1,836 research outputs found
Lie Algebroid Yang Mills with Matter Fields
Lie algebroid Yang-Mills theories are a generalization of Yang-Mills gauge
theories, replacing the structural Lie algebra by a Lie algebroid E. In this
note we relax the conditions on the fiber metric of E for gauge invariance of
the action functional. Coupling to scalar fields requires possibly nonlinear
representations of Lie algebroids. In all cases, gauge invariance is seen to
lead to a condition of covariant constancy on the respective fiber metric in
question with respect to an appropriate Lie algebroid connection.
The presentation is kept in part explicit so as to be accessible also to a
less mathematically oriented audience.Comment: 24 pages, accepted for publication in J. Geom. Phy
WZW-Poisson manifolds
We observe that a term of the WZW-type can be added to the Lagrangian of the
Poisson Sigma model in such a way that the algebra of the first class
constraints remains closed. This leads to a natural generalization of the
concept of Poisson geometry. The resulting "WZW-Poisson" manifold M is
characterized by a bivector Pi and by a closed three-form H such that
[Pi,Pi]_Schouten = .Comment: 4 pages; v2: a reference adde
Bi-spectral beam extraction in combination with a focusing feeder
Bi-spectral beam extraction combines neutrons from two different kind of
moderators into one beamline, expanding the spectral range and thereby the
utilization of an instrument. This idea can be realized by a mirror that
reflects long wavelength neutrons from an off-axis colder moderator into a
neutron guide aligned with another moderator emitting neutrons with shorter
wavelengths which will be transmitted through the mirror. The mirror used in
such systems is typically several meters long, which is a severe disadvantage
because it reduces the possible length of a focusing device in design concepts
requiring a narrow beam at a short distance from the source, as used in many
instruments under development for the planned European Spallation Source (ESS).
We propose a shortened extraction system consisting of several mirrors, and
show that such an extraction system is better suited for combination with a
feeder in an eye of the needle design, illustrated here in the context of a
possible ESS imaging beamline.Comment: Published in Nuclear Instruments and Methods in Physics Research,
Section
Algebroid Yang-Mills Theories
A framework for constructing new kinds of gauge theories is suggested.
Essentially it consists in replacing Lie algebras by Lie or Courant algebroids.
Besides presenting novel topological theories defined in arbitrary spacetime
dimensions, we show that equipping Lie algebroids E with a fiber metric having
sufficiently many E-Killing vectors leads to an astonishingly mild deformation
of ordinary Yang-Mills theories: Additional fields turn out to carry no
propagating modes. Instead they serve as moduli parameters gluing together in
part different Yang-Mills theories. This leads to a symmetry enhancement at
critical points of these fields, as is also typical for String effective field
theories.Comment: 4 pages; v3: Minor rewording of v1, version to appear in Phys. Rev.
Let
Transition from accelerated to decelerated regimes in JT and CGHS cosmologies
In this work we discuss the possibility of positive-acceleration regimes, and
their transition to decelerated regimes, in two-dimensional (2D) cosmological
models. We use general relativity and the thermodynamics in a 2D space-time,
where the gas is seen as the sources of the gravitational field. An
early-Universe model is analyzed where the state equation of van der Waals is
used, replacing the usual barotropic equation. We show that this substitution
permits the simulation of a period of inflation, followed by a
negative-acceleration era. The dynamical behavior of the system follows from
the solution of the Jackiw-Teitelboim equations (JT equations) and the
energy-momentum conservation laws. In a second stage we focus the
Callan-Giddings-Harvey-Strominger model (CGHS model); here the transition from
the inflationary period to the decelerated period is also present between the
solutions, although this result depend strongly on the initial conditions used
for the dilaton field. The temporal evolution of the cosmic scale function, its
acceleration, the energy density and the hydrostatic pressure are the physical
quantities obtained in through the analysis.Comment: To appear in Europhysics Letter
Score-based tests of differential item functioning in the two-parameter model
Measurement invariance is a fundamental assumption in item response theory models, where the relationship between a latent construct (ability) and observed item responses is of interest. Violation of this assumption would render the scale misinterpreted or cause systematic bias against certain groups of people. While a number of methods have been proposed to detect measurement invariance violations, they typically require advance definition of problematic item parameters and respondent grouping information. However, these pieces of information are typically unknown in practice. As an alternative, this paper focuses on a family of recently-proposed tests based on stochastic processes of casewise derivatives of the likelihood function (i.e., scores). These score-based tests only require estimation of the null model (when measurement invariance is assumed to hold), and they have been previously applied in factor-analytic, continuous data contexts as well as in models of the Rasch family. In this paper, we aim to extend these tests to two parameter item response models estimated via maximum likelihood. The tests' theoretical background and implementation are detailed, and the tests' abilities to identify problematic item parameters are studied via simulation. An empirical example illustrating the tests' use in practice is also provided
Nuclear magnetic resonance measurements reveal the origin of the Debye process in monohydroxy alcohols
Monohydroxy alcohols show a structural relaxation and at longer time scales a
Debye-type dielectric peak. From spin-lattice relaxation experiments using
different nuclear probes an intermediate, slower-than-structural dynamics is
identified for n-butanol. Based on these findings and on diffusion
measurements, a model of self-restructuring, transient chains is proposed. The
model is demonstrated to explain consistently the so far puzzling observations
made for this class of hydrogen-bonded glass forming liquids.Comment: 4 pages, 4 figure
Evaluating Microarray-based Classifiers: An Overview
For the last eight years, microarray-based class prediction has been the subject of numerous publications in medicine, bioinformatics and statistics journals. However, in many articles, the assessment of classification accuracy is carried out using suboptimal procedures and is not paid much attention. In this paper, we carefully review various statistical aspects of classifier evaluation and validation from a practical point of view. The main topics addressed are accuracy measures, error rate estimation procedures, variable selection, choice of classifiers and validation strategy
Score‐based measurement invariance checks for Bayesian maximum‐a‐posteriori estimates in item response theory
A family of score-based tests has been proposed in recent years for assessing the invariance of model parameters in several models of item response theory (IRT). These tests were originally developed in a maximum likelihood framework. This study discusses analogous tests for Bayesian maximum-a-posteriori estimates and multiple-group IRT models. We propose two families of statistical tests, which are based on an approximation using a pooled variance method, or on a simulation approach based on asymptotic results. The resulting tests were evaluated by a simulation study, which investigated their sensitivity against differential item functioning with respect to a categorical or continuous person covariate in the two- and three-parametric logistic models. Whereas the method based on pooled variance was found to be useful in practice with maximum likelihood as well as maximum-a-posteriori estimates, the simulation-based approach was found to require large sample sizes to lead to satisfactory results
Generalized 2d dilaton gravity with matter fields
We extend the classical integrability of the CGHS model of 2d dilaton gravity
[1] to a larger class of models, allowing the gravitational part of the action
to depend more generally on the dilaton field and, simultaneously, adding
fermion- and U(1)-gauge-fields to the scalar matter. On the other hand we
provide the complete solution of the most general dilaton-dependent 2d gravity
action coupled to chiral fermions. The latter analysis is generalized to a
chiral fermion multiplet with a non-abelian gauge symmetry as well as to the
(anti-)self-dual sector df = *df (df = -*df) of a scalar field f.Comment: 37 pages, Latex; typos and Eqs. (44,45) corrected; paragraph on p.
26, referring to a work of S. Solodukhin, reformulated; references adde
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