13,837 research outputs found
First study of the three-gluon static potential in Lattice QCD
We estimate the potential energy for a system of three static gluons in
Lattice QCD. This is relevant for the different models of three-body glueballs
have been proposed in the literature, either for gluons with a constituent
mass, or for massless ones. A Wilson loop adequate to the static hybrid
three-body system is developed. We study different spacial geometries, to
compare the starfish model with the triangle model, for the three-gluon
potential. We also study two different colour structures, symmetric and
antisymmetric, and compare the respective static potentials. A first simulation
is performed in a periodic Lattice, with and fm.Comment: 8 pages, 10 figure
Schwinger-Dyson equations and the quark-antiquark static potential
In lattice QCD, a confining potential for a static quark-antiquark pair can
be computed with the Wilson loop technique. This potential, dominated by a
linear potential at moderate distances, is consistent with the confinement with
a flux tube, an extended and scalar system also directly observable in lattice
QCD. Quantized flux tubes have also been observed in another class of
confinement, the magnetic confinement in type II superconductors. On the other
hand the solution of Schwinger Dyson Equations, say with the Landau gauge
fixing and the truncation of the series of Feynman diagrams, already at the
rainbow level for the self energy and at the ladder level for the Bethe
Salpeter equation, provides a signal of a possible inverse quartic potential in
momentum space derived from one gluon and one ghost exchange, consistent with
confinement. Here we address the successes, difficulties and open problems of
the matching of these two different perspectives of confinement, the
Schwinger-Dyson perspective versus the flux tube perspective.Comment: 12 pages, 18 figures; talk presented at QCD-TNT, Trento, 7-11 sep
200
On Quantum Special Kaehler Geometry
We compute the effective black hole potential V of the most general N=2, d=4
(local) special Kaehler geometry with quantum perturbative corrections,
consistent with axion-shift Peccei-Quinn symmetry and with cubic leading order
behavior. We determine the charge configurations supporting axion-free
attractors, and explain the differences among various configurations in
relations to the presence of ``flat'' directions of V at its critical points.
Furthermore, we elucidate the role of the sectional curvature at the
non-supersymmetric critical points of V, and compute the Riemann tensor (and
related quantities), as well as the so-called E-tensor. The latter expresses
the non-symmetricity of the considered quantum perturbative special Kaehler
geometry.Comment: 1+43 pages; v2: typo corrected in the curvature of Jordan symmetric
sequence at page 2
How Does a Fundamental String Stretch its Horizon?
It has recently been shown that if we take into account a class of higher
derivative corrections to the effective action of heterotic string theory, the
entropy of the black hole solution representing elementary string states
correctly reproduces the statistical entropy computed from the degeneracy of
elementary string states. So far the form of the solution has been analyzed at
distance scales large and small compared to the string scale. We analyze the
solution that interpolates between these two limits and point out a subtlety in
constructing such a solution due to the presence of higher derivative terms in
the effective action. We also study the T-duality transformation rules to
relate the moduli fields of the effective field theory to the physical
compactification radius in the presence of higher derivative corrections and
use these results to find the physical radius of compactification near the
horizon of the black hole. The radius approaches a finite value even though the
corresponding modulus field vanishes. Finally we discuss the non-leading
contribution to the black hole entropy due to space-time quantum corrections to
the effective action and the ambiguity involved in comparing this result to the
statistical entropy.Comment: LaTeX file, 38 pages; v2: minor changes and added reference
First study of the gluon-quark-antiquark static potential in SU(3) Lattice QCD
We study the long distance interaction for hybrid hadrons, with a static
gluon, a quark and an antiquark with lattice QCD techniques. A Wilson loop
adequate to the static hybrid three-body system is developed and, using a 24^3
x 48 periodic lattice with beta=6.2 and a ~ 0.075 fm, two different geometries
for the gluon-quark segment and the gluon-antiquark segment are investigated.
