Self-consistent solution of the Schwinger-Dyson equations for the nucleon and meson propagators

The Schwinger-Dyson equations for the nucleon and meson propagators are solved self-consistently in an approximation that goes beyond the Hartree-Fock approximation. The traditional approach consists in solving the nucleon Schwinger-Dyson equation with bare meson propagators and bare meson-nucleon vertices; the corrections to the meson propagators are calculated using the bare nucleon propagator and bare nucleon-meson vertices. It is known that such an approximation scheme produces the appearance of ghost poles in the propagators. In this paper the coupled system of Schwinger-Dyson equations for the nucleon and the meson propagators are solved self-consistently including vertex corrections. The interplay of self-consistency and vertex corrections on the ghosts problem is investigated. It is found that the self-consistency does not affect significantly the spectral properties of the propagators. In particular, it does not affect the appearance of the ghost poles in the propagators.Comment: REVTEX, 7 figures (available upon request), IFT-P.037/93, DOE/ER/40427-12-N9

Coupled virus - bacteria interactions and ecosystem function in an engineered microbial system

Viruses are thought to control bacterial abundance, affect community composition and influence ecosystem function in natural environments. Yet their dynamics have seldom been studied in engineered systems, or indeed in any system, for long periods of time. We measured virus abundance in a full-scale activated sludge plant every week for two years. Total bacteria and ammonia oxidising bacteria (AOB) abundances, bacterial community profiles, and a suite of environmental and operational parameters were also monitored. Mixed liquor virus abundance fluctuated over an order of magnitude (3.18 × 108 – 3.41 × 109 virus’s mL-1) and that variation was statistically significantly associated with total bacterial and AOB abundance, community composition, and effluent concentrations of COD and NH4+- N and thus system function. This suggests viruses play a far more important role in the dynamics of activated sludge systems than previously realised and could be one of the key factors controlling bacterial abundance, community structure and functional stability and may cause reactors to fail. These finding are based on statistical associations, not mechanistic models. Nevertheless, viral associations with abiotic factors, such as pH, make physical sense giving credence to these findings and highlighting the role that physical factors play in virus ecology. Further work is needed to identify and quantify specific bacteriophage and their hosts to enable us to develop mechanistic models of the ecology of viruses in wastewater treatment systems. However, since we have shown that viruses can be related to effluent quality and virus quantification is simple and cheap, practitioners would probably benefit from quantifying viruses now

Photoproduction of mesons in nuclei at GeV energies

In a transport model that combines initial state interactions of the photon with final state interactions of the produced particles we present a calculation of inclusive photoproduction of mesons in nuclei in the energy range from 1 to 7 GeV. We give predictions for the photoproduction cross sections of pions, etas, kaons, antikaons, and $\pi^+\pi^-$ invariant mass spectra in ^{12}C and ^{208}Pb. The effects of nuclear shadowing and final state interaction of the produced particles are discussed in detail.Comment: Text added in summary in general reliability of the method, references updated. Phys. Rev. C (2000) in pres

Fluctuations and Dissipation of Coherent Magnetization

A quantum mechanical model is used to derive a generalized Landau-Lifshitz equation for a magnetic moment, including fluctuations and dissipation. The model reproduces the Gilbert-Brown form of the equation in the classical limit. The magnetic moment is linearly coupled to a reservoir of bosonic degrees of freedom. Use of generalized coherent states makes the semiclassical limit more transparent within a path-integral formulation. A general fluctuation-dissipation theorem is derived. The magnitude of the magnetic moment also fluctuates beyond the Gaussian approximation. We discuss how the approximate stochastic description of the thermal field follows from our result. As an example, we go beyond the linear-response method and show how the thermal fluctuations become anisotropy-dependent even in the uniaxial case.Comment: 22 page

Reconstructing the 3-D Trajectories of CMEs in the Inner Heliosphere

A method for the full three-dimensional (3-D) reconstruction of the trajectories of coronal mass ejections (CMEs) using Solar TErrestrial RElations Observatory (STEREO) data is presented. Four CMEs that were simultaneously observed by the inner and outer coronagraphs (COR1 and 2) of the Ahead and Behind STEREO satellites were analysed. These observations were used to derive CME trajectories in 3-D out to ~15Rsun. The reconstructions using COR1/2 data support a radial propagation model. Assuming pseudo-radial propagation at large distances from the Sun (15-240Rsun), the CME positions were extrapolated into the Heliospheric Imager (HI) field-of-view. We estimated the CME velocities in the different fields-of-view. It was found that CMEs slower than the solar wind were accelerated, while CMEs faster than the solar wind were decelerated, with both tending to the solar wind velocity.Comment: 17 pages, 10 figures, 1 appendi

