2,256 research outputs found
The Hartree ensemble approximation revisited: The "symmetric phase"
The Hartree ensemble approximation is studied in the ``symmetric phase'' of
1+1 dimensional lambda phi^4 theory. In comparison with the ``broken phase''
studied previously, it is shown that the dynamical evolution of observables
such as the particle distribution, energy exchange and auto-correlation
functions, is substantially slower. Approximate thermalization is found only
for relatively large energy densities and couplings.Comment: 17 pages RevTeX, 16 figures, 3 tables, uses amsmath and feynmp.
Extended some sections, reordered Sec.IV, added 3 refs, numerical typo
corrected, published versio
Exact and Truncated Dynamics in Nonequilibrium Field Theory
Nonperturbative dynamics of quantum fields out of equilibrium is often
described by the time evolution of a hierarchy of correlation functions, using
approximation methods such as Hartree, large N, and nPI-effective action
techniques. These truncation schemes can be implemented equally well in a
classical statistical system, where results can be tested by comparison with
the complete nonlinear evolution obtained by numerical methods. For a 1+1
dimensional scalar field we find that the early-time behaviour is reproduced
qualitatively by the Hartree dynamics. The inclusion of direct scattering
improves this to the quantitative level. We show that the emergence of
nonthermal temperature profiles at intermediate times can be understood in
terms of the fixed points of the evolution equations in the Hartree
approximation. The form of the profile depends explicitly on the initial
ensemble. While the truncated evolution equations do not seem to be able to get
away from the fixed point, the full nonlinear evolution shows thermalization
with a (surprisingly) slow relaxation.Comment: 30 pages with 12 eps figures, minor changes; to appear in Phys.Rev.
Looking for defects in the 2PI correlator
Truncations of the 2PI effective action are seen as a promising way of
studying non-equilibrium dynamics in quantum field theories. We probe their
applicability in the non-perturbative setting of topological defect formation
in a symmetry-breaking phase transition, by comparing full classical lattice
field simulations and the 2PI formulation for classical fields in an O()
symmetric scalar field theory. At next-to-leading order in 1/N, the 2PI
formalism fails to reproduce any signals of defects in the two-point function.
This suggests that one should be careful when applying the 2PI formalism for
symmetry breaking phase transitions.Comment: 22 pages, 6 figure
Dynamics of broken symmetry lambda phi^4 field theory
We study the domain of validity of a Schwinger-Dyson (SD) approach to
non-equilibrium dynamics when there is broken symmetry. We perform exact
numerical simulations of the one- and two-point functions of lambda phi^4 field
theory in 1+1 dimensions in the classical domain for initial conditions where <
phi(x) > not equal to 0. We compare these results to two self-consistent
truncations of the SD equations which ignore three-point vertex function
corrections. The first approximation, which sets the three-point function to
one (the bare vertex approximation (BVA)) gives an excellent description for <
phi(x) > = phi(t). The second approximation which ignores higher in 1/N
corrections to the 2-PI generating functional (2PI -1/N expansion) is not as
accurate for phi(t). Both approximations have serious deficiencies in
describing the two-point function when phi(0) > .4.Comment: 10 pages, 6 figure
Three flow regimes of viscous jet falling onto a moving surface
A stationary viscous jet falling from an oriented nozzle onto a moving
surface is studied, both theoretically and experimentally. We distinguish three
flow regimes and classify them by the convexity of the jet shape (concave,
vertical and convex). The fluid is modeled as a Newtonian fluid, and the model
for the flow includes viscous effects, inertia and gravity. By studying the
characteristics of the conservation of momentum for a dynamic jet, the boundary
conditions for each flow regime are derived, and the flow regimes are
characterized in terms of the process and material parameters. The model is
solved by a transformation into an algebraic equation. We make a comparison
between the model and experiments, and obtain qualitative agreement
Пространственные метафоры “воды” и “воздуха” в поэзии Ольги Седаковой
В данной работе исследуются способы реализации образов воды и воздуха, их
семантическое наполнение в структуре поэтического текста.
Ключевые слова: образ, трансформация, текст.У даній статті досліджуються способи втілення образів води та повітря у структурі
поетичного тексту та їх семантична наповненість.
Ключові слова: образ, трансформація, текст.The article dials with the ways of water and air images conveying in the structure of poetical and
their semantic value.
Keywords: image, transformation, text
Falling of a viscous jet onto a moving surface
We analyze the stationary flow of a jet of Newtonian fluid that is drawn by
gravity onto a moving surface. The situation is modeled by a third-order ODE on
a domain of unknown length and with an additional integral condition; by
solving part of the equation explicitly we can reformulate the problem as a
first-order ODE, again with an integral constraint. We show that there are two
flow regimes, and characterize the associated regions in the three-dimensional
parameter space in terms of an easily calculable quantity. In a qualitative
sense the results from the model are found to correspond with experimental
observations.Comment: 16 pages, 11 figure
Transport coefficients from the 2PI effective action
We show that the lowest nontrivial truncation of the two-particle irreducible
(2PI) effective action correctly determines transport coefficients in a weak
coupling or 1/N expansion at leading (logarithmic) order in several
relativistic field theories. In particular, we consider a single real scalar
field with cubic and quartic interactions in the loop expansion, the O(N) model
in the 2PI-1/N expansion, and QED with a single and many fermion fields.
Therefore, these truncations will provide a correct description, to leading
(logarithmic) order, of the long time behavior of these systems, i.e. the
approach to equilibrium. This supports the promising results obtained for the
dynamics of quantum fields out of equilibrium using 2PI effective action
techniques.Comment: 5 pages, explanation in introduction expanded, summary added; to
appear in PR
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