Search

We propose and study a model of scale-free growing networks that gives a degree distribution dominated by a power-law behavior with a model-dependent, hence tunable, exponent. The model represents a hybrid of the growing networks based on popularity-driven and fitness-driven preferential attachments. As the network grows, a newly added node establishes $m$ new links to existing nodes with a probability $p$ based on popularity of the existing nodes and a probability $1-p$ based on fitness of the existing nodes. An explicit form of the degree distribution $P(p,k)$ is derived within a mean field approach. For reasonably large $k$, $P(p,k) \sim k^{-\gamma(p)}{\cal F}(k,p)$, where the function ${\cal F}$ is dominated by the behavior of $1/\ln(k/m)$ for small values of $p$ and becomes $k$-independent as $p \to 1$, and $\gamma(p)$ is a model-dependent exponent. The degree distribution and the exponent $\gamma(p)$ are found to be in good agreement with results obtained by extensive numerical simulations., Comment: 12 pages, 2 figures, submitted to PRE

Amorphous ${\mathrm{SiC}}_{x}{\mathrm{N}}_{y}$ films with various compositions were deposited by ion-beam sputtering. The bonding characteristics, Young's modulus, and density of the films were investigated using x-ray photoelectron spectroscopy (XPS), Fourier-transform infrared (FTIR) spectroscopy, surface acoustic wave spectroscopy, x-ray reflection, and molecular-dynamics (MD) simulations. It was observed that the Young's modulus decreases from $260\ifmmode\pm\else\textpm\fi{}20$ to $85\ifmmode\pm\else\textpm\fi{}12\mathrm{GPa}$ while the density decreases from $3.45\ifmmode\pm\else\textpm\fi{}0.2$ to $2.3\ifmmode\pm\else\textpm\fi{}0.3{\mathrm{g}/\mathrm{c}\mathrm{m}}^{3}$ as the carbon content of the films increases from 0% to 68%. FTIR and XPS spectra indicate an increasing proportion of double and triple bonds with higher carbon content of the films. These experimental results were compared with the Young's moduli and the infrared spectra obtained from density-functional-based MD simulations. Also for these model calculations the Young's modulus drops from $237\ifmmode\pm\else\textpm\fi{}54$ to $109\ifmmode\pm\else\textpm\fi{}25\mathrm{GPa}$ as the carbon content increases from 0% to 69%. It can be seen that the formation of $\mathrm{C}\mathrm{=}\mathrm{C},$ $\mathrm{C}\mathrm{=}\mathrm{N},$ $\mathrm{C}\mathrm{\ensuremath{\equiv}}\mathrm{C},$ and $\mathrm{C}\mathrm{\ensuremath{\equiv}}\mathrm{N}$ bonds, together with the occurrence of terminating nitrogen atoms for the films with higher carbon content, are responsible for the degradation of the ${\mathrm{sp}}^{3}$ network, and therefore for a lower Young's modulus and density.

From three-prong decays of tau leptons produced in the MAC detector at PEP, the lifetime of the tau is determined to be (4.9 +- 2.0) x 10/sup -13/ sec.

We have studied the production of acollinear muon pairs in e/sup +/e/sup -/ interactions using the MAC detector at PEP. The results are totally accounted for by the QED processes e/sup +/e/sup -/..-->..e/sup +/e/sup -/..mu../sup +/..mu../sup -/ and e/sup +/e/sup -/..--> mu../sup +/..mu../sup -/..gamma.. in which only the muon pairs are detected. We have also searched for the production of the supersymmetric partners of the muon. The existence of a spin-0 supersymmetric partner of the muon with a mass

Polymer-coated glass microspheres filled with DT fuel and argon seed gas were irradiated on the SHIVA 1.06-..mu..m laser using 4-kJ 200 ps (full width at half maximum), Gaussian pulses. Measured light absorption, x-ray spectrum, neutron yields, and x-ray continuum images compare favorably with detailed computer simulations. Pusher neutron activation and argon line imaging diagnostics were utilized to measure fuel density. Fuel densities of 1-3 g/cm/sup 3/ or 5-15 times liquid DT density were inferred.

Photon-counting experiments have been conducted to test the conjecture that at sufficiently high laser irradiance, atomic inner-shell excitations can be induced by laser-driven outer-shell oscillations. Using KrF light at an irradiance /approx/3/times/10/sup 17/ W/cm/sup 2/ on low-density noble-gas targets, we detect no prompt photons which are characteristic of inner-shell processes.

The reactions e/sup +/e/sup -/..--> mu../sup +/..mu../sup -/..gamma gamma.. and e/sup +/e/sup -/..--> mu../sup +/..mu../sup -/..gamma.. have been studied at ..sqrt..s = 29 GeV with the MAC detector at the SLAC storage ring. The measured cross sections, charge asymmetry, and invariant-mass distributions are in good agreement with the predictions of QED theory. Limits are derived for the production of excited muon states.

