We report a slow rise of luminescence in GaAs following subpicosecond photoexcitation and show that it results from a slow return of electrons from the L to the Γ valley. By fitting our data with an ensemble Monte Carlo calculation, we determine the Γ-L deformation potential to be (6.5±1.5)×${10}^8$ eV/cm. We show that the electrons returning to the Γ valley act as a source of heating for the photoexcited plasma. We further show the importance of electron-electron scattering and inadequacy of a simple phonon-cascade model, even at a density as low as 5×${10}^{16}$ ${\mathrm{cm}}^{\mathrm{-}3}$.

The excess current of a point contact on a Ag-Pb bilayer has been measured for several thicknesses of the Ag film. The excess current is due to Andreev reflection and contains information about the position dependence of the superconducting order parameter near the interface. If the Ag film is very thin, the excess current is that of a normal-metal–superconductor point contact, though slightly changed because of the depression of the order parameter at the surface of the bilayer. For larger thicknesses, the combination of the proximity effect and the limited mean free path of the electrons yields very different current-voltage characteristics.

An adaptive neural network with asymmetric connections is introduced. This network is related to the Hopfield network with graded neurons and uses a recurrent generalization of the δ rule of Rumelhart, Hinton, and Williams to modify adaptively the synaptic weights. The new network bears a resemblance to the master/slave network of Lapedes and Farber but it is architecturally simpler.

A Monte Carlo simulation of two-dimensional vesicles, modeled as closed, planar, self-avoiding tethered chains is presented. The effects of an internal pressure increment, Δp, and a bending rigidity, κ, on the sizes and shapes is studied. For κ=0 scaling behavior is found, which, in the asymptotic deflated regime, Δp<0, matches the behavior of branched polymers. For κ>0 various nontrivial equilibrium shapes or cytotypes appear and dynamical behavior reminiscent of nonlinear flickering in real vesicles is found.

The production of η and ω mesons in τ decay has been studied with the CLEO detector. A sample of 42 000 $e^{+}$$e^{\mathrm{-}}$→${\tau }^{+}$${\tau }^{\mathrm{-}}$ events at a mean √s of 10.5 GeV is used in the analysis. The branching ratio B(${\tau }^{\mathrm{-}}$→ω${\pi }^{\mathrm{-}}$${\nu }_{\tau }$) is measured to be (1.60±0.27±0.41)%. No signal for η production from τ decay is observed, resulting in 95%-confidence-level upper limits of 1.8% for B(${\tau }^{\mathrm{-}}$→η${\pi }^{\mathrm{-}}$${\nu }_{\tau }$), 2.1% for B(${\tau }^{\mathrm{-}}$→η${\pi }^{\mathrm{-}}$${\pi }^0$${\nu }_{\tau }$), and 2.1% for inclusive η production in τ decay. No evidence is found for decays proceeding via second-class currents.

We have found a simple powerful Ansatz, interpreting the Lund symmetric-fragmentation function as a universal production density, which predicts multiplicities and momentum and $P_T^2$ distributions of the many meson and baryon flavors observed in $e^{+}$$e^{\mathrm{-}}$ collisions at $E_{\mathrm{c}.\mathrm{m}.\mathrm{}\mathrm{\simeq }30}$ GeV almost perfectly with only two ‘‘natural’’ parameters of the Lund symmetric-fragmentation function. The model depends only on hadron masses and not on quark-level suppression factors such as s/u or qq/q. At a minimum, it provides an easy basis for the prediction of particle production rates and distributions, and may provide insight into the actual production mechanism.

I discuss a model of jet energy flow in which the jet is a continuous distribution of massless particles, uniform in the jet-jet center of mass. In the laboratory frame, all quark jets look the same near the jet direction. The transverse energy of a jet has a 1/${\mathrm{r}}^3$ dependence on distance from the jet direction (${\mathrm{r}}^2$=Δ${\mathrm{y}}^2$+Δ${\mathrm{\phi }}^2$), with a small section in the center that depends on the shape of the jet in the center of mass. Only the height of the center depends on the jet energy.

We present a theoretical derivation of the Stech Ansatz for the quark mass matrices within a supersymmetric SO(10) model. The model does not involve any horizontal discrete symmetries. Supersymmetry and dominance of the right-handed neutrino in the loop are crucial for the success of the model. We also comment on the solution of the strong CP problem in the context of this model.

The ${\mathrm{Q}}^2$ evolution of quark and gluon distributions predicted by perturbative QCD is reexamined by consideration of the complex moments of these distributions. This analytic continuation leads us to derive several interesting new sum rules which depend on the number of flavors.

We observe both the ${\mathit{J}}^{\mathit{P}\mathit{C}}$=$1^{++}$ ${\mathit{f}}_1$(1285) and the ${\mathit{J}}^{\mathit{P}\mathit{C}}$=$0^{\mathrm{-}+}$ η’(958) in tagged two-photon interactions and the η’(958) in untagged interactions. The measured $Q^2$ dependence and decay distribution support the $f_1$(1285) spin and parity assignment. The radiative width of the $f_1$(1285) is measured as (${\mathit{M}}^2$/${\mathit{Q}}^2$)${\mathrm{\Gamma }}_{\gamma \gamma \mathrm{*}}$=9.4±2.5±1.7 keV, on the assumption of a ρ-pole form factor.

