7,137 research outputs found
Isotropically polarized speckle patterns
The polarization of the light scattered by an optically dense, random
solution of dielectric nanoparticles shows peculiar properties when the
scatterers exhibit strong electric and magnetic polarizabilities. While the
distribution of the scattering intensity in these systems shows the typical
irregular speckle patterns, the helicity of the incident light can be fully
conserved when the electric and magnetic polarizabilities of the scatterers are
equal. We show that the multiple scattering of helical beams by a random
dispersion of "dual" dipolar nano-spheres leads to a speckle pattern exhibiting
a perfect isotropic constant polarization, a situation that could be useful in
coherent control of light as well as in lasing in random media.Comment: 5 pages, 3 figure
Sinorhizobium Meliloti, A Bacterium Lacking The Autoinducer-2 (AI-2) Synthase, Responds To AI-2 Supplied By Other Bacteria
Many bacterial species respond to the quorum-sensing signal autoinducer-2 (AI-2) by regulating different niche-specific genes. Here, we show that Sinorhizobium meliloti, a plant symbiont lacking the gene for the AI-2 synthase, while not capable of producing AI-2 can nonetheless respond to AI-2 produced by other species. We demonstrate that S. meliloti has a periplasmic binding protein that binds AI-2. The crystal structure of this protein (here named SmlsrB) with its ligand reveals that it binds (2R,4S)-2-methyl-2,3,3,4-tetrahydroxytetrahydrofuran (R-THMF), the identical AI-2 isomer recognized by LsrB of Salmonella typhimurium. The gene encoding SmlsrB is in an operon with orthologues of the lsr genes required for AI-2 internalization in enteric bacteria. Accordingly, S. meliloti internalizes exogenous AI-2, and mutants in this operon are defective in AI-2 internalization. S. meliloti does not gain a metabolic benefit from internalizing AI-2, suggesting that AI-2 functions as a signal in S. meliloti. Furthermore, S. meliloti can completely eliminate the AI-2 secreted by Erwinia carotovora, a plant pathogen shown to use AI-2 to regulate virulence. Our findings suggest that S. meliloti is capable of \u27eavesdropping\u27 on the AI-2 signalling of other species and interfering with AI-2-regulated behaviours such as virulence
The Gaseous Extent of Galaxies and the Origin of \lya Absorption Systems. III. Hubble Space Telescope Imaging of \lya-Absorbing Galaxies at z < 1
We present initial results of a program to obtain and analyze HST WFPC2
images of galaxies identified in an imaging and spectroscopic survey of faint
galaxies in fields of HST spectroscopic target QSOs. We measure properties of
87 galaxies, of which 33 are associated with corresponding \lya absorption
systems and 24 do not produce corresponding \lya absorption lines to within
sensitive upper limits. Considering only galaxy and absorber pairs that are
likely to be physically associated and excluding galaxy and absorber pairs
within 3000 \kms of the background QSOs leaves 26 galaxy and absorber pairs and
seven galaxies that do not produce corresponding \lya absorption lines to
within sensitive upper limits. Redshifts of the galaxy and absorber pairs range
from 0.0750 to 0.8912 with a median of 0.3718, and impact parameter separations
of the galaxy and absorber pairs range from 12.4 to kpc with a
median of kpc. The primary result of the analysis is that the
amount of gas encountered along the line of sight depends on the galaxy impact
parameter and B-band luminosity but does not depend strongly on the galaxy
average surface brightness, disk-to-bulge ratio, or redshift. This result
confirms and improves upon the anti-correlation between \lya absorption
equivalent width and galaxy impact parameter found previously by Lanzetta et
al. (1995). There is no evidence that galaxy interactions play an important
role in distributing tenuous gas around galaxies in most cases. Galaxies might
account for all \lya absorption systems with \AA, but this depends on
the unknown luminosity function and gaseous cross sections of low-luminosity
galaxies as well as on the uncertainties of the observed number density of \lya
absorption systems.Comment: Minor changes. Figure 1 stays intact and is available at
ftp://ftp.ess.sunysb.edu/pub/lanzetta/wfpc
Neural Modeling and Control of Diesel Engine with Pollution Constraints
The paper describes a neural approach for modelling and control of a
turbocharged Diesel engine. A neural model, whose structure is mainly based on
some physical equations describing the engine behaviour, is built for the
rotation speed and the exhaust gas opacity. The model is composed of three
interconnected neural submodels, each of them constituting a nonlinear
multi-input single-output error model. The structural identification and the
parameter estimation from data gathered on a real engine are described. The
neural direct model is then used to determine a neural controller of the
engine, in a specialized training scheme minimising a multivariable criterion.
