15,854 research outputs found
AdS/CFT and large-N volume independence
We study the Eguchi-Kawai reduction in the strong-coupling domain of gauge
theories via the gravity dual of N=4 super-Yang-Mills on R^3xS^1. We show that
D-branes geometrize volume independence in the center-symmetric vacuum and give
supergravity predictions for the range of validity of reduced large-N models at
strong coupling.Comment: 4 pages, 2 figures; references and comments adde
Towards an understanding of third-order galaxy-galaxy lensing
Third-order galaxy-galaxy lensing (G3L) is a next generation galaxy-galaxy
lensing technique that either measures the excess shear about lens pairs or the
excess shear-shear correlations about lenses. It is clear that these statistics
assess the three-point correlations between galaxy positions and projected
matter density. For future applications of these novel statistics, we aim at a
more intuitive understanding of G3L to isolate the main features that possibly
can be measured. We construct a toy model ("isolated lens model"; ILM) for the
distribution of galaxies and associated matter to determine the measured
quantities of the two G3L correlation functions and traditional galaxy-galaxy
lensing (GGL) in a simplified context. The ILM presumes single lens galaxies to
be embedded inside arbitrary matter haloes that, however, are statistically
independent ("isolated") from any other halo or lens position. In the ILM, the
average mass-to-galaxy number ratio of clusters of any size cannot change. GGL
and galaxy clustering alone cannot distinguish an ILM from any more complex
scenario. The lens-lens-shear correlator in combination with second-order
statistics enables us to detect deviations from a ILM, though. This can be
quantified by a difference signal defined in the paper. We demonstrate with the
ILM that this correlator picks up the excess matter distribution about galaxy
pairs inside clusters. The lens-shear-shear correlator is sensitive to
variations among matter haloes. In principle, it could be devised to constrain
the ellipticities of haloes, without the need for luminous tracers, or maybe
even random halo substructure. [Abridged]Comment: 14 pages, 3 figures, 1 table, accepted by A&A; some
"lens-shear-shear" were falsely "lens-lens-shear
Many-body Propagator Theory with Three-Body Interactions: a Path to Exotic Open Shell Isotopes
Ab-initio predictions of nuclei with masses up to A~100 or more is becoming
possible thanks to novel advances in computations and in the formalism of
many-body physics. Some of the most fundamental issues include how to deal with
many-nucleon interactions, how to calculate degenerate--open shell--systems,
and pursuing ab-initio approaches to reaction theory. Self-consistent Green's
function (SCGF) theory is a natural approach to address these challenges. Its
formalism has recently been extended to three- and many-body interactions and
reformulated within the Gorkov framework to reach semi-magic open shell
isotopes. These exciting developments, together with the predictive power of
chiral nuclear Hamiltonians, are opening the path to understanding large
portions of the nuclear chart, especially within the and shells. The
present talk reviews the most recent advances in ab-initio nuclear structure
and many-body theory that have been possible through the SCGF approach.Comment: 17 pages, 11 figures; Proceeding of the 17th International Conference
on Recent Progress in Many-Body-Theories; Journal of Physics: Conference
Series (JPCS). Very minor corrections, published versio
Interference induced thermoelectric switching and heat rectification in quantum Hall junctions
Interference represents one of the most striking manifestation of quantum
physics in low-dimensional systems. Despite evidences of quantum interference
in charge transport have been known for a long time, only recently signatures
of interference induced thermal properties have been reported, paving the way
for the phase-coherent manipulation of heat in mesoscopic devices. In this work
we show that anomalous thermoelectric properties and efficient heat
rectification can be achieved by exploiting the phase-coherent edge states of
quantum Hall systems. By considering a tunneling geometry with multiple quantum
point contacts, we demonstrate that the interference paths effectively break
the electron-hole symmetry, allowing for a thermoelectric charge current
flowing either from hot to cold or viceversa, depending on the details of the
tunnel junction. Correspondingly, an interference induced heat current is
predicted, and we are able to explain these results in terms of an intuitive
physical picture. Moreover, we show that heat rectification can be achieved by
coupling two quantum Hall systems with different filling factors, and that this
effect can be enhanced by exploiting the interference properties of the tunnel
junction.Comment: 9 pages, 7 figure
Active and passive fields face to face
The statistical properties of active and passive scalar fields transported by
the same turbulent flow are investigated. Four examples of active scalar have
been considered: temperature in thermal convection, magnetic potential in
two-dimensional magnetohydrodynamics, vorticity in two-dimensional Ekman
turbulence and potential temperature in surface flows. In the cases of
temperature and vorticity, it is found that the active scalar behavior is akin
to that of its co-evolving passive counterpart. The two other cases indicate
that this similarity is in fact not generic and differences between passive and
active fields can be striking: in two-dimensional magnetohydrodynamics the
magnetic potential performs an inverse cascade while the passive scalar
cascades toward the small-scales; in surface flows, albeit both perform a
direct cascade, the potential temperature and the passive scalar have different
scaling laws already at the level of low-order statistical objects. These
dramatic differences are rooted in the correlations between the active scalar
input and the particle trajectories. The role of such correlations in the issue
of universality in active scalar transport and the behavior of dissipative
anomalies is addressed.Comment: 36 pages, 20 eps figures, for the published version see
http://www.iop.org/EJ/abstract/1367-2630/6/1/07
Tractability through Exchangeability: A New Perspective on Efficient Probabilistic Inference
Exchangeability is a central notion in statistics and probability theory. The
assumption that an infinite sequence of data points is exchangeable is at the
core of Bayesian statistics. However, finite exchangeability as a statistical
property that renders probabilistic inference tractable is less
well-understood. We develop a theory of finite exchangeability and its relation
to tractable probabilistic inference. The theory is complementary to that of
independence and conditional independence. We show that tractable inference in
probabilistic models with high treewidth and millions of variables can be
understood using the notion of finite (partial) exchangeability. We also show
that existing lifted inference algorithms implicitly utilize a combination of
conditional independence and partial exchangeability.Comment: In Proceedings of the 28th AAAI Conference on Artificial Intelligenc
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