Asymptotic channels and gauge transformations of the time-dependent Dirac equation for extremely relativistic heavy-ion collisions

We discuss the two-center, time-dependent Dirac equation describing the dynamics of an electron during a peripheral, relativistic heavy-ion collision at extreme energies. We derive a factored form, which is exact in the high-energy limit, for the asymptotic channel solutions of the Dirac equation, and elucidate their close connection with gauge transformations which transform the dynamics into a representation in which the interaction between the electron and a distant ion is of short range. We describe the implications of this relationship for solving the time-dependent Dirac equation for extremely relativistic collisions.Comment: 12 pages, RevTeX, 2 figures, submitted to PR

A Conformally Invariant Holographic Two-Point Function on the Berger Sphere

We apply our previous work on Green's functions for the four-dimensional quaternionic Taub-NUT manifold to obtain a scalar two-point function on the homogeneously squashed three-sphere (otherwise known as the Berger sphere), which lies at its conformal infinity. Using basic notions from conformal geometry and the theory of boundary value problems, in particular the Dirichlet-to-Robin operator, we establish that our two-point correlation function is conformally invariant and corresponds to a boundary operator of conformal dimension one. It is plausible that the methods we use could have more general applications in an AdS/CFT context.Comment: 1+49 pages, no figures. v2: Several typos correcte

New Insights into White-Light Flare Emission from Radiative-Hydrodynamic Modeling of a Chromospheric Condensation

(abridged) The heating mechanism at high densities during M dwarf flares is poorly understood. Spectra of M dwarf flares in the optical and near-ultraviolet wavelength regimes have revealed three continuum components during the impulsive phase: 1) an energetically dominant blackbody component with a color temperature of T $\sim$ 10,000 K in the blue-optical, 2) a smaller amount of Balmer continuum emission in the near-ultraviolet at lambda $<$ 3646 Angstroms and 3) an apparent pseudo-continuum of blended high-order Balmer lines. These properties are not reproduced by models that employ a typical "solar-type" flare heating level in nonthermal electrons, and therefore our understanding of these spectra is limited to a phenomenological interpretation. We present a new 1D radiative-hydrodynamic model of an M dwarf flare from precipitating nonthermal electrons with a large energy flux of $10^{13}$ erg cm$^{-2}$ s$^{-1}$. The simulation produces bright continuum emission from a dense, hot chromospheric condensation. For the first time, the observed color temperature and Balmer jump ratio are produced self-consistently in a radiative-hydrodynamic flare model. We find that a T $\sim$ 10,000 K blackbody-like continuum component and a small Balmer jump ratio result from optically thick Balmer and Paschen recombination radiation, and thus the properties of the flux spectrum are caused by blue light escaping over a larger physical depth range compared to red and near-ultraviolet light. To model the near-ultraviolet pseudo-continuum previously attributed to overlapping Balmer lines, we include the extra Balmer continuum opacity from Landau-Zener transitions that result from merged, high order energy levels of hydrogen in a dense, partially ionized atmosphere. This reveals a new diagnostic of ambient charge density in the densest regions of the atmosphere that are heated during dMe and solar flares.Comment: 50 pages, 2 tables, 13 figures. Accepted for publication in the Solar Physics Topical Issue, "Solar and Stellar Flares". Version 2 (June 22, 2015): updated to include comments by Guest Editor. The final publication is available at Springer via http://dx.doi.org/10.1007/s11207-015-0708-

Search for direct production of charginos and neutralinos in events with three leptons and missing transverse momentum in √s = 7 TeV pp collisions with the ATLAS detector

A search for the direct production of charginos and neutralinos in final states with three electrons or muons and missing transverse momentum is presented. The analysis is based on 4.7 fb−1 of proton–proton collision data delivered by the Large Hadron Collider and recorded with the ATLAS detector. Observations are consistent with Standard Model expectations in three signal regions that are either depleted or enriched in Z-boson decays. Upper limits at 95% confidence level are set in R-parity conserving phenomenological minimal supersymmetric models and in simplified models, significantly extending previous results

Measurement of D*+/- meson production in jets from pp collisions at sqrt(s) = 7 TeV with the ATLAS detector

This paper reports a measurement of D*+/- meson production in jets from proton-proton collisions at a center-of-mass energy of sqrt(s) = 7 TeV at the CERN Large Hadron Collider. The measurement is based on a data sample recorded with the ATLAS detector with an integrated luminosity of 0.30 pb^-1 for jets with transverse momentum between 25 and 70 GeV in the pseudorapidity range |eta| < 2.5. D*+/- mesons found in jets are fully reconstructed in the decay chain: D*+ -> D0pi+, D0 -> K-pi+, and its charge conjugate. The production rate is found to be N(D*+/-)/N(jet) = 0.025 +/- 0.001(stat.) +/- 0.004(syst.) for D*+/- mesons that carry a fraction z of the jet momentum in the range 0.3 < z < 1. Monte Carlo predictions fail to describe the data at small values of z, and this is most marked at low jet transverse momentum.Comment: 10 pages plus author list (22 pages total), 5 figures, 1 table, matches published version in Physical Review

The assessment of materials for crossflow nanofiltration of organic/organic liquids and the development of scale-up options

