5,970 research outputs found

### Self-similarities in the frequency-amplitude space of a loss-modulated CO$_2$ laser

We show the standard two-level continuous-time model of loss-modulated CO$_2$
lasers to display the same regular network of self-similar stability islands
known so far to be typically present only in discrete-time models based on
mappings. For class B laser models our results suggest that, more than just
convenient surrogates, discrete mappings in fact could be isomorphic to
continuous flows.Comment: (5 low-res color figs; for ALL figures high-res PDF:
http://www.if.ufrgs.br/~jgallas/jg_papers.html

### Onset of fluidization in vertically shaken granular material

When granular material is shaken vertically one observes convection, surface
fluidization, spontaneous heap formation and other effects. There is a
controversial discussion in literature whether there exists a threshold for the
Froude number $\Gamma=A_0\omega_0^2/g$ below which these effects cannot be
observed anymore. By means of theoretical analysis and computer simulation we
find that there is no such single threshold. Instead we propose a modified
criterion which coincides with critical Froude number $\Gamma_c=1$ for small
driving frequency $\omega_0$.Comment: 7 pages, 5 figure

### Size segregation and convection

The size segregation of granular materials in a vibrating container is
investigated using Molecular Dynamics. We find that the rising of larger
particles is accompanied by the existence of convection cells even in the case
of the lowest possible frequencies. The convection can, however, also be
triggered by the larger particle itself. The possibility of rising through this
mechanism strongly depends on the depth of the larger particle.Comment: 7 pages, 4 figure

### Coefficient of restitution for elastic disks

We calculate the coefficient of restitution, $\epsilon$, starting from a
microscopic model of elastic disks. The theory is shown to agree with the
approach of Hertz in the quasistatic limit, but predicts inelastic collisions
for finite relative velocities of two approaching disks. The velocity
dependence of $\epsilon$ is calculated numerically for a wide range of
velocities. The coefficient of restitution furthermore depends on the elastic
constants of the material via Poisson's number. The elastic vibrations absorb
kinetic energy more effectively for materials with low values of the shear
modulus.Comment: 25 pages, 12 Postscript figures, LaTex2

### Patterns and Collective Behavior in Granular Media: Theoretical Concepts

Granular materials are ubiquitous in our daily lives. While they have been a
subject of intensive engineering research for centuries, in the last decade
granular matter attracted significant attention of physicists. Yet despite a
major efforts by many groups, the theoretical description of granular systems
remains largely a plethora of different, often contradicting concepts and
approaches. Authors give an overview of various theoretical models emerged in
the physics of granular matter, with the focus on the onset of collective
behavior and pattern formation. Their aim is two-fold: to identify general
principles common for granular systems and other complex non-equilibrium
systems, and to elucidate important distinctions between collective behavior in
granular and continuum pattern-forming systems.Comment: Submitted to Reviews of Modern Physics. Full text with figures (2Mb
pdf) avaliable at
http://mti.msd.anl.gov/AransonTsimringReview/aranson_tsimring.pdf Community
responce is appreciated. Comments/suggestions send to [email protected]

### The D0 Run IIb Luminosity Measurement

An assessment of the recorded integrated luminosity is presented for data
collected with the D0 detector at the Fermilab Tevatron Collider from June 2006
to September 2011 (Run IIb). In addition, a measurement of the effective cross
section for inelastic interactions, also referred to as the luminosity
constant, is reported. This measurement incorporates new features that lead to
a substantial improvement in the precision of the result. A luminosity constant
of \sigma_{LM} = 48.3\pm1.9\pm0.6 mb is obtained, where the first uncertainty
is due to the accuracy of the inelastic cross section used by both CDF and D0,
and the second uncertainty is due to D0 sources. The recorded luminosity for
the highest E_T jet trigger is L_rec = 9.2 \pm 0.4 fb^{-1}, with a relative
uncertainty of 4.3%.Comment: 20 pages, 23 figure

### Fast shower simulation in the ATLAS calorimeter

The time to simulate pp collisions in the ATLAS detector is largely dominated by the showering of electromagnetic particles in the heavy parts of the detector, especially the electromagnetic barrel and endcap calorimeters. Two procedures have been developed to accelerate the processing time of electromagnetic particles in these regions: (1) a fast shower parameterisation and (2) a frozen shower library. Both work by generating the response of the calorimeter to electrons and positrons with Geant 4, and then reintroduce the response into the simulation at runtime.
In the fast shower parameterisation technique, a parameterisation is tuned to single electrons and used later by simulation. In the frozen shower technique, actual showers from low-energy particles are used in the simulation. Full Geant 4 simulation is used to develop showers down to ~1 GeV, at which point the shower is terminated by substituting a frozen shower. Judicious use of both techniques over the entire electromagnetic portion of the ATLAS calorimeter produces an important improvement of CPU time. We discuss the algorithms and their performance in this paper

### Angle of Repose and Angle of Marginal Stability: Molecular Dyanmics of Granular Particles

We present an implementation of realistic static friction in molecular
dynamics (MD) simulations of granular particles. In our model, to break
contacts between two particles, one has to apply a finite amount of force,
determined by the Coulomb criterion. Using a two dimensional model, we show
that piles generated by avalanches have a {\it finite} angle of repose
$\theta_R$ (finite slopes). Furthermore, these piles are stable under tilting
by an angle smaller than a non-zero tilting angle $\theta_T$, showing that
$\theta_R$ is different from the angle of marginal stability $\theta_{MS}$,
which is the maximum angle of stable piles. These measured angles are compared
to a theoretical approximation. We also measure $\theta_{MS}$ by continuously
adding particles on the top of a stable pile.Comment: 14 pages, Plain Te

### Anomalous density dependence of static friction in sand

We measured experimentally the static friction force $F_s$ on the surface of
a glass rod immersed in dry sand. We observed that $F_s$ is extremely sensitive
to the closeness of packing of grains. A linear increase of the grain-density
yields to an exponentially increasing friction force. We also report on a novel
periodicity of $F_s$ during gradual pulling out of the rod. Our observations
demonstrate the central role of grain bridges and arches in the macroscopic
properties of granular packings.Comment: plain tex, 6 pages, to appear in Phys.Rev.

### Characterisation of Medipix3 Silicon Detectors in a Charged-Particle Beam

While designed primarily for X-ray imaging applications, the Medipix3 ASIC
can also be used for charged-particle tracking. In this work, results from a
beam test at the CERN SPS with irradiated and non-irradiated sensors are
presented and shown to be in agreement with simulation, demonstrating the
suitability of the Medipix3 ASIC as a tool for characterising pixel sensors.Comment: 16 pages, 13 figure

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