4,821 research outputs found

    A relativistic coupled-channel formalism for the pion form factor

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    The electromagnetic form factor of a confined quark-antiquark pair is calculated within the framework of point-form relativistic quantum mechanics. The dynamics of theexchanged photon is explicitly taken into account by treating theelectromagnetic scattering of an electron by a meson as a relativistic two-channel problem for a Bakamjian-Thomas type mass operator. This approach guarantees Poincare invariance. Using a Feshbach reduction the coupled-channel problem can be converted into a one-channel problem for the elastic electron-meson channel. By comparing the one-photon-exchange optical potential at the constituent and hadronic levels, we are able to unambiguously identify the electromagnetic meson form factor. Violations of cluster-separability properties, which are inherent in the Bakamjian-Thomas approach, become negligible for sufficiently large invariant mass of the electron-meson system. In the limit of an infinitely large invariant mass, an equivalence with form-factor calculations done in front-form relativistic quantum mechanics is established analytically.Comment: 3 pages, 1 figure, submitted to EPJ Web of Conference

    Transient Nucleation near the Mean-Field Spinodal

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    Nucleation is considered near the pseudospinodal in a one-dimensional Ōē4\phi^4 model with a non-conserved order parameter and long-range interactions. For a sufficiently large system or a system with slow relaxation to metastable equilibrium, there is a non-negligible probability of nucleation occurring before reaching metastable equilibrium. This process is referred to as transient nucleation. The critical droplet is defined to be the configuration of maximum likelihood that is dynamically balanced between the metastable and stable wells. Time-dependent droplet profiles and nucleation rates are derived, and theoretical results are compared to computer simulation. The analysis reveals a distribution of nucleation times with a distinct peak characteristic of a nonstationary nucleation rate. Under the quench conditions employed, transient critical droplets are more compact than the droplets found in metastable equilibrium simulations and theoretical predictions.Comment: 7 Pages, 5 Figure

    Electron spin phase relaxation of phosphorus donors in nuclear spin enriched silicon

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    We report a pulsed EPR study of the phase relaxation of electron spins bound to phosphorus donors in isotopically purified 29^Si and natural abundance Si single crystals measured at 8 K.Comment: 5 pages, 3 figure

    Sea ice-atmosphere interaction: Application of multispectral satellite data in polar surface energy flux estimates

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    The application of multi-spectral satellite data to estimate polar surface energy fluxes is addressed. To what accuracy and over which geographic areas large scale energy budgets can be estimated are investigated based upon a combination of available remote sensing and climatological data sets. The general approach was to: (1) formulate parameterization schemes for the appropriate sea ice energy budget terms based upon the remotely sensed and/or in-situ data sets; (2) conduct sensitivity analyses using as input both natural variability (observed data in regional case studies) and theoretical variability based upon energy flux model concepts; (3) assess the applicability of these parameterization schemes to both regional and basin wide energy balance estimates using remote sensing data sets; and (4) assemble multi-spectral, multi-sensor data sets for at least two regions of the Arctic Basin and possibly one region of the Antarctic. The type of data needed for a basin-wide assessment is described and the temporal coverage of these data sets are determined by data availability and need as defined by parameterization scheme. The titles of the subjects are as follows: (1) Heat flux calculations from SSM/I and LANDSAT data in the Bering Sea; (2) Energy flux estimation using passive microwave data; (3) Fetch and stability sensitivity estimates of turbulent heat flux; and (4) Surface temperature algorithm

