2,249 research outputs found
Optimal Image Reconstruction in Radio Interferometry
We introduce a method for analyzing radio interferometry data which produces
maps which are optimal in the Bayesian sense of maximum posterior probability
density, given certain prior assumptions. It is similar to maximum entropy
techniques, but with an exact accounting of the multiplicity instead of the
usual approximation involving Stirling's formula. It also incorporates an Occam
factor, automatically limiting the effective amount of detail in the map to
that justified by the data. We use Gibbs sampling to determine, to any desired
degree of accuracy, the multi-dimensional posterior density distribution. From
this we can construct a mean posterior map and other measures of the posterior
density, including confidence limits on any well-defined function of the
posterior map.Comment: 41 pages, 11 figures. High resolution figures 8 and 9 available at
http://www.astro.uiuc.edu/~bwandelt/SuttonWandelt200
History, College of Medicine: 1959-1968. Chapter 16: Department of Physiological Chemistry
Prepared for the Centennial of The Ohio State University
Keloid Formation A Horse
In equines the formation of exuberant granulation tissue is of common occurrence in wounds which are attended by considerable irritation. However, certain individuals have what is known as blastoma constitution , which is a tendency to produce enormous amounts of connective tissue from comparatively slight irritation. Frequent sequelae of these exuberations are keloids, which are not true tumors but excessive formations of scar tissue
Twisting 2-cocycles for the construction of new non-standard quantum groups
We introduce a new class of 2-cocycles defined explicitly on the generators
of certain multiparameter standard quantum groups. These allow us, through the
process of twisting the familiar standard quantum groups, to generate new as
well as previously known examples of non-standard quantum groups. In particular
we are able to construct generalisations of both the Cremmer-Gervais
deformation of SL(3) and the so called esoteric quantum groups of Fronsdal and
Galindo in an explicit and straightforward manner.Comment: 21 pages, AMSLaTeX, expanded introduction and a few other minor
corrections, to appear in JM
Tensor network states and algorithms in the presence of a global SU(2) symmetry
The benefits of exploiting the presence of symmetries in tensor network
algorithms have been extensively demonstrated in the context of matrix product
states (MPSs). These include the ability to select a specific symmetry sector
(e.g. with a given particle number or spin), to ensure the exact preservation
of total charge, and to significantly reduce computational costs. Compared to
the case of a generic tensor network, the practical implementation of
symmetries in the MPS is simplified by the fact that tensors only have three
indices (they are trivalent, just as the Clebsch-Gordan coefficients of the
symmetry group) and are organized as a one-dimensional array of tensors,
without closed loops. Instead, a more complex tensor network, one where tensors
have a larger number of indices and/or a more elaborate network structure,
requires a more general treatment. In two recent papers, namely (i) [Phys. Rev.
A 82, 050301 (2010)] and (ii) [Phys. Rev. B 83, 115125 (2011)], we described
how to incorporate a global internal symmetry into a generic tensor network
algorithm based on decomposing and manipulating tensors that are invariant
under the symmetry. In (i) we considered a generic symmetry group G that is
compact, completely reducible and multiplicity free, acting as a global
internal symmetry. Then in (ii) we described the practical implementation of
Abelian group symmetries. In this paper we describe the implementation of
non-Abelian group symmetries in great detail and for concreteness consider an
SU(2) symmetry. Our formalism can be readily extended to more exotic symmetries
associated with conservation of total fermionic or anyonic charge. As a
practical demonstration, we describe the SU(2)-invariant version of the
multi-scale entanglement renormalization ansatz and apply it to study the low
energy spectrum of a quantum spin chain with a global SU(2) symmetry.Comment: 32 pages, 37 figure
Hidden unity in the quantum description of matter
We introduce an algebraic framework for interacting quantum systems that
enables studying complex phenomena, characterized by the coexistence and
competition of various broken symmetry states of matter. The approach unveils
the hidden unity behind seemingly unrelated physical phenomena, thus
establishing exact connections between them. This leads to the fundamental
concept of {\it universality} of physical phenomena, a general concept not
