339 research outputs found
The average shape of a fluctuation: universality in excursions of stochastic processes
We study the average shape of a fluctuation of a time series x(t), that is
the average value _T before x(t) first returns, at time T, to its
initial value x(0). For large classes of stochastic processes we find that a
scaling law of the form _T = T^\alpha f(t/T) is obeyed. The
scaling function f(s) is to a large extent independent of the details of the
single increment distribution, while it encodes relevant statistical
information on the presence and nature of temporal correlations in the process.
We discuss the relevance of these results for Barkhausen noise in magnetic
systems.Comment: 5 pages, 5 figures, accepted for publication in Phys. Rev. Let
Average trajectory of returning walks
We compute the average shape of trajectories of some one--dimensional
stochastic processes x(t) in the (t,x) plane during an excursion, i.e. between
two successive returns to a reference value, finding that it obeys a scaling
form. For uncorrelated random walks the average shape is semicircular,
independently from the single increments distribution, as long as it is
symmetric. Such universality extends to biased random walks and Levy flights,
with the exception of a particular class of biased Levy flights. Adding a
linear damping term destroys scaling and leads asymptotically to flat
excursions. The introduction of short and long ranged noise correlations
induces non trivial asymmetric shapes, which are studied numerically.Comment: 16 pages, 16 figures; accepted for publication in Phys. Rev.
Analytical results for generalized persistence properties of smooth processes
We present a general scheme to calculate within the independent interval
approximation generalized (level-dependent) persistence properties for
processes having a finite density of zero-crossings. Our results are especially
relevant for the diffusion equation evolving from random initial conditions,
one of the simplest coarsening systems. Exact results are obtained in certain
limits, and rely on a new method to deal with constrained multiplicative
processes. An excellent agreement of our analytical predictions with direct
numerical simulations of the diffusion equation is found.Comment: 21 pages, 4 figures, to appear in Journal of Physics
On the out of equilibrium order parameters in long-range spin-glases
We show that the dynamical order parameters can be reexpressed in terms of
the distribution of the staggered auto-correlation and response functions. We
calculate these distributions for the out of equilibrium dynamics of the
Sherrington-Kirpatrick model at long times. The results suggest that the
landscape this model visits at different long times in an out of equilibrium
relaxation process is, in a sense, self-similar. Furthermore, there is a
similarity between the landscape seen out of equilibrium at long times and the
equilibrium landscape. The calculation is greatly simplified by making use of
the superspace notation in the dynamical approach. This notation also
highlights the rather mysterious formal connection between the dynamical and
replica approaches.Comment: 25 pages, Univ. di Roma I preprint #1049 (we replaced the file by the
RevTex file, figures available upon request
Brownian forces in sheared granular matter
We present results from a series of experiments on a granular medium sheared
in a Couette geometry and show that their statistical properties can be
computed in a quantitative way from the assumption that the resultant from the
set of forces acting in the system performs a Brownian motion. The same
assumption has been utilised, with success, to describe other phenomena, such
as the Barkhausen effect in ferromagnets, and so the scheme suggests itself as
a more general description of a wider class of driven instabilities.Comment: 4 pages, 5 figures and 1 tabl
Characteristics of neonatal GBS disease during a multicentre study (2007-2010) and in the year 2012
iNTRODUCTION: The characteristics of Group B Streptococcal (GBS) early onset (EOD) and late onset (LOD) neonatal infections in Italy were analyzed. Two periods were considered, a first 3-years period (2007-2010), when notification of GBS infections was enforced under the auspices of the Italian Ministry of Health, and a second 1 year period (2012) when reporting on neonatal GBS disease continued on voluntary basis. METHODS: A standardized form was used to collect data on cases of neonatal GBS disease. They included both maternal and neonatal data. RESULTS AND DISCUSSION: The two surveys underlined that preterm deliveries, precipitous labor and negatively GBS screened mothers are common causes of EOD occurrence, possibly explained by inadequate, or lack of, intrapartum antibiotic prophylaxis. Nevertheless, measures for reducing prevention failures and EOD incidence by an higher adherence to prevention strategies, as the Centre for Disease Control recommendations, are still possible and should be encouraged
Spontaneous Polarisation Build up in a Room Temperature Polariton Laser
We observe the build up of strong (~50%) spontaneous vector polarisation in
emission from a GaN-based polariton laser excited by short optical pulses at
room temperature. The Stokes vector of emitted light changes its orientation
randomly from one excitation pulse to another, so that the time-integrated
polarisation remains zero. This behaviour is completely different to any
previous laser. We interpret this observation in terms of the spontaneous
symmetry breaking in a Bose-Einstein condensate of exciton-polaritons
Sign-time distribution for a random walker with a drifting boundary
We present a derivation of the exact sign-time distribution for a random
walker in the presence of a boundary moving with constant velocity.Comment: 5 page
Coarsening and Slow-Dynamics in Granular Compaction
We address the problem of the microscopic reorganization of a granular medium
under a compaction process in the framework of Tetris-like models. We point out
the existence of regions of spatial organization which we call domains, and
study their time evolution. It turns out that after an initial transient, most
of the activity of the system is concentrated on the boundaries between
domains. One can then describe the compaction phenomenon as a coarsening
process for the domains, and a progressive reduction of domain boundaries. We
discuss the link between the coarsening process and the slow dynamics in the
framework of a model of active walkers on active substrates.Comment: Revtex 4 pages, 4 figures, in press in PRL. More info
http://axtnt3.phys.uniroma1.it/Tetri
Steady state properties of a mean field model of driven inelastic mixtures
We investigate a Maxwell model of inelastic granular mixture under the
influence of a stochastic driving and obtain its steady state properties in the
context of classical kinetic theory. The model is studied analytically by
computing the moments up to the eighth order and approximating the
distributions by means of a Sonine polynomial expansion method. The main
findings concern the existence of two different granular temperatures, one for
each species, and the characterization of the distribution functions, whose
tails are in general more populated than those of an elastic system. These
analytical results are tested against Monte Carlo numerical simulations of the
model and are in general in good agreement. The simulations, however, reveal
the presence of pronounced non-gaussian tails in the case of an infinite
temperature bath, which are not well reproduced by the Sonine method.Comment: 23 pages, 10 figures, submitted for publicatio
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