224 research outputs found
Plasmon properties in classical lattice gauge theory
In order to investigate the features of the classical approximation at high
temperatures for real time correlation functions, the plasmon frequencies and
damping rates were recently computed numerically in the SU(2)+Higgs model and
in the pure SU(2) theory. We compare the lattice results with leading order
hard thermal loop resummed perturbation theory. In the broken phase of the
SU(2)+Higgs model, we show that the lattice results can be reproduced and that
the lattices used are too coarse to observe some important plasmon effects. In
the symmetric phase, the main qualitative features of the lattice results can
also be understood. In the pure SU(2) theory, on the other hand, there are
discrepancies which might point to larger Landau and plasmon damping effects
than indicated by perturbation theory.Comment: 17 pages, 5 figures. A few points written more clearly (e.g., in the
abstract). A note added concerning two recent papers. Conclusions unchanged.
To appear in Phys.Lett.
Finite baryon density effects on gauge field dynamics
We discuss the effective action for QCD gauge fields at finite temperatures
and densities, obtained after integrating out the hardest momentum scales from
the system. We show that a non-vanishing baryon density induces a charge
conjugation (C) odd operator to the gauge field action, proportional to the
chemical potential. Even though it is parametrically smaller than the leading C
even operator, it could have an important effect on C odd observables. The same
operator appears to be produced by classical kinetic theory, allowing in
principle for a non-perturbative study of such processes.Comment: 20 page
Can electroweak bubble walls run away?
In extensions of the Standard Model with SU(2) singlet scalar fields, there
can be regions of parameter space for which the electroweak phase transition is
first order already at the mean-field level of analysis. We show that in this
case the phase interface (bubble wall) can become ultra-relativistic, with the
relativistic gamma factor gamma = (1-v_{wall}^2)^{-1/2} growing linearly with
the wall's propagation distance. We provide a simple criterion for determining
whether the bubble wall "runs away" in this way or if gamma approaches a
terminal value.Comment: references adde
Top transport in electroweak baryogenesis
In non-supersymmetric models of electroweak baryogenesis the top quark plays
a crucial role. Its CP-violating source term can be calculated in the WKB
approximation. We point out how to resolve certain discrepancies between
computations starting from the Dirac equation and the Schwinger--Keldysh
formalism. We also improve on the transport equations, keeping the
W-scatterings at finite rate. We apply these results to a model with one Higgs
doublet, augmented by dimension-6 operators, where our refinements lead to an
increase in the baryon asymmetry by a factor of up to about 5.Comment: 17 pages, 3 figures, references adde
Derivative expansion and gauge independence of the false vacuum decay rate in various gauges
In theories with radiative symmetry breaking, the calculation of the false
vacuum decay rate requires the inclusion of higher-order terms in the
derivative expansion of the effective action. I show here that, in the case of
covariant gauges, the presence of infrared singularities forbids the consistent
calculation by keeping the lowest-order terms. The situation is remedied,
however, in the case of gauges. Using the Nielsen identities I show
that the final result is gauge independent for generic values of the gauge
parameter that are not anomalously small.Comment: Some comments and references adde
Variability and trends in total and vertically resolved stratospheric ozone
International audienceTrends in ozone columns and vertical distributions were calculated for the period 1979?2004 based on the three-dimensional ozone data set CATO (Candidoz Assimilated Three-dimensional Ozone) using a multiple linear regression model. CATO has been reconstructed from TOMS, GOME and SBUV total column ozone observations in an equivalent latitude and potential temperature framework and offers a pole to pole coverage of the stratosphere on 15 potential temperature levels. The regression model includes explanatory variables describing the influence of the quasi-biennial oscillation, volcanic eruptions, the solar cycle, the Brewer-Dobson circulation, Arctic ozone depletion, and the increase in stratospheric chlorine. The effects of displacements of the polar vortex and jet streams due to planetary waves, which may significantly affect trends at a given geographical latitude, are eliminated in the equivalent latitude framework. Ozone variability is largely explained by the QBO and stratospheric aerosol loading and the spatial structure of their influence is in good agreement with previous studies. The solar cycle signal peaks at about 30 to 35 km altitude which is lower than reported previously, and no negative signal is found in the tropical lower stratosphere. The Brewer-Dobson circulation shows a dominant contribution to interannual variability at both high and low latitudes and accounts for some of the ozone increase seen in the northern hemisphere since the mid-1990s. Arctic ozone depletion significantly affects the high northern latitudes between January and March and extends its influence to the mid-latitudes during later months. The vertical distribution of the ozone trend shows distinct negative trends at about 18 km in the lower stratosphere with largest declines over the poles, and above 35 km in the upper stratosphere. A narrow band of large negative trends extends into the tropical lower stratosphere. Assuming that the observed negative trend before 1995 continued to 2004 cannot explain the ozone changes since 1996. A model accounting for recent changes in EESC, aerosols and Eliassen-Palm flux, on the other hand, closely tracks ozone changes since 1995
