412 research outputs found
Casimir Scaling from Center Vortices: Towards an Understanding of the Adjoint String Tension
We argue that the approximate ``Casimir scaling'' of the string tensions of
higher-representation Wilson loops is an effect due to the finite thickness of
center vortex configurations. It is shown, in the context of a simple model of
the Z(2) vortex core, how vortex condensation in Yang-Mills theory can account
for both Casimir scaling in intermediate size loops, and color-screening in
larger loops. An implication of our model is that the deviations from exact
Casimir scaling, which tend to grow with loop size, become much more pronounced
as the dimensionality of the group representation increases.Comment: 13 pages, including 3 eps figures, Latex2e. Two references adde
Some Cautionary Remarks on Abelian Projection and Abelian Dominance
Some critical remarks are presented, concerning the abelian projection theory
of quark confinement.Comment: Talk presented at LATTICE96(topology) plenary session, uses psfig and
espcrc2 package
Diffusion as mixing mechanism in granular materials
We present several numerical results on granular mixtures. In particular, we
examine the efficiency of diffusion as a mixing mechanism in these systems. The
collisions are inelastic and to compensate the energy loss, we thermalize the
grains by adding a random force. Starting with a segregated system, we show
that uniform agitation (heating) leads to a uniform mixture of grains of
different sizes. We define a characteristic mixing time, , and
study theoretically and numerically its dependence on other parameters like the
density. We examine a model for bidisperse systems for which we can calculate
some physical quantities. We also examine the effect of a temperature gradient
and demonstrate the appearance of an expected segregation.Comment: 15 eps figures, include
New procedures for testing whether stock price processes are martingales
We propose procedures for testing whether stock price processes are
martingales based on limit order type betting strategies. We first show that
the null hypothesis of martingale property of a stock price process can be
tested based on the capital process of a betting strategy. In particular with
high frequency Markov type strategies we find that martingale null hypotheses
are rejected for many stock price processes
Parallax of PSR J1744-1134 and the Local Interstellar Medium
We present the annual trigonometric parallax of PSR J1744-1134 derived from
an analysis of pulse times of arrival. The measured parallax, pi = 2.8+/-0.3
mas ranks among the most precisely determined distances to any pulsar. The
parallax distance of 357+/-39 pc is over twice that derived from the dispersion
measure using the Taylor & Cordes model for the Galactic electron distribution.
The mean electron density in the path to the pulsar, n_e = (0.0088 +/- 0.0009)
cm^{-3}, is the lowest for any disk pulsar. We have compared the n_e for PSR
J1744-1134 with those for another 11 nearby pulsars with independent distance
estimates. We conclude that there is a striking asymmetry in the distribution
of electrons in the local interstellar medium. The electron column densities
for pulsars in the third Galactic quadrant are found to be systematically
higher than for those in the first. The former correlate with the position of
the well known local HI cavity in quadrant three. The excess electrons within
the cavity may be in the form of HII clouds marking a region of interaction
between the local hot bubble and a nearby superbubble.Comment: revised version accepted for publication in ApJ Letters; reanalysis
of uncertainty in parallax measure and changes to fig
Three-dimensional plant architecture and sunlit-shaded patterns: a stochastic model of light dynamics in canopies
Background and Aims Diurnal changes in solar position and intensity combined with the structural complexity of plant architecture result in highly variable and dynamic light patterns within the plant canopy. This affects productivity through the complex ways that photosynthesis responds to changes in light intensity. Current methods to characterise light dynamics, such as ray-tracing, are able to produce data with excellent spatio-temporal resolution but are computationally intensive and the resultant data are complex and high dimensional. This necessitates development of more economical models for summarising the data and for simulating realistic light patterns over the course of a day.
Methods High-resolution reconstructions of field-grown plants are assembled in various configurations to form canopies, and a forward ray-tracing algorithm is applied to the canopies to compute light dynamics at high (1 minute) temporal resolution. From the ray-tracer output, the sunlit or shaded state for each patch on the plants is determined, and these data are used to develop a novel stochastic model for the sunlit-shaded patterns. The model is designed to be straightforward to fit to data using maximum likelihood estimation, and fast to simulate from.
Key Results For a wide range of contrasting 3D canopies, the stochastic model is able to summarise, and replicate in simulations, key features of the light dynamics. When light patterns simulated from the stochastic model are used as input to a model of photoinhibition, the predicted reduction in carbon gain is similar to that from calculations based on the (extremely costly) ray-tracer data.
Conclusions The model provides a way to summarise highly complex data in a small number of parameters, and a cost-effective way to simulate realistic light patterns. Simulations from the model will be particularly useful for feeding into larger-scale photosynthesis models for calculating how light dynamics affects the photosynthetic productivity of canopies
Planeamiento territorial sostenible: un reto para el futuro de nuestras sociedades; criterios aplicados
In a large part of the 17 sustainable development objectives set as goals for humanity by the UN, sustainability can be glimpsed. As a result of the dominant socio-productive model, the only way to head towards more sustainable territories that allow achieving and maintaining the well-being of the world's population is to bear in mind the need to properly plan territorial development. This work reflects on this need and takes a step forward in the definition of the main criteria to achieve territorial sustainability at regional and local scales
The intellectual influence of economic journals: quality versus quantity
The evaluation of scientific output has a key role in the allocation of
research funds and academic positions. Decisions are often based on quality indicators
for academic journals, and over the years, a handful of scoring methods have
been proposed for this purpose. Discussing the most prominent methods (de facto
standards) we show that they do not distinguish quality from quantity at article level.
The systematic bias we find is analytically tractable and implies that the methods are
manipulable. We introduce modified methods that correct for this bias, and use them
to provide rankings of economic journals. Our methodology is transparent; our results
are replicable
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