Simulations show the effect of the pollution constraint weighting on a
trajectory tracking of the engine speed. Neural networks, which are flexible
and parsimonious nonlinear black-box models, with universal approximation
capabilities, can accurately describe or control complex nonlinear systems,
with little a priori theoretical knowledge. The presented work extends optimal
neuro-control to the multivariable case and shows the flexibility of neural
optimisers. Considering the preliminary results, it appears that neural
networks can be used as embedded models for engine control, to satisfy the more
and more restricting pollutant emission legislation. Particularly, they are
able to model nonlinear dynamics and outperform during transients the control
schemes based on static mappings.Comment: 15 page
How higher-spin gravity surpasses the spin two barrier: no-go theorems versus yes-go examples
Aiming at non-experts, we explain the key mechanisms of higher-spin
extensions of ordinary gravity. We first overview various no-go theorems for
low-energy scattering of massless particles in flat spacetime. In doing so we
dress a dictionary between the S-matrix and the Lagrangian approaches,
exhibiting their relative advantages and weaknesses, after which we high-light
potential loop-holes for non-trivial massless dynamics. We then review positive
yes-go results for non-abelian cubic higher-derivative vertices in constantly
curved backgrounds. Finally we outline how higher-spin symmetry can be
reconciled with the equivalence principle in the presence of a cosmological
constant leading to the Fradkin--Vasiliev vertices and Vasiliev's higher-spin
gravity with its double perturbative expansion (in terms of numbers of fields
and derivatives).Comment: LaTeX, 50 pages, minor changes, many refs added; version accepted for
publication in Reviews of Modern Physic
A Budget and Accounting of Metals at z~0: Results from the COS-Halos Survey
We present a budget and accounting of metals in and around star-forming
galaxies at . We combine empirically derived star formation histories
with updated supernova and AGB yields and rates to estimate the total mass of
metals produced by galaxies with present-day stellar mass of
--. On the accounting side of the ledger, we
show that a surprisingly constant 20--25% mass fraction of produced metals
remain in galaxies' stars, interstellar gas and interstellar dust, with little
dependence of this fraction on the galaxy stellar mass (omitting those metals
immediately locked up in remnants). Thus, the bulk of metals are outside of
galaxies, produced in the progenitors of today's galaxies. The COS-Halos
survey is uniquely able to measure the mass of metals in the circumgalactic
medium (to impact parameters of kpc) of low-redshift
galaxies. Using these data, we map the distribution of CGM metals as traced by
both the highly ionized OVI ion and a suite of low-ionization species; combined
with constraints on circumgalactic dust and hotter X-ray emitting gas out to
similar impact parameters, we show that % of metals produced by
galaxies can be easily accounted for out to
150 kpc. With the current data, we cannot rule out a constant mass of metals
within this fixed physical radius. This census provides a crucial boundary
condition for the eventual fate of metals in galaxy evolution models.Comment: 19 pages, 12 figures, 2 tables. ApJ, in pres
Gravitational wave bursts from cosmic (super)strings: Quantitative analysis and constraints
We discuss data analysis techniques that can be used in the search for
gravitational wave bursts from cosmic strings. When data from multiple
interferometers are available, we describe consistency checks that can be used
to greatly reduce the false alarm rates. We construct an expression for the
rate of bursts for arbitrary cosmic string loop distributions and apply it to
simple known solutions. The cosmology is solved exactly and includes the
effects of a late-time acceleration. We find substantially lower burst rates
than previous estimates suggest and explain the disagreement. Initial LIGO is
unlikely to detect field theoretic cosmic strings with the usual loop sizes,
though it may detect cosmic superstrings as well as cosmic strings and
superstrings with non-standard loop sizes (which may be more realistic). In the
absence of a detection, we show how to set upper limits based on the loudest
event. Using Initial LIGO sensitivity curves, we show that these upper limits
may result in interesting constraints on the parameter space of theories that
lead to the production of cosmic strings.Comment: Replaced with version accepted for publication in PR
Model dependence of the neutron-skin thickness on the symmetry energy
The model dependence in the correlations of the neutron-skin thickness in heavy nuclei with various symmetry-energy parameters is analyzed by using several families of systematically varied microscopic mean-field models. Such correlations show a varying degree of model dependence once the results for all the different families are combined. Some mean-field models associated with similar values of the symmetry-energy slope parameter at saturation density L , and pertaining to different families, yield a greater-than-expected spread in the neutron-skin thickness of the 208 Pb nucleus. The effective value of the symmetry-energy slope parameter L eff , determined by using the nucleon density profiles of the finite nucleus and the density derivative S ′ ( ρ ) of the symmetry energy starting from about saturation density up to low densities typical of the surface of nuclei, seems to account for the spread in the neutron-skin thickness for the models with similar L . The differences in the values of L eff are mainly due to the small differences in the nucleon density distributions of heavy nuclei in the surface region and the behavior of the symmetry energy at subsaturation densities
Dynamics of the Fisher Information Metric
We present a method to generate probability distributions that correspond to
metrics obeying partial differential equations generated by extremizing a
functional , where is the
Fisher metric. We postulate that this functional of the dynamical variable
is stationary with respect to small variations of these
variables. Our approach enables a dynamical approach to Fisher information
metric. It allows to impose symmetries on a statistical system in a systematic
way. This work is mainly motivated by the entropy approach to nonmonotonic
reasoning.Comment: 11 page
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