4,260 research outputs found
LabelTranslator: A Tool to Automatically Localize an Ontology
This demo proposal briefly presents LabelTranslator, a system that suggests translations of ontology labels, with the purpose of localizing ontologies. LabelTranslator takes as input an ontology whose labels are described in a source natural language and obtains the most probable translation of each ontology label into a target natural language.Our main contribution is the automatization of this process, which reduces human efforts to localize manually the ontology
Hybrid Quantum Cosmology: Combining Loop and Fock Quantizations
As a necessary step towards the extraction of realistic results from Loop
Quantum Cosmology, we analyze the physical consequences of including
inhomogeneities. We consider in detail the quantization of a gravitational
model in vacuo which possesses local degrees of freedom, namely, the linearly
polarized Gowdy cosmologies with the spatial topology of a three-torus. We
carry out a hybrid quantization which combines loop and Fock techniques. We
discuss the main aspects and results of this hybrid quantization, which include
the resolution of the cosmological singularity, the polymeric quantization of
the internal time, a rigorous definition of the quantum constraints and the
construction of their solutions, the Hilbert structure of the physical states,
and the recovery of a conventional Fock quantization for the inhomogeneities.Comment: 24 pages, published in International Journal of Modern Physics A,
Special Issue: Proceedings of the Second Workshop on Quantum Gravity and
Noncommutative Geometry (Lisbon, Portugal
On the improvement of the low energy neutrino factory
The low energy neutrino factory has been proposed as a very sensitive setup
for future searches for CP violation and matter effects. Here we study how its
performance is affected when the experimental specifications of the setup are
varied. Most notably, we have considered the addition of the 'platinum' nu_{mu}
-> nu_{e} channel. We find that, whilst theoretically the extra channel
provides very useful complementary information and helps to lift degeneracies,
its practical usefulness is lost when considering realistic background levels.
Conversely, an increase in statistics in the 'golden' nu_{e} -> nu_{mu} channel
and, to some extent, an improvement in the energy resolution, lead to an
important increase in the performance of the facility, given the rich energy
dependence of the 'golden' channel at these energies. We show that a low energy
neutrino factory with a baseline of 1300 km, muon energy of 4.5 GeV, and either
a 20 kton totally active scintillating detector or 100 kton liquid argon
detector, can have outstanding sensitivity to the neutrino oscillation
parameters theta13, delta and the mass hierarchy. For our estimated exposure of
2.8 x 10^{23} kton x decays per muon polarity, the low energy neutrino factory
has sensitivity to theta13 and delta for sin^{2}(2theta13) > 10^{-4} and to the
mass hierarchy for sin^{2}(2theta13) > 10^{-3}.Comment: 13 pages, 8 eps figures. Version published in PRD - experimental
section with preliminary results removed, abstract and conclusions re-written
accordingly, title changed, author list amended
Uniqueness of the Fock quantization of the Gowdy model
After its reduction by a gauge-fixing procedure, the family of linearly
polarized Gowdy cosmologies admit a scalar field description whose
evolution is governed by a Klein-Gordon type equation in a flat background in
1+1 dimensions with the spatial topology of , though in the presence of a
time-dependent potential. The model is still subject to a homogeneous
constraint, which generates -translations. Recently, a Fock quantization
of this scalar field was introduced and shown to be unique under the
requirements of unitarity of the dynamics and invariance under the gauge group
of -translations. In this work, we extend and complete this uniqueness
result by considering other possible scalar field descriptions, resulting from
reasonable field reparameterizations of the induced metric of the reduced
model. In the reduced phase space, these alternate descriptions can be obtained
by means of a time-dependent scaling of the field, the inverse scaling of its
canonical momentum, and the possible addition of a time-dependent, linear
contribution of the field to this momentum. Demanding again unitarity of the
field dynamics and invariance under the gauge group, we prove that the
alternate canonical pairs of fieldlike variables admit a Fock representation if
and only if the scaling of the field is constant in time. In this case, there
exists essentially a unique Fock representation, provided by the quantization
constructed by Corichi, Cortez, and Mena Marugan. In particular, our analysis
shows that the scalar field description proposed by Pierri does not admit a
Fock quantization with the above unitarity and invariance properties.Comment: 14 page
Quantum unitary dynamics in cosmological spacetimes
We address the question of unitary implementation of the dynamics for scalar
fields in cosmological scenarios. Together with invariance under spatial
isometries, the requirement of a unitary evolution singles out a rescaling of
the scalar field and a unitary equivalence class of Fock representations for
the associated canonical commutation relations. Moreover, this criterion
provides as well a privileged quantization for the unscaled field, even though
the associated dynamics is not unitarily implementable in that case. We discuss
the relation between the initial data that determine the Fock representations
in the rescaled and unscaled descriptions, and clarify that the S-matrix is
well defined in both cases. In our discussion, we also comment on a recently
proposed generalized notion of unitary implementation of the dynamics, making
clear the difference with the standard unitarity criterion and showing that the
two approaches are not equivalent.Comment: 18 page