Non-Markovian Decay of a Three Level Cascade Atom in a Structured Reservoir

We present a formalism that enables the study of the non-Markovian dynamics of a three-level ladder system in a single structured reservoir. The three-level system is strongly coupled to a bath of reservoir modes and two quantum excitations of the reservoir are expected. We show that the dynamics only depends on reservoir structure functions, which are products of the mode density with the coupling constant squared. This result may enable pseudomode theory to treat multiple excitations of a structured reservoir. The treatment uses Laplace transforms and an elimination of variables to obtain a formal solution. This can be evaluated numerically (with the help of a numerical inverse Laplace transform) and an example is given. We also compare this result with the case where the two transitions are coupled to two separate structured reservoirs (where the example case is also analytically solvable)

Structure Formation, Melting, and the Optical Properties of Gold/DNA Nanocomposites: Effects of Relaxation Time

We present a model for structure formation, melting, and optical properties of gold/DNA nanocomposites. These composites consist of a collection of gold nanoparticles (of radius 50 nm or less) which are bound together by links made up of DNA strands. In our structural model, the nanocomposite forms from a series of Monte Carlo steps, each involving reaction-limited cluster-cluster aggregation (RLCA) followed by dehybridization of the DNA links. These links form with a probability $p_{eff}$ which depends on temperature and particle radius $a$. The final structure depends on the number of monomers (i. e. gold nanoparticles) $N_m$, $T$, and the relaxation time. At low temperature, the model results in an RLCA cluster. But after a long enough relaxation time, the nanocomposite reduces to a compact, non-fractal cluster. We calculate the optical properties of the resulting aggregates using the Discrete Dipole Approximation. Despite the restructuring, the melting transition (as seen in the extinction coefficient at wavelength 520 nm) remains sharp, and the melting temperature $T_M$ increases with increasing $a$ as found in our previous percolation model. However, restructuring increases the corresponding link fraction at melting to a value well above the percolation threshold. Our calculated extinction cross section agrees qualitatively with experiments on gold/DNA composites. It also shows a characteristic ``rebound effect,'' resulting from incomplete relaxation, which has also been seen in some experiments. We discuss briefly how our results relate to a possible sol-gel transition in these aggregates.Comment: 12 pages, 10 figure

Complete Order alpha_s^3 Results for e^+ e^- to (gamma,Z) to Four Jets

We present the next-to-leading order (O(alpha_s^3)) perturbative QCD predictions for e^+e^- annihilation into four jets. A previous calculation omitted the O(alpha_s^3) terms suppressed by one or more powers of 1/N_c^2, where N_c is the number of colors, and the `light-by-glue scattering' contributions. We find that all such terms are uniformly small, constituting less than 10% of the correction. For the Durham clustering algorithm, the leading and next-to-leading logarithms in the limit of small jet resolution parameter y_{cut} can be resummed. We match the resummed results to our fixed-order calculation in order to improve the small y_{cut} prediction.Comment: Latex2e, 17 pages with 5 encapsulated figures. Note added regarding subsequent related work. To appear in Phys. Rev.

Enhancement of the magnetic anisotropy of nanometer-sized Co clusters: influence of the surface and of the inter-particle interactions

We study the magnetic properties of spherical Co clusters with diameters between 0.8 nm and 5.4 nm (25 to 7500$ atoms) prepared by sequential sputtering of Co and Al2O3. The particle size distribution has been determined from the equilibrium susceptibility and magnetization data and it is compared to previous structural characterizations. The distribution of activation energies was independently obtained from a scaling plot of the ac susceptibility. Combining these two distributions we have accurately determined the effective anisotropy constant Keff. We find that Keff is enhanced with respect to the bulk value and that it is dominated by a strong anisotropy induced at the surface of the clusters. Interactions between the magnetic moments of adjacent layers are shown to increase the effective activation energy barrier for the reversal of the magnetic moments. Finally, this reversal is shown to proceed classically down to the lowest temperature investigated (1.8 K).Comment: 13 figures submitted to Phys. Rev.

MoonLITE programmatic and technological update

MoonLITE is a proposed four penetrator lunar mission. Following a US/UK working group assessment, a science assessment and the first UK impact trials, a full mission-level phase A study has begun. A technological and programmatic update of the mission is given