41,405 research outputs found
Personalized content retrieval in context using ontological knowledge
Personalized content retrieval aims at improving the retrieval process by taking into account the particular interests of individual users. However, not all user preferences are relevant in all situations. It is well known that human preferences are complex, multiple, heterogeneous, changing, even contradictory, and should be understood in context with the user goals and tasks at hand. In this paper, we propose a method to build a dynamic representation of the semantic context of ongoing retrieval tasks, which is used to activate different subsets of user interests at runtime, in a way that out-of-context preferences are discarded. Our approach is based on an ontology-driven representation of the domain of discourse, providing enriched descriptions of the semantics involved in retrieval actions and preferences, and enabling the definition of effective means to relate preferences and context
Measuring the Radiative Histories of QSOs with the Transverse Proximity Effect
Since the photons that stream from QSOs alter the ionization state of the gas
they traverse, any changes to a QSO's luminosity will produce
outward-propagating ionization gradients in the surrounding intergalactic gas.
This paper shows that at redshift z~3 the gradients will alter the gas's
Lyman-alpha absorption opacity enough to produce a detectable signature in the
spectra of faint background galaxies. By obtaining noisy (S:N~4) low-resolution
(~7A) spectra of a several dozen background galaxies in an R~20' field
surrounding an isotropically radiating 18th magnitude QSO at z=3, it should be
possible to detect any order-of-magnitude changes to the QSO's luminosity over
the previous 50--100 Myr and to measure the time t_Q since the onset of the
QSO's current luminous outburst with an accuracy of ~5 Myr for t_Q<~50 Myr.
Smaller fields-of-view are acceptable for shorter QSO lifetimes. The major
uncertainty, aside from cosmic variance, will be the shape and orientation of
the QSO's ionization cone. This can be determined from the data if the number
of background sources is increased by a factor of a few. The method will then
provide a direct test of unification models for AGN.Comment: Accepted for publication in the ApJ. 16 page
Boltzmann entropy of a Newtonian Universe
A dynamical estimate is given for the Boltzmann entropy of the Universe,
under the simplifying assumptions provided by Newtonian cosmology. We first
model the cosmological fluid as the probability fluid of a quantum-mechanical
system. Next, following current ideas about the emergence of spacetime, we
regard gravitational equipotentials as isoentropic surfaces. Therefore
gravitational entropy is proportional to the vacuum expectation value of the
gravitational potential in a certain quantum state describing the matter
contents of the Universe. The entropy of the matter sector can also be
computed. While providing values of the entropy that turn out to be somewhat
higher than existing estimates, our results are in perfect compliance with the
upper bound set by the holographic principle.Comment: 15 page
Puzzles in quarkonium hadronic transitions with two pion emission
The anomalously large rates of some hadronic transitions from quarkonium are
studied using QCD multipole expansion (QCDME) in the framework of a constituent
quark model which has been successful in describing hadronic phenomenology. The
hybrid intermediate states needed in the QCDME method are calculated in a
natural extension of our constituent quark model based on the Quark Confining
String (QCS) scheme. Some of the anomalies are explained due to the presence of
an hybrid state with a mass near the mass of the decaying resonance whereas
other are justified by the presence of molecular components in the wave
function. Some unexpected results are pointed out.Comment: Conference proceedings of the XI Quark Confinement and the Hadron
Spectrum (CONFINEMENT XI). Saint Petersburg (Russia) from 8 to 12 September
201
Constraints on Lorentz Invariance Violation using INTEGRAL/IBIS observations of GRB041219A
One of the experimental tests of Lorentz invariance violation is to measure
the helicity dependence of the propagation velocity of photons originating in
distant cosmological obejcts. Using a recent determination of the distance of
the Gamma-Ray Burst GRB 041219A, for which a high degree of polarization is
observed in the prompt emission, we are able to improve by 4 orders of
magnitude the existing constraint on Lorentz invariance violation, arising from
the phenomenon of vacuum birefringence.Comment: 5 pages, 3 figures, accepted for publication as a Rapid Communication
in Physical Review
Comprehensive study of the critical behavior in the diluted antiferromagnet in a field
We study the critical behavior of the Diluted Antiferromagnet in a Field with
the Tethered Monte Carlo formalism. We compute the critical exponents
(including the elusive hyperscaling violations exponent ). Our results
provide a comprehensive description of the phase transition and clarify the
inconsistencies between previous experimental and theoretical work. To do so,
our method addresses the usual problems of numerical work (large tunneling
barriers and self-averaging violations).Comment: 4 pages, 2 figure
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