4,664 research outputs found
The IBMAP approach for Markov networks structure learning
In this work we consider the problem of learning the structure of Markov
networks from data. We present an approach for tackling this problem called
IBMAP, together with an efficient instantiation of the approach: the IBMAP-HC
algorithm, designed for avoiding important limitations of existing
independence-based algorithms. These algorithms proceed by performing
statistical independence tests on data, trusting completely the outcome of each
test. In practice tests may be incorrect, resulting in potential cascading
errors and the consequent reduction in the quality of the structures learned.
IBMAP contemplates this uncertainty in the outcome of the tests through a
probabilistic maximum-a-posteriori approach. The approach is instantiated in
the IBMAP-HC algorithm, a structure selection strategy that performs a
polynomial heuristic local search in the space of possible structures. We
present an extensive empirical evaluation on synthetic and real data, showing
that our algorithm outperforms significantly the current independence-based
algorithms, in terms of data efficiency and quality of learned structures, with
equivalent computational complexities. We also show the performance of IBMAP-HC
in a real-world application of knowledge discovery: EDAs, which are
evolutionary algorithms that use structure learning on each generation for
modeling the distribution of populations. The experiments show that when
IBMAP-HC is used to learn the structure, EDAs improve the convergence to the
optimum
Quantum walks of correlated particles
Quantum walks of correlated particles offer the possibility to study
large-scale quantum interference, simulate biological, chemical and physical
systems, and a route to universal quantum computation. Here we demonstrate
quantum walks of two identical photons in an array of 21 continuously
evanescently-coupled waveguides in a SiOxNy chip. We observe quantum
correlations, violating a classical limit by 76 standard deviations, and find
that they depend critically on the input state of the quantum walk. These
results open the way to a powerful approach to quantum walks using correlated
particles to encode information in an exponentially larger state space
Minimal surfaces and particles in 3-manifolds
We use minimal (or CMC) surfaces to describe 3-dimensional hyperbolic,
anti-de Sitter, de Sitter or Minkowski manifolds. We consider whether these
manifolds admit ``nice'' foliations and explicit metrics, and whether the space
of these metrics has a simple description in terms of Teichm\"uller theory. In
the hyperbolic settings both questions have positive answers for a certain
subset of the quasi-Fuchsian manifolds: those containing a closed surface with
principal curvatures at most 1. We show that this subset is parameterized by an
open domain of the cotangent bundle of Teichm\"uller space. These results are
extended to ``quasi-Fuchsian'' manifolds with conical singularities along
infinite lines, known in the physics literature as ``massive, spin-less
particles''.
Things work better for globally hyperbolic anti-de Sitter manifolds: the
parameterization by the cotangent of Teichm\"uller space works for all
manifolds. There is another description of this moduli space as the product two
copies of Teichm\"uller space due to Mess. Using the maximal surface
description, we propose a new parameterization by two copies of Teichm\"uller
space, alternative to that of Mess, and extend all the results to manifolds
with conical singularities along time-like lines. Similar results are obtained
for de Sitter or Minkowski manifolds.
Finally, for all four settings, we show that the symplectic form on the
moduli space of 3-manifolds that comes from parameterization by the cotangent
bundle of Teichm\"uller space is the same as the 3-dimensional gravity one.Comment: 53 pages, no figure. v2: typos corrected and refs adde
A New Scintillator Tile/Fiber Preshower Detector for the CDF Central Calorimeter
A detector designed to measure early particle showers has been installed in
front of the central CDF calorimeter at the Tevatron. This new preshower
detector is based on scintillator tiles coupled to wavelength-shifting fibers
read out by multi-anode photomultipliers and has a total of 3,072 readout
channels. The replacement of the old gas detector was required due to an
expected increase in instantaneous luminosity of the Tevatron collider in the
next few years. Calorimeter coverage, jet energy resolution, and electron and
photon identification are among the expected improvements. The final detector
design, together with the R&D studies that led to the choice of scintillator
and fiber, mechanical assembly, and quality control are presented. The detector
was installed in the fall 2004 Tevatron shutdown and started collecting
colliding beam data by the end of the same year. First measurements indicate a
light yield of 12 photoelectrons/MIP, a more than two-fold increase over the
design goals.Comment: 5 pages, 10 figures (changes are minor; this is the final version
published in IEEE-Trans.Nucl.Sci.