restricted to the domain of critical behavior. Key to our framework is the
concept of {\it languages} and the construction of {\it dictionaries} relating
them.Comment: 10 pages 2 psfigures. Appeared in Recent Progress in Many-Body
Theorie
S and D-wave phase shifts in isospin-2 pi pi scattering from lattice QCD
The isospin-2 pi pi system provides a useful testing ground for determining
elastic hadron scattering parameters from finite-volume spectra obtained using
lattice QCD computations. A reliable determination of the excited state
spectrum of two pions in a cubic box follows from variational analysis of
correlator matrices constructed using a large basis of operators. A general
operator construction is presented which respects the symmetries of a
multi-hadron system in flight. This is applied to the case of pi pi and allows
for the determination of the scattering phase-shifts at a large number of
kinematic points, in both S-wave and D-wave, within the elastic region. The
technique is demonstrated with a calculation at a pion mass of 396 MeV, where
the elastic scattering is found to be well described by a scattering length
parameterisation.Comment: Tables of little-group CGCs in ancillary file; v2: minor changes to
reflect published versio
-graded Heisenberg algebras and deformed supersymmetries
The notion of -grading on the enveloping algebra generated by products of
q-deformed Heisenberg algebras is introduced for complex number in the unit
disc. Within this formulation, we consider the extension of the notion of
supersymmetry in the enveloping algebra. We recover the ordinary
grading or Grassmann parity for associative superalgebra, and a modified
version of the usual supersymmetry. As a specific problem, we focus on the
interesting limit for which the Arik and Coon deformation of the
Heisenberg algebra allows to map fermionic modes to bosonic ones in a modified
sense. Different algebraic consequences are discussed.Comment: 2 figure
One-loop effective potential for SO(10) GUT theories in de Sitter space
Zeta-function regularization is applied to evaluate the one-loop effective
potential for SO(10) grand-unified theories in de Sitter cosmologies. When the
Higgs scalar field belongs to the 210-dimensional irreducible representation of
SO(10), attention is focused on the mass matrix relevant for the
SU(3)xSU(2)xU(1) symmetry-breaking direction, to agree with low-energy
phenomenology of the particle-physics standard model. The analysis is
restricted to those values of the tree-level-potential parameters for which the
absolute minima of the classical potential have been evaluated. As shown in the
recent literature, such minima turn out to be SO(6)xSO(4)- or
SU(3)xSU(2)xSU(2)xU(1)-invariant. Electroweak phenomenology is more naturally
derived, however, from the former minima. Hence the values of the parameters
leading to the alternative set of minima have been discarded. Within this
framework, flat-space limit and general form of the one-loop effective
potential are studied in detail by using analytic and numerical methods. It
turns out that, as far as the absolute-minimum direction is concerned, the
flat-space limit of the one-loop calculation about a de Sitter background does
not change the results previously obtained in the literature, where the
tree-level potential in flat space-time was studied. Moreover, when curvature
effects are no longer negligible in the one-loop potential, it is found that
the early universe remains bound to reach only the SO(6)xSO(4) absolute
minimum.Comment: 25 pages, plain Tex, plus Latex file of the tables appended at the
end. Published in Classical and Quantum Gravity, Vol. 11, pp. 2031-2044,
August 199
Ambient noise tomography reveals basalt and sub-basalt velocity structure beneath the Faroe Islands, North Atlantic
The Faroe Islands Passive Seismological Experiment (FIPSE) was funded by Sindri (contract C46-52-01) and formed a collaborative project between Dr. David Cornwell, Prof. Richard England (University of Leicester) and Prof. Graham Stuart (University of Leeds). Seismological equipment was loaned from the NERC geophysical equipment facility (GEF, loan 918), with field assistance from David Hawthorn and data processing assistance from Victoria Lane (SEIS-UK). We acknowledge the help, advice and support of JarĂ°feingi, especially Thomas Varming, Uni Petersen, Bartal HĂžjgaard, Romica Ăster and Heri Ziska. RannvĂĄ M. Arge and Magni JĂžkladal are thanked for their assistance with fieldwork. Research undertaken in this article was supported by the Carnegie Trust for the Universities of Scotland, via a Collaborative Research Grant. Rosie Fletcher is thanked for her comments, which greatly improved the text.Peer reviewedPostprin
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