2,038 research outputs found
Steady state, relaxation and first-passage properties of a run-and-tumble particle in one-dimension
We investigate the motion of a run-and-tumble particle (RTP) in one
dimension. We find the exact probability distribution of the particle with and
without diffusion on the infinite line, as well as in a finite interval. In the
infinite domain, this probability distribution approaches a Gaussian form in
the long-time limit, as in the case of a regular Brownian particle. At
intermediate times, this distribution exhibits unexpected multi-modal forms. In
a finite domain, the probability distribution reaches a steady state form with
peaks at the boundaries, in contrast to a Brownian particle. We also study the
relaxation to the steady state analytically. Finally we compute the survival
probability of the RTP in a semi-infinite domain. In the finite interval, we
compute the exit probability and the associated exit times. We provide
numerical verifications of our analytical results
Self-propelled particles with selective attraction-repulsion interaction - From microscopic dynamics to coarse-grained theories
In this work we derive and analyze coarse-grained descriptions of
self-propelled particles with selective attraction-repulsion interaction, where
individuals may respond differently to their neighbours depending on their
relative state of motion (approach versus movement away). Based on the
formulation of a nonlinear Fokker-Planck equation, we derive a kinetic
description of the system dynamics in terms of equations for the Fourier modes
of a one-particle density function. This approach allows effective numerical
investigation of the stability of possible solutions of the system. The
detailed analysis of the interaction integrals entering the equations
demonstrates that divergences at small wavelengths can appear at arbitrary
expansion orders.
Further on, we also derive a hydrodynamic theory by performing a closure at
the level of the second Fourier mode of the one-particle density function. We
show that the general form of equations is in agreement with the theory
formulated by Toner and Tu.
Finally, we compare our analytical predictions on the stability of the
disordered homogeneous solution with results of individual-based simulations.
They show good agreement for sufficiently large densities and non-negligible
short-ranged repulsion. Disagreements of numerical results and the hydrodynamic
theory for weak short-ranged repulsion reveal the existence of a previously
unknown phase of the model consisting of dense, nematically aligned filaments,
which cannot be accounted for by the present Toner and Tu type theory of polar
active matter.Comment: revised version, 37pages, 11 figure
The Brownian Mean Field model
We discuss the dynamics and thermodynamics of the Brownian Mean Field (BMF)
model which is a system of N Brownian particles moving on a circle and
interacting via a cosine potential. It can be viewed as the canonical version
of the Hamiltonian Mean Field (HMF) model. We first complete the description of
this system in the mean field approximation. Then, we take fluctuations into
account and study the stochastic evolution of the magnetization both in the
homogeneous phase and in the inhomogeneous phase. We discuss its behavior close
to the critical point
Levy flights in quenched random force fields
Levy flights, characterized by the microscopic step index f, are for f<2 (the
case of rare events) considered in short range and long range quenched random
force fields with arbitrary vector character to first loop order in an
expansion about the critical dimension 2f-2 in the short range case and the
critical fall-off exponent 2f-2 in the long range case. By means of a dynamic
renormalization group analysis based on the momentum shell integration method,
we determine flows, fixed point, and the associated scaling properties for the
probability distribution and the frequency and wave number dependent diffusion
coefficient. Unlike the case of ordinary Brownian motion in a quenched force
field characterized by a single critical dimension or fall-off exponent d=2,
two critical dimensions appear in the Levy case. A critical dimension (or
fall-off exponent) d=f below which the diffusion coefficient exhibits anomalous
scaling behavior, i.e, algebraic spatial behavior and long time tails, and a
critical dimension (or fall-off exponent) d=2f-2 below which the force
correlations characterized by a non trivial fixed point become relevant. As a
general result we find in all cases that the dynamic exponent z, characterizing
the mean square displacement, locks onto the Levy index f, independent of
dimension and independent of the presence of weak quenched disorder.Comment: 27 pages, Revtex file, 17 figures in ps format attached, submitted to
Phys. Rev.
Evolution of galaxies due to self-excitation
These lectures will cover methods for studying the evolution of galaxies
since their formation. Because the properties of a galaxy depend on its
history, an understanding of galaxy evolution requires that we understand the
dynamical interplay between all components. The first part will emphasize
n-body simulation methods which minimize sampling noise. These techniques are
based on harmonic expansions and scale linearly with the number of bodies,
similar to Fourier transform solutions used in cosmological simulations.
Although fast, until recently they were only efficiently used for small number
of geometries and background profiles. These same techniques may be used to
study the modes and response of a galaxy to an arbitrary perturbation. In
particular, I will describe the modal spectra of stellar systems and role of
damped modes which are generic to stellar systems in interactions and appear to
play a significant role in determining the common structures that we see. The
general development leads indirectly to guidelines for the number of particles
necessary to adequately represent the gravitational field such that the modal
spectrum is resolvable. I will then apply these same excitation to
understanding the importance of noise to galaxy evolution.Comment: 24 pages, 7 figures, using Sussp.sty (included). Lectures presented
at the NATO Advanced Study Institute, "The Restless Universe: Applications of
Gravitational N-Body Dynamics to Planetary, Stellar and Galactic Systems,"
Blair Atholl, July 200
Symplectic tomography as classical approach to quantum systems
By using a generalization of the optical tomography technique we describe the
dynamics of a quantum system in terms of equations for a purely classical
probability distribution which contains complete information about the system.Comment: 12 pages, LATEX,preprint of Camerino University, to appear in
Phys.Lett.A (1996
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