Quantum Cylindrical Waves and Sigma Models
We analyze cylindrical gravitational waves in vacuo with general polarization
and develop a viewpoint complementary to that presented recently by Niedermaier
showing that the auxiliary sigma model associated with this family of waves is
not renormalizable in the standard perturbative sense.Comment: 11 pages (DIN A4), accepted in International Journal of Modern
Physics
A uniqueness criterion for the Fock quantization of scalar fields with time dependent mass
A major problem in the quantization of fields in curved spacetimes is the
ambiguity in the choice of a Fock representation for the canonical commutation
relations. There exists an infinite number of choices leading to different
physical predictions. In stationary scenarios, a common strategy is to select a
vacuum (or a family of unitarily equivalent vacua) by requiring invariance
under the spacetime symmetries. When stationarity is lost, a natural
generalization consists in replacing time invariance by unitarity in the
evolution. We prove that, when the spatial sections are compact, the criterion
of a unitary dynamics, together with the invariance under the spatial
isometries, suffices to select a unique family of Fock quantizations for a
scalar field with time dependent mass.Comment: 11 pages, version accepted for publication in Classical and Quantum
Gravit
Quantum Gowdy model: A uniqueness result
Modulo a homogeneous degree of freedom and a global constraint, the linearly
polarised Gowdy cosmologies are equivalent to a free scalar field
propagating in a fixed nonstationary background. Recently, a new field
parameterisation was proposed for the metric of the Gowdy spacetimes such that
the associated scalar field evolves in a flat background in 1+1 dimensions with
the spatial topology of , although subject to a time dependent potential.
Introducing a suitable Fock quantisation for this scalar field, a quantum
theory was constructed for the Gowdy model in which the dynamics is implemented
as a unitary transformation. A question that was left open is whether one might
adopt a different, nonequivalent Fock representation by selecting a distinct
complex structure. The present work proves that the chosen Fock quantisation is
in fact unique (up to unitary equivalence) if one demands unitary
implementation of the dynamics and invariance under the group of constant
translations. These translations are precisely those generated by the global
constraint that remains on the Gowdy model. It is also shown that the proof of
uniqueness in the choice of complex structure can be applied to more general
field dynamics than that corresponding to the Gowdy cosmologies.Comment: 28 pages, minor changes, version accepted for publication in
Classical and Quantum Gravit
Criteria for the determination of time dependent scalings in the Fock quantization of scalar fields with a time dependent mass in ultrastatic spacetimes
For Klein-Gordon fields, it is well known that there exist an infinite number
of nonequivalent Fock representations of the canonical commutation relations
and, therefore, of inequivalent quantum theories. A context in which this kind
of ambiguities arises and prevents the derivation of robust results is, e.g.,
in the quantum analysis of cosmological perturbations. In these situations,
typically, a suitable scaling of the field by a time dependent function leads
to a description in an auxiliary static background, though the nonstationarity
still shows up in a time dependent mass. For such a field description, and
assuming the compactness of the spatial sections, we recently proved in three
or less spatial dimensions that the criteria of a natural implementation of the
spatial symmetries and of a unitary time evolution are able to select a unique
class of unitarily equivalent vacua, and hence of Fock representations. In this
work, we succeed to extend our uniqueness result to the consideration of all
possible field descriptions that can be reached by a time dependent canonical
transformation which, in particular, involves a scaling of the field by a
function of time. This kind of canonical transformations modify the dynamics of
the system and introduce a further ambiguity in its quantum description,
exceeding the choice of a Fock representation. Remarkably, for any compact
spatial manifold in less than four dimensions, we show that our criteria
eliminate any possible nontrivial scaling of the field other than that leading
to the description in an auxiliary static background. Besides, we show that
either no time dependent redefinition of the field momentum is allowed or, if
this may happen, the redefinition does not introduce any Fock representation
that cannot be obtained by a unitary transformation.Comment: 37 pages. Modified title. Improved discussion concerning the spatial
symmetry group. New section (section VI
Analysis of defect structure in silicon. Silicon sheet growth development for the large area silicon sheet task of the Low-Cost Solar array Project
One hundred ninety-three silicon sheet samples, approximately 880 square centimeters, were analyzed for twin boundary density, dislocation pit density, and grain boundary length. One hundred fifteen of these samples were manufactured by a heat exchanger method, thirty-eight by edge defined film fed growth, twenty-three by the silicon on ceramics process, and ten by the dendritic web process. Seven solar cells were also step-etched to determine the internal defect distribution on these samples. Procedures were developed or the quantitative characterization of structural defects such as dislocation pits, precipitates, twin & grain boundaries using a QTM 720 quantitative image analyzing system interfaced with a PDP 11/03 mini computer. Characterization of the grain boundary length per unit area for polycrystalline samples was done by using the intercept method on an Olympus HBM Microscope
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