2,005 research outputs found
Elliptic flow due to radiation in heavy-ion collisions
In this paper we demonstrate that radiation patterns could cause flow-like
behaviour without any reference to hydrodynamic description. For that purpose
we use a statistical ensemble of radiating dipoles, motivated by the
investigation of the equivalent photon yield produced by decelerating charges.
For the elliptic asymmetry factor we find a reasonable agreement with
experimental data.Comment: 5 pages, 5 figures, v2: discussion on the physical interpretation of
the form factor F is adde
Low-Dimensional Long-Range Topological Charge Structure in the QCD Vacuum
While sign-coherent 4-dimensional structures cannot dominate topological
charge fluctuations in the QCD vacuum at all scales due to reflection
positivity, it is possible that enhanced coherence exists over extended
space-time regions of lower dimension. Using the overlap Dirac operator to
calculate topological charge density, we present evidence for such structure in
pure-glue SU(3) lattice gauge theory. It is found that a typical equilibrium
configuration is dominated by two oppositely-charged sign-coherent connected
structures (``sheets'') covering about 80% of space-time. Each sheet is built
from elementary 3-d cubes connected through 2-d faces, and approximates a
low-dimensional curved manifold (or possibly a fractal structure) embedded in
the 4-d space. At the heart of the sheet is a ``skeleton'' formed by about 18%
of the most intense space-time points organized into a global long-range
structure, involving connected parts spreading over maximal possible distances.
We find that the skeleton is locally 1-dimensional and propose that its
geometrical properties might be relevant for understanding the possible role of
topological charge fluctuations in the physics of chiral symmetry breaking.Comment: 4 pages RevTeX, 4 figures; v2: 6 pages, 5 figures, more explanations
provided, figure and references added, published versio
Instanton dominance of topological charge fluctuations in QCD?
We consider the local chirality of near-zero eigenvectors from Wilson-Dirac
and clover improved Wilson-Dirac lattice operators as proposed recently by
Horv\'ath et al. We studied finer lattices and repaired for the loss of
orthogonality due to the non-normality of the Wilson-Dirac matrix. As a result
we do see a clear double peak structure on lattices with resolutions higher
than 0.1 fm. We found that the lattice artifacts can be considerably reduced by
exploiting the biorthogonal system of left and right eigenvectors. We conclude
that the dominance of instantons on topological charge fluctuations is not
ruled out by local chirality measurements.Comment: 10 pages, 6 figure
Topological Structure of the QCD Vacuum Revealed by Overlap Fermions
Overlap fermions preserve a remnant of chiral symmetry on the lattice. They
are a powerful tool to investigate the topological structure of the vacuum of
Yang-Mills theory and full QCD. Recent results concerning the localization of
topological charge and the localization and local chirality of the overlap
eigenmodes are reported. The charge distribution is radically different, if a
spectral cut-off for the Dirac eigenmodes is applied. The density q(x) is
changing from the scale-a charge density (with full lattice resolution) to the
ultraviolet filtered charge density. The scale-a density, computed on the Linux
cluster of LRZ, has a singular, sign-coherent global structure of co-dimension
1 first described by the Kentucky group. We stress, however, the cluster
properties of the UV filtered topological density resembling the instanton
picture. The spectral cut-off can be mapped to a bosonic smearing procedure.
The UV filtered field strength reveals a high degree of (anti)selfduality at
"hot spots" of the action. The fermionic eigenmodes show a high degree of local
chirality. The lowest modes are seen to be localized in low-dimensional
space-time regions.Comment: 13 pages, 11 figures, accepted to appear in the Proceedings of "HLRB,
KONWIHR and Linux-Cluster: Review, Results and Future Projects Workshop",
Leibniz Rechenzentrum Munich, December 200
Effect of a columnar defect on the shape of slow-combustion fronts
We report experimental results for the behavior of slow-combustion fronts in
the presence of a columnar defect with excess or reduced driving, and compare
them with those of mean-field theory. We also compare them with simulation
results for an analogous problem of driven flow of particles with hard-core
repulsion (ASEP) and a single defect bond with a different hopping probability.
The difference in the shape of the front profiles for excess vs. reduced
driving in the defect, clearly demonstrates the existence of a KPZ-type of
nonlinear term in the effective evolution equation for the slow-combustion
fronts. We also find that slow-combustion fronts display a faceted form for
large enough excess driving, and that there is a corresponding increase then in
the average front speed. This increase in the average front speed disappears at
a non-zero excess driving in agreement with the simulated behavior of the ASEP
model.Comment: 7 pages, 7 figure
Calorons, instantons and constituent monopoles in SU(3) lattice gauge theory
We analyze the zero-modes of the Dirac operator in quenched SU(3) gauge
configurations at non-zero temperature and compare periodic and anti-periodic
temporal boundary conditions for the fermions. It is demonstrated that for the
different boundary conditions often the modes are localized at different
space-time points and have different sizes. Our observations are consistent
with patterns expected for Kraan - van Baal solutions of the classical
Yang-Mills equations. These solutions consist of constituent monopoles and the
zero-modes are localized on different constituents for different boundary
conditions. Our findings indicate that the excitations of the QCD vacuum are
more structured than simple instanton-like lumps.Comment: Remarks added. To appear in Phys. Rev.
Extension of PRISM by Synthesis of Optimal Timeouts in Fixed-Delay CTMC
We present a practically appealing extension of the probabilistic model
checker PRISM rendering it to handle fixed-delay continuous-time Markov chains
(fdCTMCs) with rewards, the equivalent formalism to the deterministic and
stochastic Petri nets (DSPNs). fdCTMCs allow transitions with fixed-delays (or
timeouts) on top of the traditional transitions with exponential rates. Our
extension supports an evaluation of expected reward until reaching a given set
of target states. The main contribution is that, considering the fixed-delays
as parameters, we implemented a synthesis algorithm that computes the
epsilon-optimal values of the fixed-delays minimizing the expected reward. We
provide a performance evaluation of the synthesis on practical examples
Local Chirality of Low-Lying Dirac Eigenmodes and the Instanton Liquid Model
The reasons for using low-lying Dirac eigenmodes to probe the local structure
of topological charge fluctuations in QCD are discussed, and it is pointed out
that the qualitative double-peaked behavior of the local chiral orientation
probability distribution in these modes is necessary, but not sufficient for
dominance of instanton-like fluctuations. The results with overlap Dirac
operator in Wilson gauge backgrounds at lattice spacings ranging from a~0.04 fm
to a~0.12 fm are reported, and it is found that the size and density of local
structures responsible for double-peaking of the distribution are in
disagreement with the assumptions of the Instanton Liquid Model. More
generally, our results suggest that vacuum fluctuations of topological charge
are not effectively dominated by locally quantized (integer-valued) lumps in
QCD.Comment: 29 pages, 13 figures; v2: minor improvements in presentation, results
and conclusions unchanged, version to appear in PR
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