145 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
Kinetic pathways of the Nematic-Isotropic phase transition as studied by confocal microscopy on rod-like viruses
We investigate the kinetics of phase separation for a mixture of rodlike
viruses (fd) and polymer (dextran), which effectively constitutes a system of
attractive rods. This dispersion is quenched from a flow-induced fully nematic
state into the region where the nematic and the isotropic phase coexist. We
show experimental evidence that the kinetic pathway depends on the overall
concentration. When the quench is made at high concentrations, the system is
meta-stable and we observe typical nucleation-and-growth. For quenches at low
concentration the system is unstable and the system undergoes a spinodal
decomposition. At intermediate concentrations we see the transition between
both demixing processes, where we locate the spinodal point.Comment: 11 pages, 6 figures, accepted in J. Phys.: Condens. Matter as
symposium paper for the 6th Liquid Matter Conference in Utrech
Particle Production and Effective Thermalization in Inhomogeneous Mean Field Theory
As a toy model for dynamics in nonequilibrium quantum field theory we
consider the abelian Higgs model in 1+1 dimensions with fermions. In the
approximate dynamical equations, inhomogeneous classical (mean) Bose fields are
coupled to quantized fermion fields, which are treated with a mode function
expansion. The effective equations of motion imply e.g. Coulomb scattering, due
to the inhomogeneous gauge field. The equations are solved numerically. We
define time dependent fermion particle numbers with the help of the single-time
Wigner function and study particle production starting from inhomogeneous
initial conditions. The particle numbers are compared with the Fermi-Dirac
distribution parametrized by a time dependent temperature and chemical
potential. We find that the fermions approximately thermalize locally in time.Comment: 16 pages + 6 eps figures, some clarifications and two references
added, typos corrected; to appear in Phys.Rev.
Divergences in Real-Time Classical Field Theories at Non-Zero Temperature
The classical approximation provides a non-perturbative approach to
time-dependent problems in finite temperature field theory. We study the
divergences in hot classical field theory perturbatively. At one-loop, we show
that the linear divergences are completely determined by the classical
equivalent of the hard thermal loops in hot quantum field theories, and that
logarithmic divergences are absent. To deal with higher-loop diagrams, we
present a general argument that the superficial degree of divergence of
classical vertex functions decreases by one with each additional loop: one-loop
contributions are superficially linearly divergent, two-loop contributions are
superficially logarithmically divergent, and three- and higher-loop
contributions are superficially finite. We verify this for two-loop SU(N)
self-energy diagrams in Feynman and Coulomb gauges. We argue that hot,
classical scalar field theory may be completely renormalized by local (mass)
counterterms, and discuss renormalization of SU(N) gauge theories.Comment: 31 pages with 7 eps figure
Far-from-equilibrium quantum many-body dynamics
The theory of real-time quantum many-body dynamics as put forward in Ref.
[arXiv:0710.4627] is evaluated in detail. The formulation is based on a
generating functional of correlation functions where the Keldysh contour is
closed at a given time. Extending the Keldysh contour from this time to a later
time leads to a dynamic flow of the generating functional. This flow describes
the dynamics of the system and has an explicit causal structure. In the present
work it is evaluated within a vertex expansion of the effective action leading
to time evolution equations for Green functions. These equations are applicable
for strongly interacting systems as well as for studying the late-time
behaviour of nonequilibrium time evolution. For the specific case of a bosonic
N-component phi^4 theory with contact interactions an s-channel truncation is
identified to yield equations identical to those derived from the 2PI effective
action in next-to-leading order of a 1/N expansion. The presented approach
allows to directly obtain non-perturbative dynamic equations beyond the widely
used 2PI approximations.Comment: 20 pp., 6 figs; submitted version with added references and typos
corrected
Tachyonic preheating using 2PI-1/N dynamics and the classical approximation
We study the process of tachyonic preheating using approximative quantum
equations of motion derived from the 2PI effective action. The O(N) scalar
(Higgs) field is assumed to experience a fast quench which is represented by an
instantaneous flip of the sign of the mass parameter. The equations of motion
are solved numerically on the lattice, and the Hartree and 1/N-NLO
approximations are compared to the classical approximation. Classical dynamics
is expected to be valid, since the occupation numbers can rise to large values
during tachyonic preheating. We find that the classical approximation performs
excellently at short and intermediate times, even for couplings in the larger
region currently allowed for the SM Higgs. This is reassuring, since all
previous numerical studies of tachyonic preheating and baryogenesis during
tachyonic preheating have used classical dynamics. We also compare different
initializations for the classical simulations.Comment: 32 pages, 21 figures. Published version: Some details added, section
added, references added, conclusions unchange
Behavioral Priming: It's All in the Mind, but Whose Mind?
The perspective that behavior is often driven by unconscious determinants has become widespread in social psychology. Bargh, Chen, and Burrows' (1996) famous study, in which participants unwittingly exposed to the stereotype of age walked slower when exiting the laboratory, was instrumental in defining this perspective. Here, we present two experiments aimed at replicating the original study. Despite the use of automated timing methods and a larger sample, our first experiment failed to show priming. Our second experiment was aimed at manipulating the beliefs of the experimenters: Half were led to think that participants would walk slower when primed congruently, and the other half was led to expect the opposite. Strikingly, we obtained a walking speed effect, but only when experimenters believed participants would indeed walk slower. This suggests that both priming and experimenters' expectations are instrumental in explaining the walking speed effect. Further, debriefing was suggestive of awareness of the primes. We conclude that unconscious behavioral priming is real, while real, involves mechanisms different from those typically assumed to cause the effect
Driving pressure during general anesthesia for open abdominal surgery (DESIGNATION) : study protocol of a randomized clinical trial
Background
Intraoperative driving pressure (Delta P) is associated with development of postoperative pulmonary complications (PPC). When tidal volume (V-T) is kept constant, Delta P may change according to positive end-expiratory pressure (PEEP)-induced changes in lung aeration. Delta P may decrease if PEEP leads to a recruitment of collapsed lung tissue but will increase if PEEP mainly causes pulmonary overdistension. This study tests the hypothesis that individualized high PEEP, when compared to fixed low PEEP, protects against PPC in patients undergoing open abdominal surgery.
Methods
The "Driving prESsure durIng GeNeral AnesThesIa for Open abdomiNal surgery trial" (DESIGNATION) is an international, multicenter, two-group, double-blind randomized clinical superiority trial. A total of 1468 patients will be randomly assigned to one of the two intraoperative ventilation strategies. Investigators screen patients aged >= 18 years and with a body mass index <= 40 kg/m(2), scheduled for open abdominal surgery and at risk for PPC. Patients either receive an intraoperative ventilation strategy with individualized high PEEP with recruitment maneuvers (RM) ("individualized high PEEP") or one in which PEEP of 5 cm H2O without RM is used ("low PEEP"). In the "individualized high PEEP" group, PEEP is set at the level at which Delta P is lowest. In both groups of the trial, V-T is kept at 8 mL/kg predicted body weight. The primary endpoint is the occurrence of PPC, recorded as a collapsed composite of adverse pulmonary events. Discussion DESIGNATION will be the first randomized clinical trial that is adequately powered to compare the effects of individualized high PEEP with RM versus fixed low PEEP without RM on the occurrence of PPC after open abdominal surgery. The results of DESIGNATION will support anesthesiologists in their decisions regarding PEEP settings during open abdominal surgery
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