7,346 research outputs found
Dethinning Extensive Air Shower Simulations
We describe a method for restoring information lost during statistical
thinning in extensive air shower simulations. By converting weighted particles
from thinned simulations to swarms of particles with similar characteristics,
we obtain a result that is essentially identical to the thinned shower, and
which is very similar to non-thinned simulations of showers. We call this
method dethinning. Using non-thinned showers on a large scale is impossible
because of unrealistic CPU time requirements, but with thinned showers that
have been dethinned, it is possible to carry out large-scale simulation studies
of the detector response for ultra-high energy cosmic ray surface arrays. The
dethinning method is described in detail and comparisons are presented with
parent thinned showers and with non-thinned showers
The Stochastic Dynamics of Rectangular and V-shaped Atomic Force Microscope Cantilevers in a Viscous Fluid and Near a Solid Boundary
Using a thermodynamic approach based upon the fluctuation-dissipation theorem
we quantify the stochastic dynamics of rectangular and V-shaped microscale
cantilevers immersed in a viscous fluid. We show that the stochastic cantilever
dynamics as measured by the displacement of the cantilever tip or by the angle
of the cantilever tip are different. We trace this difference to contributions
from the higher modes of the cantilever. We find that contributions from the
higher modes are significant in the dynamics of the cantilever tip-angle. For
the V-shaped cantilever the resulting flow field is three-dimensional and
complex in contrast to what is found for a long and slender rectangular
cantilever. Despite this complexity the stochastic dynamics can be predicted
using a two-dimensional model with an appropriately chosen length scale. We
also quantify the increased fluid dissipation that results as a V-shaped
cantilever is brought near a solid planar boundary.Comment: 10 pages, 15 images, corrected equation (8
Isospin splittings of meson and baryon masses from three-flavor lattice QCD + QED
Lattice QCD simulations are now reaching a precision where isospin breaking
effects become important. Previously, we have developed a program to
systematically investigate the pattern of flavor symmetry beaking within QCD
and successfully applied it to meson and baryon masses involving up, down and
strange quarks. In this Letter we extend the calculations to QCD + QED and
present our first results on isospin splittings in the pseudoscalar meson and
baryon octets. In particular, we obtain the nucleon mass difference of
M_n-M_p=1.35(18)(8)\,\mbox{MeV} and the electromagnetic contribution to the
pion splitting M_{\pi^+}-M_{\pi^0}=4.60(20)\,\mbox{MeV}. Further we report
first determination of the separation between strong and electromagnetic
contributions in the scheme.Comment: 14 pages, 10 figures, text and figures added, Journal versio
Network formation of tissue cells via preferential attraction to elongated structures
Vascular and non-vascular cells often form an interconnected network in
vitro, similar to the early vascular bed of warm blooded embryos. Our
time-lapse recordings show that the network forms by extending sprouts, i.e.,
multicellular linear segments. To explain the emergence of such structures, we
propose a simple model of preferential attraction to stretched cells. Numerical
simulations reveal that the model evolves into a quasi-stationary pattern
containing linear segments, which interconnect above the critical volume
fraction of 0.2. In the quasi-stationary state the generation of new branches
offset the coarsening driven by surface tension. In agreement with empirical
data, the characteristic size of the resulting polygonal pattern is
density-independent within a wide range of volume fractions
Consequences of asteroid fragmentation during impact hazard mitigation
The consequences of the fragmentation of an Earth-threatening asteroid due to an attempted deflection are examined in this paper. The minimum required energy for a successful impulsive deflection of a threatening object is computed and compared to the energy required to break up a small size asteroid. The results show that the fragmentation of an asteroid that underwent an impulsive deflection, such as a kinetic impact or a nuclear explosion, is a very plausible event.Astatistical model is used to approximate the number and size of the fragments as well as the distribution of velocities at the instant after the deflection attempt takes place. This distribution of velocities is a function of the energy provided by the deflection attempt, whereas the number and size of the asteroidal fragments is a function of the size of the largest fragment. The model also takes into account the gravity forces that could lead to a reaggregation of the asteroid after fragmentation. The probability distribution of the pieces after the deflection is then propagated forward in time until the encounter with Earth. A probability damage factor (i.e., expected damage caused by a given size fragment multiplied by its impact probability) is then computed and analyzed for different plausible scenarios, characterized by different levels of deflection energies and lead times
The Neuromuscular Origins of Kinematic Variability during Perturbed Walking
We investigated the neuromuscular contributions to kinematic variability and thus step to step adjustments in posture and foot placement across a range of walking speeds in response to optical flow perturbations of different amplitudes using a custom virtual environment. We found that perturbations significantly increased step width, decreased step length, and elicited larger trunk sway compared to normal walking. However, perturbation-induced effects on the corresponding variabilities of these measurements were much more profound. Consistent with our hypotheses, we found that: (1) perturbations increased EMG activity of the gluteus medius and postural control muscles during leg swing, and increased antagonist leg muscle coactivation during limb loading in early stance, and (2) changes in the magnitude of step to step adjustments in postural sway and lateral foot placement positively correlated with those of postural control and gluteus medius muscle activities, respectively, in response to perturbations. However, (3) interactions between walking speed and susceptibility to perturbations, when present, were more complex than anticipated. Our study provides important mechanistic neuromuscular insight into walking balance control and important reference values for the emergence of balance impairment
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