We confirm the observation of a spin-1 resonance at 1423 MeV in the $K_S^0$$K^{\pm }$${\pi }^{\mp }$ system produced in single-tagged two-photon interactions. The Dalitz plot indicates that this resonance decays primarily via a $K^{\mathrm{*}}$K intermediate state. We measure a radiative width times branching ratio ${\mathit{B}}_{\mathit{KK}\mathrm{¯}\mathrm{\pi }}$(${\mathit{M}}^2$/${\mathit{Q}}^2$)${\mathrm{\Gamma }}_{\gamma \gamma \mathrm{*}}$=3.2±1.4±0.6 keV on the assumption of a ρ-pole form factor.

The two-neutrino mode of double-beta decay in ${}_{}{}^{82}{}_{}{}^{}\mathrm{Se}$ has been observed in a time-projection chamber at a half-life of ${(1.1\mathrm{}}_{\mathrm{-}0.3}^{+0.8\mathrm{}}$)×${10}^{20}$ yr (68% confidence level). This result from direct counting confirms the earlier geochemical measurements and helps provide a standard by which to test the double-beta-decay matrix elements of nuclear theory. It is the rarest natural decay process ever observed directly in the laboratory.

Two-dimensional γ-ray energy correlations have been measured at high spins in ${}_{}{}^{146}{}_{}{}^{}\mathrm{Gd}$ with the anti-Compton spectrometer array OSIRIS. Pronounced ridges have been found consisting of stretched E2 transitions. The separation of the ridges gives a dynamical moment of inertia 2${\mathrm{\theta }}_{\mathrm{band}}^{\mathrm{}\left(2\right)\mathrm{}}$=(150±6)${\mathrm{ħ}}^2$/MeV, suggesting a superdeformation with an axis ratio c/a=1.7 (${\beta }_2$≊0.6). The quadrupole moment of the superdeformed nucleus is $Q_0$${=11\mathrm{}}_{\mathrm{-}4}^{+14\mathrm{}}$ e⋅b. .AE

A model-independent multipole analysis of ${}_{}{}^{238}{}_{}{}^{}\mathrm{U}$(e,e’f) coincidence data, taken at four momentum transfers (0.2≤$q_{\mathrm{eff}\mathrm{\le }0.7}$ ${\mathrm{fm}}^{\mathrm{-}1}$; ω=4–22 MeV) yields both E1, E2/E0, and E3 form factors and strength distributions. The E2/E0 strength distribution in the fission channel shows two distinct bumps centered at ω≃10 and 14 MeV, exhausting up to 12 MeV (19±2)% of the isoscalar E2 sum rule. The extracted form factors can be described within a hydrodynamical model by use of parameters $c_{\mathrm{tr}/c_0=1.2\mathrm{}}$ and 1.0 for E1 and E2, respectively.

Potential-energy surfaces are obtained for the three dynamical-symmetry chains of SO(8) within the fermion-dynamical-symmetry model. A remarkable feature is that the SO(7) dynamical symmetry is transitional and β soft. The transitional or ‘‘critical’’ behavior of SO(7) is vividly demonstrated.

Spectroscopic studies of highly charged xenon ions (up to ${\mathrm{Xe}}^{9+}$), produced by electron–xenon-atom collisions, have been carried out in coincidence with electrons ejected at 90° to the incident electron direction and having energies between 20 and 550 eV. Relative values of partial-double-differential ionization cross sections, $d^2$${\sigma }^{\left(n\right)}$/de dΩ, for ${\mathrm{Xe}}^{+}$ to ${\mathrm{Xe}}^{8+}$ have been measured for several incident electron energies between 1.0 and 8.0 keV.

We have studied highly excited autoionizing states in atomic sodium, using two laser beams and synchrotron radiation. We have observed the electron spectra resulting from the decay of these 2${\mathrm{p}}^5$3s4p, 2${\mathrm{p}}^5$3s4d, and 2${\mathrm{p}}^5$3s5s states to the 2${\mathrm{p}}^6$ ${\mathrm{}}^1$${\mathrm{S}}_0$ ground state of ${\mathrm{Na}}^{+}$. The excitation energies and the relative oscillator strengths of inner-shell transitions in the excited atoms have been measured.

A fourth-order interference technique has been used to measure the time intervals between two photons, and by implication the length of the photon wave packet, produced in the process of parametric down-conversion. The width of the time-interval distribution, which is largely determined by an interference filter, is found to be about 100 fs, with an accuracy that could, in principle, be less than 1 fs.

We have measured the reduction in optical-mode size produced by optical guiding in a free-electron laser. The intracavity transverse mode is smaller than the diffraction-limited mode in the small-signal as a result of strong guiding regime, but it grows larger in saturation as a result of reduced gain. Comparison with theory shows agreement except for a discrepancy in the shape of the mode as saturation begins.

Multiple-pass gain is reported at 2177 and 2163 Å in C iii ions in a vacuum-arc discharge, pumped by Mn vi line radiation from a laser-produced Mn plasma. These Be-like uv lasers pumped by resonant photoexcitation are prototypes for soft x-ray lasers in higher-Z, isoelectronic analogs.