6,601 research outputs found
Calculating effective gun policies
Following recent shootings in the USA, a debate has erupted, one side
favoring stricter gun control, the other promoting protection through more
weapons. We provide a scientific foundation to inform this debate, based on
mathematical, epidemiological models that quantify the dependence of
firearm-related death rates of people on gun policies. We assume a shooter
attacking a single individual or a crowd. Two strategies can minimize deaths in
the model, depending on parameters: either a ban of private firearms
possession, or a policy allowing the general population to carry guns. In
particular, the outcome depends on the fraction of offenders that illegally
possess a gun, on the degree of protection provided by gun ownership, and on
the fraction of the population who take up their right to own a gun and carry
it with them when attacked, parameters that can be estimated from statistical
data. With the measured parameters, the model suggests that if the gun law is
enforced at a level similar to that in the United Kingdom, gun-related deaths
are minimized if private possession of firearms is banned. If such a policy is
not practical or possible due to constitutional or cultural constraints, the
model and parameter estimation indicate that a partial reduction in firearm
availability can lead to a reduction in gun-induced death rates, even if they
are not minimized. Most importantly, our analysis identifies the crucial
parameters that determine which policy reduces the death rates, providing
guidance for future statistical studies that will be necessary for more refined
quantitative predictions
Black holes and non-relativistic quantum systems
We describe black holes in d+3 dimensions, whose thermodynamic properties
correspond to those of a scale invariant non-relativistic d+1 dimensional
quantum system with dynamical exponent z=2. The gravitational model involves a
massive abelian vector field and a scalar field, in addition to the metric. The
energy per particle in the dual theory is , exactly as in a
non-interacting Fermi gas, while the ratio of shear viscosity to entropy
density is .Comment: 8 pages; v2: discussion modifie
Spectroscopic diagnostic for the mineralogy of large dust grains
We examine the thermal infrared spectra of large dust grains of different
chemical composition and mineralogy. Strong resonances in the optical
properties result in detectable spectral structure even when the grain is much
larger than the wavelength at which it radiates. We apply this to the thermal
infrared spectra of compact amorphous and crystalline silicates. The weak
resonances of amorphous silicates at 9.7 and 18 micron virtually disappear for
grains larger than about 10 micron. In contrast, the strong resonances of
crystalline silicates produce emission dips in the infrared spectra of large
grains; these emission dips are shifted in wavelength compared to the emission
peaks commonly seen in small crystalline silicate grains. We discuss the effect
of a fluffy or compact grain structure on the infrared emission spectra of
large grains, and apply our theory to the dust shell surrounding Vega.Comment: Submitted to A&A Letter
Dissipation of dark matter
Fluids often display dissipative properties. We explore dissipation in the
form of bulk viscosity in the cold dark matter fluid. We constrain this model
using current data from supernovae, baryon acoustic oscillations and the cosmic
microwave background. Considering the isotropic and homogeneous background
only, viscous dark matter is allowed to have a bulk viscosity
Pas, also consistent with the expected integrated Sachs-Wolfe effect
(which plagues some models with bulk viscosity). We further investigate the
small-scale formation of viscous dark matter halos, which turns out to place
significantly stronger constraints on the dark matter viscosity. The existence
of dwarf galaxies is guaranteed only for much smaller values of the dark matter
viscosity, Pas.Comment: 10 pages, 3 figures, published in PR
Violation of the Holographic Viscosity Bound in a Strongly Coupled Anisotropic Plasma
We study the conductivity and shear viscosity tensors of a strongly coupled
N=4 super-Yang-Mills plasma which is kept anisotropic by a theta parameter that
depends linearly on one of the spatial dimensions. Its holographic dual is
given by an anisotropic axion-dilaton-gravity background and has recently been
proposed by Mateos and Trancanelli as a model for the pre-equilibrium stage of
quark-gluon plasma in heavy-ion collisions. By applying the membrane paradigm
which we also check by numerical evaluation of Kubo formula and lowest lying
quasinormal modes, we find that the shear viscosity purely transverse to the
direction of anisotropy saturates the holographic viscosity bound, whereas
longitudinal shear viscosities are smaller, providing the first such example
not involving higher-derivative theories of gravity and, more importantly, with
fully known gauge-gravity correspondence.Comment: 4 pages, 2 figures; v3: references added, version to appear in Phys.
