Field Theoretic Description of Ultrarelativistic Electron-Positron Plasmas

Ultrarelativistic electron-positron plasmas can be produced in high-intensity laser fields and play a role in various astrophysical situations. Their properties can be calculated using QED at finite temperature. Here we will use perturbative QED at finite temperature for calculating various important properties, such as the equation of state, dispersion relations of collective plasma modes of photons and electrons, Debye screening, damping rates, mean free paths, collision times, transport coefficients, and particle production rates, of ultrarelativistic electron-positron plasmas. In particular, we will focus on electron-positron plasmas produced with ultra-strong lasers.Comment: 13 pages, 7 figures, 1 table, published versio

Priming of Depth-Rotated Objects Depends on Attention and Part Changes

Three priming experiments investigated the role of attention and view changes when common objects were rotated in depth. Objects were shown in prime-probe trial pairs. Experiment 1 extended findings by Stankiewicz, Hummel, and Cooper (1998) showing that attended objects primed themselves in the same but not in a reflected view, whereas ignored objects only primed themselves in the same view. In Experiment 2, depth-rotations produced changes in the visible part structure between prime and probe view of an object. Priming after depth-rotation was more reduced for attended objects than for ignored objects. Experiment 3 showed that other depth rotations that did not change the perceived part structure revealed a priming pattern similar to that in Experiment 1, with equivalent reduction in priming for attended and ignored objects. These data indicate that recognition of attended objects is mediated by a part-based (analytic) representation together with a view-based (holistic) representation, whereas ignored images are recognized in a strictly view-dependent fashion

Photoproduction of η\eta mesons off neutrons from a deuteron target

A formalism is developed for the partial wave analysis of data on meson photoproduction off deuterons and applied to photoproduction of $\eta$ and $\pi^0$ mesons. Different interpretations of a dip-bump structure of the $\eta$ photoproduction cross section in the 1670 MeV region are presented and discussed. Helicity amplitudes for two low-mass $S_{11}$ states are determined.Comment: 11 pages, 13 figure

Interference phenomena in the JP=1/2−J^P=1/2^--wave in η\eta photoproduction

The recent precise experimental results for the photoproduction of $\eta$-mesons off the neutron measured with the Crystal Ball/TAPS calorimeter at the MAMI accelerator have been investigated in detail in the framework of the Bonn-Gatchina coupled channel model. The main result is that the narrow structure observed in the excitation function of $\gamma n \rightarrow n\eta$ can be reproduced fully with a particular interference pattern in the $J^P=1/2^-$ partial wave. Introduction of the narrow resonance $N(1685)$ with the properties reported in earlier publications deteriorates the quality of the fit.Comment: 10 pages, 13 figures, accepted for publication in EPJ

Elastic and thermodynamic properties of the shape-memory alloy AuZn

The current work reports on the elastic shear moduli, internal friction, and the specific heat of the B2 cubic ordered alloy AuZn as a function of temperature. Measurements were made on single-crystal and polycrystalline samples using Resonant Ultrasound Spectroscopy (RUS), semi-adiabatic calorimetry and stress-strain measurements. Our results confirm that this alloy exhibits the shape-memory effect and a phase transition at 64.75 K that appears to be continuous (second-order) from the specific heat data. It is argued that the combination of equiatomic composition and a low transformation temperature constrain the chemical potential and its derivatives to exhibit behavior that lies at the borderline between that of a first-order (discontinuous) and a continuous phase transition. The acoustic dissipation does not peak at the transtion temperature as expected, but shows a maximum well into the low-temperature phase. The Debye temeprature value of 219 K, obtained from the low-temperature specific heat data is in favorable agreement with that determined from the acoustic data (207 K) above the transition.Comment: 25 pages, 6 figures, submitted to Phys. Rev.

Covariant transport approach for strongly interacting partonic systems

The dynamics of partons, hadrons and strings in relativistic nucleus-nucleus collisions is analyzed within the novel Parton-Hadron-String Dynamics (PHSD) transport approach, which is based on a dynamical quasiparticle model for partons (DQPM) matched to reproduce recent lattice-QCD results - including the partonic equation of state - in thermodynamic equilibrium. Scalar- and vector-interaction densities are extracted from the DQPM as well as effective scalar- and vector-mean fields for the partons. The transition from partonic to hadronic degrees of freedom is described by covariant transition rates for the fusion of quark-antiquark pairs or three quarks (antiquarks), respectively, obeying flavor current-conservation, color neutrality as well as energy-momentum conservation. Since the dynamical quarks and antiquarks become very massive close to the phase transition, the formed resonant 'pre-hadronic' color-dipole states ($q\bar{q}$ or $qqq$) are of high invariant mass, too, and sequentially decay to the groundstate meson and baryon octets increasing the total entropy. When applying the PHSD approach to Pb+Pb colllisions at 158 A$\cdot$GeV we find a significant effect of the partonic phase on the production of multi-strange antibaryons due to a slightly enhanced $s{\bar s}$ pair production from massive time-like gluon decay and a larger formation of antibaryons in the hadronization process.Comment: 12 pages, 6 figures, to be published in the Proceedings of the 26th Winter Workshop on `Nuclear Dynamics', Ochto Rios, Jamaica, 2-9 January, 2010

Out of equilibrium quantum field dynamics of an initial thermal state after a change in the external field

The effects of the initial temperature in the out of equilibrium quantum field dynamics in the presence of an homogeneous external field are investigated. We consider an initial thermal state of temperature T for a constant external field J. A subsequent sign flip of the external field, J to -J, gives rise to an out of equilibrium nonperturbative quantum field dynamics. The dynamics is studied here for the symmetry broken lambda(Phi^2)^2 scalar N component field theory in the large N limit. We find a dynamical effective potential for the expectation value that helps to understand the dynamics. The dynamics presents two regimes defined by the presence or absence of a temporal trapping close to the metastable equilibrium position of the potential. The two regimes are separated by a critical value of the external field that depends on the initial temperature. The temporal trapping is shorter for larger initial temperatures or larger external fields. Parametric resonances and spinodal instabilities amplify the quantum fluctuations in the field components transverse to the external field. When there is a temporal trapping this is the main mechanism that allows the system to escape from the metastable state for large N. Subsequently backreaction stops the growth of the quantum fluctuations and the system enters a quasiperiodic regime.Comment: LaTeX, 19 pages, 12 .eps figures, improved version to appear in Phys Rev

Emittance growth in linear induction accelerators

The Dual-Axis Radiographic Hydrotest (DARHT) facility uses bremsstrahlung radiation source spots produced by the focused electron beams from two linear induction accelerators (LIAs) to radiograph large hydrodynamic experiments driven by high explosives. Radiographic resolution is determined by the size of the source spot, and beam emittance is the ultimate limitation to spot size. On the DARHT Axis-II LIA we measure an emittance higher than predicted by theoretical simulations, and even though this axis produces sub-millimeter source spots, we are exploring ways to improve the emittance. Some of the possible causes for the discrepancy have been investigated using particle-in-cell (PIC) codes, although most of these are discounted based on beam measurements. The most likely source of emittance growth is a mismatch of the beam to the magnetic transport, which can cause beam halo.Comment: 20th Int. Conf. on High-Power Particle Beams, Washington, DC, May, 201