21,515 research outputs found
Modeling temporal fluctuations in avalanching systems
We demonstrate how to model the toppling activity in avalanching systems by
stochastic differential equations (SDEs). The theory is developed as a
generalization of the classical mean field approach to sandpile dynamics by
formulating it as a generalization of Itoh's SDE. This equation contains a
fractional Gaussian noise term representing the branching of an avalanche into
small active clusters, and a drift term reflecting the tendency for small
avalanches to grow and large avalanches to be constricted by the finite system
size. If one defines avalanching to take place when the toppling activity
exceeds a certain threshold the stochastic model allows us to compute the
avalanche exponents in the continum limit as functions of the Hurst exponent of
the noise. The results are found to agree well with numerical simulations in
the Bak-Tang-Wiesenfeld and Zhang sandpile models. The stochastic model also
provides a method for computing the probability density functions of the
fluctuations in the toppling activity itself. We show that the sandpiles do not
belong to the class of phenomena giving rise to universal non-Gaussian
probability density functions for the global activity. Moreover, we demonstrate
essential differences between the fluctuations of total kinetic energy in a
two-dimensional turbulence simulation and the toppling activity in sandpiles.Comment: 14 pages, 11 figure
Avalanches, Scaling and Coherent Noise
We present a simple model of a dynamical system driven by externally-imposed
coherent noise. Although the system never becomes critical in the sense of
possessing spatial correlations of arbitrarily long range, it does organize
into a stationary state characterized by avalanches with a power-law size
distribution. We explain the behavior of the model within a time-averaged
approximation, and discuss its potential connection to the dynamics of
earthquakes, the Gutenberg-Richter law, and to recent experiments on avalanches
in rice piles.Comment: 17 pages, 4 Postscript figures, written in LaTeX using RevTeX and
epsfig.st
Critical fluctuations and anomalous transport in soft Yukawa-Langevin systems
Simulation of a Langevin-dynamics model demonstrates emergence of critical
fluctuations and anomalous grain transport which have been observed in
experiments on "soft" quasi-two-dimensional dusty plasma clusters. It has been
suggested that these anomalies derive from particular non-equilibrium physics,
but our model does not contain such physics: the grains are confined by an
external potential, interact via static Yukawa forces, and are subject to
stochastic heating and dissipation from neutrals. One remarkable feature is
emergence of leptokurtic probability distributions of grain displacements
on time-scales , where is the
time at which the standard deviation
approaches the mean inter-grain distance . Others are development of
humps in the distributions on multiples of , anomalous Hurst exponents,
and transitions from leptokurtic towards Gaussian displacement distributions on
time scales . The latter is a signature of intermittency,
here interpreted as a transition from bursty transport associated with hopping
on intermediate time scales to vortical flows on longer time scales.Comment: 12 pages, 9 figure
2D orbital-like magnetic order in
In high temperature copper oxides superconductors, a novel magnetic order
associated with the pseudogap phase has been identified in two different
cuprate families over a wide region of temperature and doping. We here report
the observation below 120 K of a similar magnetic ordering in the archetypal
cuprate (LSCO) system for x=0.085. In contrast to the
previous reports, the magnetic ordering in LSCO is {\it\bf only} short range
with an in-plane correlation length of 10 \AA\ and is bidimensional
(2D). Such a less pronounced order suggests an interaction with other
electronic instabilities. In particular, LSCO also exhibits a strong tendency
towards stripes ordering at the expense of the superconducting state.Comment: 4 figures, submitted to Phys. Rev. Let
Simulating spin-3/2 particles at colliders
Support for interactions of spin-3/2 particles is implemented in the
FeynRules and ALOHA packages and tested with the MadGraph 5 and CalcHEP event
generators in the context of three phenomenological applications. In the first,
we implement a spin-3/2 Majorana gravitino field, as in local supersymmetric
models, and study gravitino and gluino pair-production. In the second, a
spin-3/2 Dirac top-quark excitation, inspired from compositness models, is
implemented. We then investigate both top-quark excitation and top-quark
pair-production. In the third, a general effective operator for a spin-3/2
Dirac quark excitation is implemented, followed by a calculation of the angular
distribution of the s-channel production mechanism.Comment: 20 pages, 7 figure
Immune reconstitution syndrome presenting as probable AIDS-related lymphoma: a case report
We report an unusual case of HIV-related immune reconstitution inflammatory syndrome, presenting as suspected AIDS-related lymphoma. Symptoms, initial investigations including fine-needle biopsy and 18F-FDG PET/CT scan were highly compatible with high grade AIDS-related lymphoma, however subsequently IRIS was diagnosed. We discuss pitfalls in the interpretation of diagnostic results in ARL versus IRIS
Dynamic SU(2) Lattice Gauge Theory at Finite Temperature
The dynamic relaxation process for the (2+1)--dimensional SU(2) lattice gauge
theory at critical temperature is investigated with Monte Carlo methods. The
critical initial increase of the Polyakov loop is observed. The dynamic
exponents and as well as the static critical exponent
are determined from the power law behaviour of the Polyakov loop, the
auto-correlation and the second moment at the early stage of the time
evolution. The results are well consistent and universal short-time scaling
behaviour of the dynamic system is confirmed. The values of the exponents show
that the dynamic SU(2) lattice gauge theory is in the same dynamic universality
class as the dynamic Ising model.Comment: 10 pages with 2 figure
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