Rev. Let
The anatomy and paleoecology of the boselaphine Miotragocerus pannoniae from the late Miocene Höwenegg locality (Hegau, Germany)
Point trajectory planning of flexible redundant robot manipulators using genetic algorithms
The paper focuses on the problem of point-to-point trajectory planning for flexible redundant robot manipulators (FRM) in joint space. Compared with irredundant flexible manipulators, a FRM possesses additional possibilities during point-to-point trajectory planning due to its kinematics redundancy. A trajectory planning method to minimize vibration and/or executing time of a point-to-point motion is presented for FRMs based on Genetic Algorithms (GAs). Kinematics redundancy is integrated into the presented method as planning variables. Quadrinomial and quintic polynomial are used to describe the segments that connect the initial, intermediate, and final points in joint space. The trajectory planning of FRM is formulated as a problem of optimization with constraints. A planar FRM with three flexible links is used in simulation. Case studies show that the method is applicable
From ab initio quantum chemistry to molecular dynamics: The delicate case of hydrogen bonding in ammonia
The ammonia dimer (NH3)2 has been investigated using high--level ab initio
quantum chemistry methods and density functional theory (DFT). The structure
and energetics of important isomers is obtained to unprecedented accuracy
without resorting to experiment. The global minimum of eclipsed C_s symmetry is
characterized by a significantly bent hydrogen bond which deviates from
linearity by about 20 degrees. In addition, the so-called cyclic C_{2h}
structure is extremely close in energy on an overall flat potential energy
surface. It is demonstrated that none of the currently available (GGA,
meta--GGA, and hybrid) density functionals satisfactorily describe the
structure and relative energies of this nonlinear hydrogen bond. We present a
novel density functional, HCTH/407+, designed to describe this sort of hydrogen
bond quantitatively on the level of the dimer, contrary to e.g. the widely used
BLYP functional. This improved functional is employed in Car-Parrinello ab
initio molecular dynamics simulations of liquid ammonia to judge its
performance in describing the associated liquid. Both the HCTH/407+ and BLYP
functionals describe the properties of the liquid well as judged by analysis of
radial distribution functions, hydrogen bonding structure and dynamics,
translational diffusion, and orientational relaxation processes. It is
demonstrated that the solvation shell of the ammonia molecule in the liquid
phase is dominated by steric packing effects and not so much by directional
hydrogen bonding interactions. In addition, the propensity of ammonia molecules
to form bifurcated and multifurcated hydrogen bonds in the liquid phase is
found to be negligibly small.Comment: Journal of Chemical Physics, in press (305335JCP
Probing new physics in electroweak penguins through B_d and B_s decays
An enhanced electroweak penguin amplitude due to the presence of unknown new
physics can explain the discrepancies found between theory and experiment in
the B -> pi K decays, in particular in A_CP(B^- -> pi^0 K^-) - A_CP(\bar{B}^0
-> pi^+ K^-), but the current precision of the theoretical and experimental
results does not allow to draw a firm conclusion. We argue that the \bar{B}_s
-> phi rho^0 and \bar{B}_s -> phi pi^0 decays offer an additional tool to
investigate this possibility. These purely isospin-violating decays are
dominated by electroweak penguins and we show that in presence of a new physics
contribution their branching ratio can be enhanced by about an order of
magnitude, without violating any constraints from other hadronic B decays. This
makes them very interesting modes for LHCb and future B factories. In
arXiv:1011.6319 we have performed both a model-independent analysis and a study
within realistic New Physics models such as a modified-Z^0-penguin scenario, a
model with an additional Z' boson and the MSSM. In this article we summarise
the most important results of our study.Comment: 8 pages, 5 figures, LaTeX. Talk given at Discrete2010, Rome, 6-11
December 2010; References adde
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