    Plant spectra as integrative measures of plant phenotypes

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    Spectroscopy at the leaf and canopy scales has attracted considerable interest in plant ecology over the past decades. Using reflectance spectra, ecologists can infer plant traits and strategies‚ÄĒand the community- or ecosystem-level processes they correlate with‚ÄĒat individual or community levels, covering more individuals and larger areas than traditional field surveys. Because of the complex entanglement of structural and chemical factors that generate spectra, it can be tricky to understand exactly what phenotypic information they contain. We discuss common approaches to estimating plant traits from spectra‚ÄĒradiative transfer and empirical models‚ÄĒand elaborate on their strengths and limitations in terms of the causal influences of various traits on the spectrum. Many chemical traits have broad, shallow and overlapping absorption features, and we suggest that covariance among traits may have an important role in giving empirical models the flexibility to estimate such traits. While trait estimates from reflectance spectra have been used to test ecological hypotheses over the past decades, there is also a growing body of research that uses spectra directly, without estimating specific traits. By treating positions of species in multidimensional spectral space as analogous to trait space, researchers can infer processes that structure plant communities using the information content of the full spectrum, which may be greater than any standard set of traits. We illustrate this power by showing that co-occurring grassland species are more separable in spectral space than in trait space and that the intrinsic dimensionality of spectral data is comparable to fairly comprehensive trait datasets. Nevertheless, using spectra this way may make it harder to interpret patterns in terms of specific biological processes. Synthesis. Plant spectra integrate many aspects of plant form and function. The information in the spectrum can be distilled into estimates of specific traits, or the spectrum can be used in its own right. These two approaches may be complementary‚ÄĒthe former being most useful when specific traits of interest are known in advance and reliable models exist to estimate them, and the latter being most useful under uncertainty about which aspects of function matter most

    Coherent manipulation of electron spins up to ambient temperatures in Cr5+^{5+}(S=1/2) doped K3_3NbO8_8

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    We report coherent spin manipulation on Cr5+^{5+} (\emph{S} = 1/2, \emph{I} = 0) doped K3_3NbO8_8, which constitutes a dilute two-level model relevant for use as a spin qubit. Rabi oscillations are observed for the first time in a spin system based on transition metal oxides up to room temperature. At liquid helium temperature the phase coherence relaxation time \emph{T2T_2} reaches ‚ąľ10\sim 10 őľ\mus and, with a Rabi frequency of 20 MHz, yields a single qubit figure of merit \emph{QMQ_M} of about 500. This shows that a diluted ensemble of Cr5+^{5+} (\emph{S} = 1/2) doped K3_3NbO8_8 is a potential candidate for solid-state quantum information processing.Comment: 4 page

    Transparent ICD and DRG Coding Using Information Technology: Linking and Associating Information Sources with the eXtensible Markup Language

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    With the introduction of ICD-10 as the standard for diagnostics, it becomes necessary to develop an electronic representation of its complete content, inherent semantics, and coding rules. The authors' design relates to the current efforts by the CEN/TC 251 to establish a European standard for hierarchical classification systems in health care. The authors have developed an electronic representation of ICD-10 with the eXtensible Markup Language (XML) that facilitates integration into current information systems and coding software, taking different languages and versions into account. In this context, XML provides a complete processing framework of related technologies and standard tools that helps develop interoperable applications. XML provides semantic markup. It allows domain-specific definition of tags and hierarchical document structure. The idea of linking and thus combining information from different sources is a valuable feature of XML. In addition, XML topic maps are used to describe relationships between different sources, or "semantically associated‚ÄĚ parts of these sources. The issue of achieving a standardized medical vocabulary becomes more and more important with the stepwise implementation of diagnostically related groups, for example. The aim of the authors' work is to provide a transparent and open infrastructure that can be used to support clinical coding and to develop further software applications. The authors are assuming that a comprehensive representation of the content, structure, inherent semantics, and layout of medical classification systems can be achieved through a document-oriented approac

    Trapped ion mobility spectrometry and PASEF enable in-depth lipidomics from minimal sample amounts

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    A comprehensive characterization of the lipidome from limited starting material remains very challenging. Here we report a high-sensitivity lipidomics workflow based on nanoflow liquid chromatography and trapped ion mobility spectrometry (TIMS). Taking advantage of parallel accumulation-serial fragmentation (PASEF), we fragment on average 15 precursors in each of 100 ms TIMS scans, while maintaining the full mobility resolution of co-eluting isomers. The acquisition speed of over 100 Hz allows us to obtain MS/MS spectra of the vast majority of isotope patterns. Analyzing 1 mu L of human plasma, PASEF increases the number of identified lipids more than three times over standard TIMS-MS/MS, achieving attomole sensitivity. Building on high intra- and inter-laboratory precision and accuracy of TIMS collisional cross sections (CCS), we compile 1856 lipid CCS values from plasma, liver and cancer cells. Our study establishes PASEF in lipid analysis and paves the way for sensitive, ion mobility-enhanced lipidomics in four dimensions
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