Collisions of particles in locally AdS spacetimes I. Local description and global examples
We investigate 3-dimensional globally hyperbolic AdS manifolds containing
"particles", i.e., cone singularities along a graph . We impose
physically relevant conditions on the cone singularities, e.g. positivity of
mass (angle less than on time-like singular segments). We construct
examples of such manifolds, describe the cone singularities that can arise and
the way they can interact (the local geometry near the vertices of ).
We then adapt to this setting some notions like global hyperbolicity which are
natural for Lorentz manifolds, and construct some examples of globally
hyperbolic AdS manifolds with interacting particles.Comment: This is a rewritten version of the first part of arxiv:0905.1823.
That preprint was too long and contained two types of results, so we sliced
it in two. This is the first part. Some sections have been completely
rewritten so as to be more readable, at the cost of slightly less general
statements. Others parts have been notably improved to increase readabilit
Multiple Scale Reorganization of Electrostatic Complexes of PolyStyrene Sulfonate and Lysozyme
We report on a SANS investigation into the potential for these structural
reorganization of complexes composed of lysozyme and small PSS chains of
opposite charge if the physicochemical conditions of the solutions are changed
after their formation. Mixtures of solutions of lysozyme and PSS with high
matter content and with an introduced charge ratio [-]/[+]intro close to the
electrostatic stoichiometry, lead to suspensions that are macroscopically
stable. They are composed at local scale of dense globular primary complexes of
radius ~ 100 {\AA}; at a higher scale they are organized fractally with a
dimension 2.1. We first show that the dilution of the solution of complexes,
all other physicochemical parameters remaining constant, induces a macroscopic
destabilization of the solutions but does not modify the structure of the
complexes at submicronic scales. This suggests that the colloidal stability of
the complexes can be explained by the interlocking of the fractal aggregates in
a network at high concentration: dilution does not break the local aggregate
structure but it does destroy the network. We show, secondly, that the addition
of salt does not change the almost frozen inner structure of the cores of the
primary complexes, although it does encourage growth of the complexes; these
coalesce into larger complexes as salt has partially screened the electrostatic
repulsions between two primary complexes. These larger primary complexes remain
aggregated with a fractal dimension of 2.1. Thirdly, we show that the addition
of PSS chains up to [-]/[+]intro ~ 20, after the formation of the primary
complex with a [-]/[+]intro close to 1, only slightly changes the inner
structure of the primary complexes. Moreover, in contrast to the synthesis
achieved in the one-step mixing procedure where the proteins are unfolded for a
range of [-]/[+]intro, the native conformation of the proteins is preserved
inside the frozen core
kt Effects in Direct-Photon Production
We discuss the phenomenology of initial-state parton-kt broadening in
direct-photon production and related processes in hadron collisions. After a
brief summary of the theoretical basis for a Gaussian-smearing approach, we
present a systematic study of recent results on fixed-target and collider
direct-photon production, using complementary data on diphoton and pion
production to provide empirical guidance on the required amount of kt
broadening. This approach provides a consistent description of the observed
pattern of deviation of next-to-leading order QCD calculations relative to the
direct-photon data, and accounts for the shape and normalization difference
between fixed-order perturbative calculations and the data. We also discuss the
uncertainties in this phenomenological approach, the implications of these
results on the extraction of the gluon distribution of the nucleon, and the
comparison of our findings to recent related work.Comment: LaTeX, uses revtex and epsf, 37 pages, 15 figure
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