When these segments are perpendicular, the static potential is compatible with
confinement realized with a pair of fundamental strings, one linking the gluon
to the quark and another linking the same gluon to the antiquark. When the
segments are parallel and superposed, the total string tension is larger and
agrees with the Casimir Scaling measured by Bali. This can be interpreted with
a type-II superconductor analogy for the confinement in QCD, with repulsion of
the fundamental strings and with the string tension of the first topological
excitation of the string (the adjoint string) larger than the double of the
fundamental string tension.Comment: 4 pages RevTeX, 4 figure
Estabilidade de uma estrutura de agrupamento : segmentos de clientes de uma instituição cultural
Neste trabalho implementa-se, como meio de avaliação de estabilidade de um agrupamento, uma nova proposta de validação cruzada de agrupamentos que prescinde do uso de classificadores, recorrendo à utilização de amostras ponderadas de treino e teste (Cardoso, Faceli et al. 2009).
Ilustra-se a metodologia proposta sobre um agrupamento de clientes do CCB - Centro Cultural de Belém. Este agrupamento é efetuado mediante estimação de um modelo de mistura finita. Na constituição dos grupos ou segmentos atende-se à natureza ordinal das variáveis base (medições em escala de tipo Likert), em alternativa à modelação habitual que consideraria as mesmas variáveis como métricas. Em complemento, são apontadas metodologias consideradas mais
apropriadas para a interpretação e discriminação dos grupos obtidos.This work implements, as a means of assessing the stability of a cluster, a new proposal for crossvalidation of clusters that dispenses with the use of classifiers, resorting to the use of weighted samples of training and testing (Cardoso, Facel et al. 2009) We illustrate the proposed approach over a cluster of clients of CCB – Cultura Centre of Belem (Centro Cultural de Belém). The clustering is obtained by means of an estimation of a mixture finite model. In the constitution of the clusters or segments, it it taken in consideration the ordinal nature of the clustering base variables (measurements in Likert scale) in lieu of the usual modeling that would consider the same variables as metric. In addition, we point out to some methodologies that are considered more adequate to interpret and discriminate the segments obtained
Assessing the role of optical head-mounted displays in education: an investigation of Google Glass in creating learning portfolios and providing feedback
Technology Enhanced Learning is a field that has seen impressive developments over the past few years. Educators have experimented with the use of web technologies, introduced innovative e-learning approaches, extended the role of virtual learning environments and introduced learning analytics. The authors’ research aims to investigate how ubiquitous computing and augmented reality can further support students in a range of learning activities. In particular, this paper discusses a research study in the role of Optical Head-Mounted Displays (OHMD) in education. Emphasis is given on how the technology can enhance learning through the provision of additional support via augmented reality.
The paper describes how OHMD, and more specifically Google Glass has been used by students in a Higher Education Institution as part of their assessment. The research aim is twofold as it considers (i) the role of OHMD in supporting students during the creation of learning portfolios which can be used for formative and summative assessment, and (ii) the impact OHMD technology has in providing alternative ways of feedback. The scope of the research is to assess the suitability of the technology, the benefits that can be introduced in educational contexts as well as the perceived value of the technology from the learners’ point of view.
The first part of the study described in the paper describes how learners have used OHMD to construct a portfolio of learning evidence through cooperative evaluation of their work. In particular the study involved students using Google Glass to take snaps of their work, while recording a video diary of their contribution towards group coursework. Users reflected on the experience in terms of ease of use, simplicity and usefulness. They also evaluated the effectiveness of using OHMD during specific tasks including reading, writing and browsing.
The second part of the study is focused on providing feedback by using OHMD to attach vignettes on pictures of presentations. The technology is used for commenting on presentation content and delivery, while it is investigated as an alternative for providing feedback on practical activities. The paper also provides a detailed discussion of preliminary findings from the pilot with 92 participants studying at first and final years of a University degree
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