Landau-Pomeranchuk-Migdal effect in thermal field theory
In recent studies, the production rate of photons or lepton pairs by a quark
gluon plasma has been found to be enhanced due to collinear singularities. This
enhancement pattern is very dependent on rather strict collinearity conditions
between the photon and the quark momenta. It was estimated by neglecting the
collisional width of quasi-particles. In this paper, we study the modifications
of this collinear enhancement when we take into account the possibility for the
quarks to have a finite mean free path. Assuming a mean free path of order
, we find that only low invariant mass photons are
affected. The region where collision effects are important can be interpreted
as the region where the Landau-Pomeranchuk-Migdal effect plays a role in
thermal photon production by bremsstrahlung. It is found that this effect
modifies the spectrum of very energetic photons as well. Based on these results
and on a previous work on infrared singularities, we end this paper by a
reasonable physical picture for photon production by a quark gluon plasma, that
should be useful to set directions for future technical developments.Comment: 28 pages Latex document, 9 postscript figures, typos corrected,
semantics cleanup, final version to appear in Phys. Rev.
Improved Hamiltonian for Minkowski Yang-Mills Theory
I develop an improved Hamiltonian for classical, Minkowski Yang-Mills theory,
which evolves infrared fields with corrections from lattice spacing
beginning at . I use it to investigate the response of Chern-Simons
number to a chemical potential, and to compute the maximal Lyapunov exponent.
Both quantities have small limits, in both cases within of the
limit found using the unimproved (Kogut Susskind) Hamiltonian. For the maximal
Lyapunov exponent the limits differ by about , significant at about , indicating that while a small limit exists, its value is corrupted
by lattice artefacts. For the response of Chern-Simons number the statistics
are not good enough to resolve differences, but it seems possible in
analogy with the Lyapunov exponent that the final answer depends on the lattice
regulation.Comment: Latex, 33 pages plus 2 .epsi figures included with psfig. Revised to
include new data which weakens some original conclusion
Therapeutic and educational objectives in robot assisted play for children with autism
“This material is presented to ensure timely dissemination of scholarly and technical work. Copyright and all rights therein are retained by authors or by other copyright holders. All persons copying this information are expected to adhere to the terms and constraints invoked by each author's copyright. In most cases, these works may not be reposted without the explicit permission of the copyright holder." “Copyright IEEE. Personal use of this material is permitted. However, permission to reprint/republish this material for advertising or promotional purposes or for creating new collective works for resale or redistribution to servers or lists, or to reuse any copyrighted component of this work in other works must be obtained from the IEEE.” DOI: 10.1109/ROMAN.2009.5326251This article is a methodological paper that describes the therapeutic and educational objectives that were identified during the design process of a robot aimed at robot assisted play. The work described in this paper is part of the IROMEC project (Interactive Robotic Social Mediators as Companions) that recognizes the important role of play in child development and targets children who are prevented from or inhibited in playing. The project investigates the role of an interactive, autonomous robotic toy in therapy and education for children with special needs. This paper specifically addresses the therapeutic and educational objectives related to children with autism. In recent years, robots have already been used to teach basic social interaction skills to children with autism. The added value of the IROMEC robot is that play scenarios have been developed taking children's specific strengths and needs into consideration and covering a wide range of objectives in children's development areas (sensory, communicational and interaction, motor, cognitive and social and emotional). The paper describes children's developmental areas and illustrates how different experiences and interactions with the IROMEC robot are designed to target objectives in these areas.Final Published versio
High temperature color conductivity at next-to-leading log order
The non-Abelian analog of electrical conductivity at high temperature has
previously been known only at leading logarithmic order: that is, neglecting
effects suppressed only by an inverse logarithm of the gauge coupling. We
calculate the first sub-leading correction. This has immediate application to
improving, to next-to-leading log order, both effective theories of
non-perturbative color dynamics, and calculations of the hot electroweak baryon
number violation rate.Comment: 47 pages, 6+2 figure
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