124 research outputs found
Fractal Behaviour in the O(3) Model
We study domain formation in the two-dimensional O(3) model near criticality.
The fractal dimension of these domains is determined with good statistical
accuracy.Comment: 6 pages + 3 figures (concatenated PS files, uuencoded gz-compressed
Analysing Lyapunov spectra of chaotic dynamical systems
It is shown that the asymptotic spectra of finite-time Lyapunov exponents of
a variety of fully chaotic dynamical systems can be understood in terms of a
statistical analysis. Using random matrix theory we derive numerical and in
particular analytical results which provide insights into the overall behaviour
of the Lyapunov exponents particularly for strange attractors. The
corresponding distributions for the unstable periodic orbits are investigated
for comparison.Comment: 4 pages, 4 figure
An efficient algorithm simulating a macroscopic system at the critical point
It is well known that conventional simulation algorithms are inefficient for
the statistical description of macroscopic systems exactly at the critical
point due to the divergence of the corresponding relaxation time (critical
slowing down). On the other hand the dynamics in the order parameter space is
simplified significantly in this case due to the onset of self-similarity in
the associated fluctuation patterns. As a consequence the effective action at
the critical point obtains a very simple form. In the present work we show that
this simplified action can be used in order to simulate efficiently the
statistical properties of a macroscopic system exactly at the critical point.
Using the proposed algorithm we generate an ensemble of configurations
resembling the characteristic fractal geometry of the critical system related
to the self-similar order parameter fluctuations. As an example we simulate the
one-component real scalar field theory at the transition point as a
representative system belonging to the Ising universality class.Comment: 10 pages, 6 figures, submitted for publication to Journal of
Computational Physic
Braneworld models with a non-minimally coupled phantom bulk field: a simple way to obtain the -1-crossing at late times
We investigate general braneworld models, with a non-minimally coupled
phantom bulk field and arbitrary brane and bulk matter contents. We show that
the effective dark energy of the brane-universe acquires a dynamical nature, as
a result of the non-minimal coupling which provides a mechanism for an indirect
"bulk-brane interaction" through gravity. For late-time cosmological evolution
and without resorting to special ansatzes or to specific areas of the parameter
space, we show that the -1-crossing of its equation-of-state parameter is
general and can be easily achieved. As an example we provide a simple, but
sufficiently general, approximate analytical solution, that presents the
crossing behavior.Comment: 11 pages, 2 figure
Pion production from a critical QCD phase
A theoretical scheme which relates multiparticle states generated in
ultrarelativistic nuclear collisions to a QCD phase transition is considered in
the framework of the universality class provided by the 3-D Ising model. Two
different evolution scenarios for the QGP system are examined. The statistical
mechanics of the critical state is accounted for in terms of (critical) cluster
formation consistent with suitably cast effective action functionals, one for
each considered type of expansion. Fractal properties associated with these
clusters, characterizing the density fluctuations near the QCD critical point,
are determined. Monte-Carlo simulations are employed to generate events,
pertaining to the total system, which correspond to signals associated with
unconventional sources of pion production
The Critical Endpoint of Bootstrap and Lattice QCD Matter
The critical sector of strong interactions at high temperatures is explored
in the frame of two complementary approaches: Statistical Bootstrap for the
hadronic phase and Lattice QCD for the Quark-Gluon partition function. A region
of thermodynamic instability of hadronic matter was found, as a direct
prediction of Statistical Bootstrap. As a result, critical endpoint solutions
for nonzero chemical potential were traced in the phase diagram of strongly
interacting matter. These solutions are compared with recent lattice QCD
results and their proximity to the freeze-out points of experiments with nuclei
at high energies is also discussed.Comment: 23 pages, 8 figure
Zeroing in on more photons and gluons
We discuss radiation zeros that are found in gauge tree amplitudes for
processes involving multi-photon emission. Previous results are clarified by
examples and by further elaboration. The conditions under which such amplitude
zeros occur are identical in form to those for the single-photon zeros, and all
radiated photons must travel parallel to each other. Any other neutral particle
likewise must be massless (e.g. gluon) and travel in that common direction. The
relevance to questions like gluon jet identification and computational checks
is considered. We use examples to show how certain multi-photon amplitudes
evade the zeros, and to demonstrate the connection to a more general result,
the decoupling of an external electromagnetic plane wave in the ``null zone".
Brief comments are made about zeros associated with other gauge-boson emission.Comment: 26 page
Phase-space analysis of interacting phantom cosmology
We perform a detailed phase-space analysis of various phantom cosmological
models, where the dark energy sector interacts with the dark matter one. We
examine whether there exist late-time scaling attractors, corresponding to an
accelerating universe and possessing dark energy and dark matter densities of
the same order. We find that all the examined models, although accepting stable
late-time accelerated solutions, cannot alleviate the coincidence problem,
unless one imposes a form of fine-tuning in the model parameters. It seems that
interacting phantom cosmology cannot fulfill the basic requirement that led to
its construction.Comment: 6 figures, use revtex, v2: minor corrections, references added,
accepted for publication in JCA
Collinear helium under periodic driving: stabilization of the asymmetric stretch orbit
The collinear eZe configuration of helium, with the electrons on opposite
sides of the nucleus, is studied in the presence of an external electromagnetic
(laser or microwave) field. We show that the classically unstable "asymmetric
stretch" orbit, on which doubly excited intrashell states of helium with
maximum interelectronic angle are anchored, can be stabilized by means of a
resonant driving where the frequency of the electromagnetic field equals the
frequency of Kepler-like oscillations along the orbit. A static magnetic field,
oriented parallel to the oscillating electric field of the driving, can be used
to enforce the stability of the configuration with respect to deviations from
collinearity. Quantum Floquet calculations within a collinear model of the
driven two-electron atom reveal the existence of nondispersive wave packets
localized on the stabilized asymmetric stretch orbit, for double excitations
corresponding to principal quantum numbers of the order of N > 10.Comment: 13 pages, 12 figure
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