With the aqueous applications of crossflow filtration being well established, comparable developments in the field of organic/organic liquid systems remain in their infancy. Progress within the field has been hindered by the fact that there are few systems which are both robust to hydrocarbon solvents and provide good fluxes/separations under realistic operating conditions. The authors of the current paper have explored a number of materials for crossflow filtration of organic media and found that the dense organic polymer PDMS (polydimethyl siloxane) affords the best results (see Figure 1). Building on initial results, a full assessment of the membrane performance has been undertaken. Using a laboratory set-up, a range of pure and mixed hydrocarbon streams have been passed across the PDMS to assess performance with time and under variable operating conditions. Recent papers and presentations by the afore mentioned authors have considered transport mechanisms across a 2 μm PDMS membrane supported on PAN. Results from flat sheet experiments have been used to design a larger scale unit. The operation of this system has shown excellent read across in terms of flux and selectivity. It is hoped that the work detailed within this presentation will prompt other workers in the field to consider the development of novel organic polymers to build on the applicability of filtration for organic/organic separations

Effect of swelling in non-aqueous nanofiltration with polydimethylsiloxane (PDMS) membranes

Transport mechanisms and process limitations are relatively well understood for aqueous nanofiltration systems. Much work has also been done on the use of membranes for the removal of suspended matter from organic solvents. The removal of organic solute compounds from organic solvents using membrane technology has been addressed by very few workers, and little is known of the fundamental transport and separation mechanisms. A dense polydimethylsiloxane (PDMS) composite membrane was used to assess the flux and separation performance of a range of organic solute compounds and organic solvents. Solvent flux was modelled with the Hagen- Poisuelle equation and found to fit the model well, with swelling effects being the most likely cause of some deviations. The effect of solvent type and membrane swelling on solute rejection will be discussed

Nanofiltration of organic solvents

Transport mechanisms and process limitations are relatively well understood for aqueous nanofiltration systems. Much work has also been done on the use of membranes for the removal of suspended matter from organic solvents. The removal of organic solute compounds from organic solvents using membrane technology has been addressed by very few workers, and little is known of the fundamental transport and separation mechanisms. The work aims to enhance the understanding of non-aqueous nanofiltration by focusing on the flux performance of organic solvents through a dense 2 μm polydimethylsiloxane composite membrane. The flux of alcohols, n-alkanes, i-alkanes and cyclic compounds were studied in deadend mode, at pressures of 10–900 kPa. Fluxes of 10–80 l/m2 h were obtained for alkanes and cyclic compounds, whereas alcohol flux was around two orders of magnitude lower. The results suggest that the solvent flux through polydimethylsiloxane takes place via two distinct mechanisms – namely hydraulic and chemical transport. Hydraulic transport appears to dominate at pressures above 300 kPa, whereas chemical transport becomes more apparent at lower pressures. Comparison of the hydraulic transport data with a Hagen-Poisuelle model gives good agreement for similar solvents. Swelling effects caused by solvent-membrane interactions are identified as playing a major role in solvent flux behaviour, and compressibility effects are also thought to account for deviations from the Hagen-Poisuelle model. Viscous flow was verified by a nonseparation of mixtures of n-alkane and cyclic compounds, which suggests that the polydimethylsiloxane layer cannot sustain a dense structure when used in organic solvent nanofiltration applications

Solvent flux through dense polymeric nanofiltration membranes

This work examines the flux performance of organic solvents through a polydimethylsiloxane (PDMS) composite membrane. A selection of n-alkanes, i-alkanes and cyclic compounds were studied in deadend permeation experiments at pressures up to 900 kPa to give fluxes for pure solvents and mixtures between 10 and 100 l m-2 h-1. Results for the chosen alkanes and aromatics, and subsequent modelling using the Hagen-Poiseuille equation, suggest that solvent transport through PDMS can be successfully interpreted via a predominantly hydraulic mechanism. It is suggested that the mechanism has a greater influence at higher pressures and the modus operandi is supported by the non-separation of binary solvent mixtures and a dependency on viscosity and membrane thickness. The effects of swelling that follow solvent-membrane interactions show that the relative magnitudes of the Hildebrand solubility parameter for the active membrane layer and the solvent(s) are a good indicator of permeation level. Solvents constituting a group (e.g. all n-alkanes) induced similar flux behaviours when corrections were made for viscosity and affected comparable swelling properties in the PDMS membrane layer

Non-aqueous nanofiltration: solute rejection in low-polarity binary systems

The separation characteristics of a dense polydimethylsiloxane (PDMS) membrane were studied using alkyl and aromatic solvents and low-polarity, sulphur bearing, organometallic and polynuclear aromatic (PNA) solute compounds. Rejection was found to be dependent on transmembrane pressure, crossflow rate (hydrodynamic conditions), solute size and the degree of swelling induced by the solvent. Rejection increased progressively with pressure whilst a threshold condition was observed above which further increases in crossflow had a negligible influence on rejection. Measurements over the molecular weight range 84-612 g/mol showed the membrane to have a molecular weight cut-off in the region 350-400 g/mol to all but one of the tested PNA compounds (i.e. rubrene). An additional correlation using molecular dimensions instead of molecular weight showed the cut-off size of the membrane to be in the region of 1-2 nm, with all data falling on a well defined rejection/size curve. Solvent type influenced membrane swelling to an extent dependent on the relative magnitude of the solubility parameters for the solvent and PDMS; similar values led to more swelling, higher fluxes and lower rejections. Results support the concept of viscous solvent flow whilst solute transport could be either predominantly viscous or a combination of viscous and diffusive. With larger molecules a size exclusion mechanism was dominant. A new model is proposed that takes account of solute transport by a combination of viscous and diffusive mechanisms and this is shown to well represent the experimental data