19,343 research outputs found
Perfect Lattice Topology: The Quantum Rotor as a Test Case
Lattice actions and topological charges that are classically and quantum
mechanically perfect (i.e. free of lattice artifacts) are constructed
analytically for the quantum rotor. It is demonstrated that the Manton action
is classically perfect while the Villain action is quantum perfect. The
geometric construction for the topological charge is only perfect at the
classical level. The quantum perfect lattice topology associates a topological
charge distribution, not just a single charge, with each lattice field
configuration. For the quantum rotor with the classically perfect action and
topological charge, the remaining cut-off effects are exponentially suppressed.Comment: 12 pages, including two figures. ordinary LaTeX, requires fps.sty;
Submitted to Phys. Lett.
A New Class of Group Field Theories for 1st Order Discrete Quantum Gravity
Group Field Theories, a generalization of matrix models for 2d gravity,
represent a 2nd quantization of both loop quantum gravity and simplicial
quantum gravity. In this paper, we construct a new class of Group Field Theory
models, for any choice of spacetime dimension and signature, whose Feynman
amplitudes are given by path integrals for clearly identified discrete gravity
actions, in 1st order variables. In the 3-dimensional case, the corresponding
discrete action is that of 1st order Regge calculus for gravity (generalized to
include higher order corrections), while in higher dimensions, they correspond
to a discrete BF theory (again, generalized to higher order) with an imposed
orientation restriction on hinge volumes, similar to that characterizing
discrete gravity. The new models shed also light on the large distance or
semi-classical approximation of spin foam models. This new class of group field
theories may represent a concrete unifying framework for loop quantum gravity
and simplicial quantum gravity approaches.Comment: 48 pages, 4 figures, RevTeX, one reference adde
Gravitational Scattering in the ADD-model at High and Low Energies
Gravitational scattering in the ADD-model is considered at both sub- and
transplanckian energies using a common formalism. By keeping a physical cut-off
in the KK tower associated with virtual KK exchange, such as the cut-off
implied from a finite brane width, troublesome divergences are removed from the
calculations in both energy ranges. The scattering behavior depends on three
different energy scales: the fundamental Planck mass, the collision energy and
the inverse brane width. The result for energies low compared to the effective
cut-off (inverse brane width) is a contact-like interaction. At high energies
the gravitational scattering associated with the extra dimensional version of
Newton's law is recovered
Calibration artefacts in radio interferometry. I. Ghost sources in WSRT data
This work investigates a particular class of artefacts, or ghost sources, in
radio interferometric images. Earlier observations with (and simulations of)
the Westerbork Synthesis Radio Telescope (WSRT) suggested that these were due
to calibration with incomplete sky models. A theoretical framework is derived
that validates this suggestion, and provides predictions of ghost formation in
a two-source scenario. The predictions are found to accurately match the result
of simulations, and qualitatively reproduce the ghosts previously seen in
observational data. The theory also provides explanations for many previously
puzzling features of these artefacts (regular geometry, PSF-like sidelobes,
seeming independence on model flux), and shows that the observed phenomenon of
flux suppression affecting unmodelled sources is due to the same mechanism. We
demonstrate that this ghost formation mechanism is a fundamental feature of
calibration, and exhibits a particularly strong and localized signature due to
array redundancy. To some extent this mechanism will affect all observations
(including those with non-redundant arrays), though in most cases the ghosts
remain hidden below the noise or masked by other instrumental artefacts. The
implications of such errors on future deep observations are discussed.Comment: 19 pages, 15 figures, submitted to MNRA
Evaluating Graph Signal Processing for Neuroimaging Through Classification and Dimensionality Reduction
Graph Signal Processing (GSP) is a promising framework to analyze
multi-dimensional neuroimaging datasets, while taking into account both the
spatial and functional dependencies between brain signals. In the present work,
we apply dimensionality reduction techniques based on graph representations of
the brain to decode brain activity from real and simulated fMRI datasets. We
introduce seven graphs obtained from a) geometric structure and/or b)
functional connectivity between brain areas at rest, and compare them when
performing dimension reduction for classification. We show that mixed graphs
using both a) and b) offer the best performance. We also show that graph
sampling methods perform better than classical dimension reduction including
Principal Component Analysis (PCA) and Independent Component Analysis (ICA).Comment: 5 pages, GlobalSIP 201
- …