Compression of DT-neon-seeded, glass microballoons has been observed in CO/sub 2/-laser experiments at Los Alamos Scientific Laboratory. DT densities of the order of 20% of liquid density have been attained. Detailed comparisons of the neon densities determined from the measured neon source size and from the fit of observed spectra by Stark-broadening calculations have been made. The excellent agreement obtained with these methods indicates that the extrapolation of the microfield effects used in Stark-broadening calculations to high densities is reasonable.

Interferometric data resolved to 1 ..mu..m and 15 psec confirm the significant role of radiation pressure during high-intensity laser-plasma interactions. Specifically observed manifestations include electron density shelves, and small- and large-scale deformations of transverse isodensity of surfaces.

The energy-energy correlation cross section for hadrons produced in electron-positron annihilation at a center-of-mass energy of 29 GeV has been measured with the MAC detector at SLAC. The result is corrected for the effects of detector resolution, acceptance, and initial-state radiation. The correlation is measured in two independent ways on the same data sample: the energy weights and angles are obtained either from the energy flow in the finely segmented total absorption calorimeters or from the momenta of charged tracks in the central drift chamber. This procedure helps reduce systematic errors by cross-checking the effects of the detector on the measurement, particularly important because the corrections depend on complex Monte Carlo simulations. The results are compared with the predictions of Monte Carlo models of complete second-order perturbative quantum chromodynamics and fragmentation, with the following conclusions: (1) fitting the asymmetry for large correlation angles gives values for ..cap alpha../sub S/ of 0.120 +- 0.006 in perturbation theory, 0.185 +- 0.013 in the Lund string model, and values which vary from 0.105 to 0.140 ( +- 0.01) in the incoherent jet models, depending on the gluon fragmentation scheme and the algorithm used for momentum conservation; and (2) the string fragmentation model providesmore » a satisfactory description of the measured energy-energy correlation cross section, whereas incoherent jet formation does not.« less

A search for supersymmetric electron production via the reaction ${e}^{+}{e}^{\ensuremath{-}}\ensuremath{\rightarrow}{e}^{\ifmmode\pm\else\textpm\fi{}}\stackrel{\ifmmode \tilde{}\else \~{}\fi{}}{\ensuremath{\gamma}}{\stackrel{\ifmmode \tilde{}\else \~{}\fi{}}{e}}^{\ensuremath{\mp}}$ followed by the decay ${\stackrel{\ifmmode \tilde{}\else \~{}\fi{}}{e}}^{\ensuremath{\mp}}\ensuremath{\rightarrow}{e}^{\ensuremath{\mp}}\stackrel{\ifmmode \tilde{}\else \~{}\fi{}}{\ensuremath{\gamma}}$ has been performed with the MAC detector at the electron-positron storage ring PEP. No candidates were found in a sample corresponding to an integrated luminosity of 36.4 ${\mathrm{pb}}^{\ensuremath{-}1}$. For a massless $\stackrel{\ifmmode \tilde{}\else \~{}\fi{}}{\ensuremath{\gamma}}$ this corresponds to a lower limit on the $\stackrel{\ifmmode \tilde{}\else \~{}\fi{}}{e}$ mass of 22.4 GeV/${\mathit{c}}^{2}$ at the 95% confidence level.

Direct photon production in hadronic events from e/sup +/e/sup -/ annihilation has been studied at ..sqrt..s = 29 GeV with use of the MAC detector at the PEP storage ring. A charge asymmetry A = (-12.3 +- 3.5)% is observed in the final-state jets. The cross section and the charge asymmetry are in good agreement with the predictions of the fractionally charged quark-parton model. Both the charge asymmetry and total yield have been used to determine values of quark charges. Limits have been established for anomalous sources of direct photons.

Temporally resolved sub-kiloelectronvolt x-ray emission pulses have been obtained from disk targets illuminated with 1.06-\ensuremath{\mu}m wavelength laser light at 5\ifmmode\times\else\texttimes\fi{}${10}^{14}$ W/${\mathrm{cm}}^{2}$. Striking variations in the x-ray pulse shape from Be, Ti, Sn, Au, and U targets indicate the onset of strongly inhibited electron thermal conduction during the rise of the laser intensity. The laser intensity threshold for this strong inhibition is found to increase with target atomic number $Z$.

A search for monojet production in e/sup +/e/sup -/ annihilation has been performed at ..sqrt..s = 29 GeV with the MAC detector at PEP. No events beyond those expected from tau decays have been found. The result has been interpreted as an upper limit for Z/sup 0/ decays into monojets, giving an upper limit for the branching ratio of 0.5% at the 90% confidence level. This limit excludes an interpretation that the monojets discovered by Arnison et al. are due to Z/sup 0/ decays into light Higgs pairs.

An enclosing "box" calorimeter has been used to measure the polarization and angular dependence of 1.06-\ensuremath{\mu}m laser-light absorption under experimental conditions approximating those assumed by Estabrook, Valeo, and Kruer in their simulations; i.e., \ensuremath{\sim}${10}^{16}$ W/${\mathrm{cm}}^{2}$ plane waves incident on a planar plasma. A clear resonance absorption maximum was observed for $p$-but not for $s$-polarized incident radiation as predicted.