

We report frequency-dependent measurements of the specific heat $c_p$ thermal conductivity $\kappa$ of ${(o-\mathrm{t}\mathrm{e}\mathrm{r}\mathrm{p}\mathrm{h}\mathrm{e}\mathrm{n}\mathrm{y}\mathrm{l})}_{1-x}{(o-\mathrm{p}\mathrm{h}\mathrm{e}\mathrm{n}\mathrm{y}\mathrm{l}\mathrm{p}\mathrm{h}\mathrm{e}\mathrm{n}\mathrm{o}\mathrm{l})}_x$ mixtures near the glass transition. $\kappa$ is flat and frequency independent and all of the dynamics is contained in $c_p$. The mean relaxation time $\tau$ shows non-Arrhenius behavior and the distribution of times broadens with decreasing temperature. The divergence of both $\tau$ and the width of the distribution is consistent with the Kauzmann temperature.

High-resolution calorimetric investigations have been carried out on the chiral and racemic versions of 4-(3-methyl-2-chlorobutanoyloxy)-4′-heptyloxybiphenyl. While the chiral compound shows a weakly first-order $\mathrm{S}\mathrm{m}\mathrm{e}\mathrm{c}\mathrm{t}\mathrm{i}\mathrm{c}-A-\mathrm{c}\mathrm{h}\mathrm{i}\mathrm{r}\mathrm{a}\mathrm{l}-\mathrm{S}\mathrm{m}\mathrm{e}\mathrm{c}\mathrm{t}\mathrm{i}\mathrm{c}-C$ transition, the racemic one exhibits a continuous $\mathrm{S}\mathrm{m}\mathrm{e}\mathrm{c}\mathrm{t}\mathrm{i}\mathrm{c}-A-\mathrm{S}\mathrm{m}\mathrm{e}\mathrm{c}\mathrm{t}\mathrm{i}\mathrm{c}-C$ transition. The latter transition is found to be at a mean-field tricritical point, within our experimental resolution.

Using scanning tunneling microscopy and spectroscopy we have investigated the growth of a ⅓ - monolayer Ag film on Si(111)7×7 at substrate temperatures of 90 and 130°C. An Ag-induced subunit has been identified on top of three dangling bonds of the second atomic layer, which may be considered as the critical nucleus of Ag condensation. At 130°C Ag mostly condenses in the form of triangularshaped flat islands of equal size, which are preferentially located on faulted halves of the 7×7 unit cells and are explained by a close-packed structure of Ag atoms. On the Ag islands the local density of states near the Fermi level is reduced compared to the uncovered 7×7 unit cells.