22,430 research outputs found
Stochastic expansions using continuous dictionaries: L\'{e}vy adaptive regression kernels
This article describes a new class of prior distributions for nonparametric
function estimation. The unknown function is modeled as a limit of weighted
sums of kernels or generator functions indexed by continuous parameters that
control local and global features such as their translation, dilation,
modulation and shape. L\'{e}vy random fields and their stochastic integrals are
employed to induce prior distributions for the unknown functions or,
equivalently, for the number of kernels and for the parameters governing their
features. Scaling, shape, and other features of the generating functions are
location-specific to allow quite different function properties in different
parts of the space, as with wavelet bases and other methods employing
overcomplete dictionaries. We provide conditions under which the stochastic
expansions converge in specified Besov or Sobolev norms. Under a Gaussian error
model, this may be viewed as a sparse regression problem, with regularization
induced via the L\'{e}vy random field prior distribution. Posterior inference
for the unknown functions is based on a reversible jump Markov chain Monte
Carlo algorithm. We compare the L\'{e}vy Adaptive Regression Kernel (LARK)
method to wavelet-based methods using some of the standard test functions, and
illustrate its flexibility and adaptability in nonstationary applications.Comment: Published in at http://dx.doi.org/10.1214/11-AOS889 the Annals of
Statistics (http://www.imstat.org/aos/) by the Institute of Mathematical
Statistics (http://www.imstat.org
Strongly interacting neutrinos as the highest energy cosmic rays
We show that all features of the ultrahigh energy cosmic ray spectrum from
10^{17} eV to 10^{21} eV can be described with a simple power-like injection
spectrum of protons under the assumption that the neutrino-nucleon
cross-section is significantly enhanced at center of mass energies above
\approx 100 TeV. In our scenario, the cosmogenic neutrinos produced during the
propagation of protons through the cosmic microwave background initiate air
showers in the atmosphere, just as the protons. The total air shower spectrum
induced by protons and neutrinos shows excellent agreement with the
observations. A particular possibility for a large neutrino-nucleon
cross-section exists within the Standard Model through electroweak
instanton-induced processes.Comment: 8 pages, 4 figures, talk given at Beyond the Desert '03, Castle
Ringberg, 9-14 June, 200
Domain structure in the monoclinic Pm phase of Pb(Mg1/3Nb2/3)O3 - PbTiO3 single crystals
The domain structure of (1-x)Pb(Mg1/3Nb2/3)O3- xPbTiO3 single crystals with
composition x ~ 0.33 in the range of the morphotropic phase boundary (MPB) was
studied. Based on the analysis of spontaneous strain compatibility and charge
of domain walls, we have established the permissible domain arrangements for
the ferroelectric phases of different symmetry, which are expected to occur in
the range of the MPB. Examination of (001)-oriented unpoled and electrically
poled (along the [001] direction) crystal plates in polarizing microscope
reveals a monophase state with the domain structure compatible with the
structure theoretically predicted for the Mc monoclinic phase (space group Pm),
which was recently discovered in the compositions close to the MPB by x-ray and
neutron diffraction studies. In the unpoled crystal the 180 deg. walls between
the domains whose spontaneous polarization vectors are parallel to the plane of
the crystal plate (i.e. a-domains) are observed. The domain structure of the
poled crystal is predominantly composed of crystallographically prominent Wf
walls parallel to (001) (i.e. the plane of the crystal plate) and inclined S
walls parallel to [110] or [1-10] direction. In poled and unpoled samples the
optical rotatory polarization effect is found, which is related to the
inhomogeneity of the sample resulting from the mixture of domains.Comment: PDF file, 30 pages, 15 figures (3 in color) collected on pp.18-3
Maximized string order parameters in the valence bond solid states of quantum integer spin chains
We propose a set of maximized string order parameters to describe the hidden
topological order in the valence bond solid states of quantum integer spin-S
chains. These optimized string order parameters involve spin-twist angles
corresponding to rotations around or -axes, suggesting a
hidden symmetry. Our results also suggest that a local
triplet excitation in the valence bond solid states carries a
topological charge measured by these maximized string order parameters.Comment: 5 pages, 1 figur
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