25,605 research outputs found
An Improved Estimator for the Correlation Function of 2D Nonlinear Sigma Models
I present a new improved estimator for the correlation function of 2D
nonlinear sigma models. Numerical tests for the 2D XY model and the 2D
O(3)-invariant vector model were performed. For small physical volume, i.e. a
lattice size small compared to the to the bulk correlation length, a reduction
of the statistical error of the finite system correlation length by a factor of
up to 30 compared to the cluster-improved estimator was observed. This
improvement allows for a very accurate determination of the running coupling
proposed by M. L"uscher et al. for 2D O(N)-invariant vector models.Comment: 20 pages, LaTeX + 2 ps figures, CERN-TH.7375/9
Cosmic-ray acceleration at collisionless astrophysical shocks using Monte-Carlo simulations
Context. The diffusive shock acceleration mechanism has been widely accepted
as the acceleration mechanism for galactic cosmic rays. While self-consistent
hybrid simulations have shown how power-law spectra are produced, detailed
information on the interplay of diffusive particle motion and the turbulent
electromagnetic fields responsible for repeated shock crossings are still
elusive. Aims. The framework of test-particle theory is applied to investigate
the effect of diffusive shock acceleration by inspecting the obtained
cosmic-ray energy spectra. The resulting energy spectra can be obtained this
way from the particle motion and, depending on the prescribed turbulence model,
the influence of stochastic acceleration through plasma waves can be studied.
Methods. A numerical Monte-Carlo simulation code is extended to include
collisionless shock waves. This allows one to trace the trajectories of test
particle while they are being accelerated. In addition, the diffusion
coefficients can be obtained directly from the particle motion, which allows
for a detailed understanding of the acceleration process. Results. The classic
result of an energy spectrum with is only reproduced for parallel
shocks, while, for all other cases, the energy spectral index is reduced
depending on the shock obliqueness. Qualitatively, this can be explained in
terms of the diffusion coefficients in the directions that are parallel and
perpendicular to the shock front.Comment: 12 pages, 15 figures, accepted for publication in A&
Robust Control of Uncertain Markov Decision Processes with Temporal Logic Specifications
We present a method for designing robust controllers for dynamical systems with linear temporal logic specifications. We abstract the original system by a finite Markov Decision Process (MDP) that has transition probabilities in a specified uncertainty set. A robust control policy for the MDP is generated that maximizes the worst-case probability of satisfying the specification over all transition probabilities in the uncertainty set. To do this, we use a procedure from probabilistic model checking to combine the system model with an automaton representing the specification. This new MDP is then transformed into an equivalent form that satisfies assumptions for stochastic shortest path dynamic programming. A robust version of dynamic programming allows us to solve for a -suboptimal robust control policy with time complexity times that for the non-robust case. We then implement this control policy on the original dynamical system
Sedimentation and polar order of active bottom-heavy particles
Self-propelled particles in an external gravitational field have been shown
to display both an increased sedimentation length and polar order even without
particle interactions. Here, we investigate self-propelled particles which
additionally are bottom-heavy, that is they feel a torque aligning them to swim
against the gravitational field. For bottom-heavy particles the gravitational
field has the two opposite effects of i) sedimentation and ii) upward alignment
of the particles' swimming direction. We perform a multipole expansion of the
one-particle distribution with respect to orientation and derive expressions
for sedimentation length and mean particle orientation which we check against
Brownian Dynamics simulations. For large strength of gravity or small particle
speeds and aligning torque, we observe sedimentation with increased
sedimentation length compared with passive colloids but also active colloids
without bottom-heaviness. Increasing, for example, swimming speed the
sedimentation profile is inverted and the particles swim towards the top wall
of the enclosing box. We find maximal orientational order at intermediate
swimming speeds for both cases of particles with bottom-heaviness and those
without. Ordering unsurprisingly is increased for the bottom-heavy particles,
but this difference disappears at higher levels of activity and for very high
activities ordering goes to zero in both cases.Comment: 6 pages, 3 figure
Critical exponents of a three dimensional O(4) spin model
By Monte Carlo simulation we study the critical exponents governing the
transition of the three-dimensional classical O(4) Heisenberg model, which is
considered to be in the same universality class as the finite-temperature QCD
with massless two flavors. We use the single cluster algorithm and the
histogram reweighting technique to obtain observables at the critical
temperature. After estimating an accurate value of the inverse critical
temperature \Kc=0.9360(1), we make non-perturbative estimates for various
critical exponents by finite-size scaling analysis. They are in excellent
agreement with those obtained with the expansion method with
errors reduced to about halves of them.Comment: 25 pages with 8 PS figures, LaTeX, UTHEP-28
Surface properties of ocean fronts
Background information on oceanic fronts is presented and the results of several models which were developed to study the dynamics of oceanic fronts and their effects on various surface properties are described. The details of the four numerical models used in these studies are given in separate appendices which contain all of the physical equations, program documentation and running instructions for the models
Expression of baculovirus P35 prevents cell death in Drosophila
The baculovirus P35 protein functions to prevent apoptotic death of infected cells. We have expressed P35 in the developing embryo and eye of the fly Drosophila melanogaster. P35 eliminates most, if not all, normally occurring cell death in these tissues, as well as X-irradiation-induced death. Excess pupal eye cells that are normally eliminated by apoptosis develop into pigment cells when their death is prevented by P35 expression. Our results suggest that one mechanism by which viruses prevent the death of the host cell is to block a cell death pathway that mediates normally occurring cell death. Identification of molecules that interact biochemically or genetically with P35 in Drosophila should provide important insights into how cell death is regulated
Spectral Formation in X-Ray Pulsar Accretion Columns
We present the first self-consistent model for the dynamics and the radiative
transfer occurring in bright X-ray pulsar accretion columns, with a special
focus on the role of the shock in energizing the emerging X-rays. The pressure
inside the accretion column of a luminous X-ray pulsar is dominated by the
photons, and consequently the equations describing the coupled
radiative-dynamical structure must be solved simultaneously. Spectral formation
in these sources is therefore a complex, nonlinear phenomenon. We obtain the
analytical solution for the Green's function describing the upscattering of
monochromatic radiation injected into the column from the thermal mound located
near the base of the flow. The Green's function is convolved with a Planck
distribution to model the X-ray spectrum resulting from the reprocessing of
blackbody photons produced in the thermal mound. These photons diffuse through
the infalling gas and eventually escape out the walls of the column, forming
the observed X-ray spectrum. We show that the resulting column-integrated,
phase-averaged spectrum has a power-law shape at high energies and a blackbody
shape at low energies, in agreement with the observational data for many X-ray
pulsars.Comment: Accepted for publication in ApJ Letters. Several typos noticed during
the proof review were correcte
- ā¦