7,743 research outputs found
Excited heavy tetraquarks with hidden charm
The masses of the excited heavy tetraquarks with hidden charm are calculated
within the relativistic diquark-antidiquark picture. The dynamics of the light
quark in a heavy-light diquark is treated completely relativistically. The
diquark structure is taken into account by calculating the diquark-gluon form
factor. New experimental data on charmonium-like states above open charm
threshold are discussed. The obtained results indicate that X(3872), Y(4260),
Y(4360), Z(4248), Z(4433) and Y(4660) could be tetraquark states with hidden
charm.Comment: 11 page
Streamer branching rationalized by conformal mapping techniques
Spontaneous branching of discharge channels is frequently observed, but not
well understood. We recently proposed a new branching mechanism based on
simulations of a simple continuous discharge model in high fields. We here
present analytical results for such streamers in the Lozansky-Firsov limit
where they can be modelled as moving equipotential ionization fronts. This
model can be analyzed by conformal mapping techniques which allow the reduction
of the dynamical problem to finite sets of nonlinear ordinary differential
equations. The solutions illustrate that branching is generic for the intricate
head dynamics of streamers in the Lozansky-Firsov-limit.Comment: 4 pages, 2 figure
Why isolated streamer discharges hardly exist above the breakdown field in atmospheric air
We investigate streamer formation in the troposphere, in electric fields
above the breakdown threshold. With fully three-dimensional particle
simulations, we study the combined effect of natural background ionization and
of photoionization on the discharge morphology. In previous investigations
based on deterministic fluid models without background ionization, so-called
double-headed streamers emerged. But in our improved model, many electron
avalanches start to grow at different locations. Eventually the avalanches
collectively screen the electric field in the interior of the discharge. This
happens after what we call the `ionization screening time', for which we give
an analytical estimate. As this time is comparable to the streamer formation
time, we conclude that isolated streamers are unlikely to exist in fields well
above breakdown in atmospheric air.Comment: Changed citation information. 6 pages, 4 figures, Geophysical
Research Letters, Vol. 40, 2417-2422, 201
Magnetically Controlled Spasmodic Accretion During Star Formation. II. Results
The problem of the late accretion phase of the evolution of an axisymmetric,
isothermal magnetic disk surrounding a forming star has been formulated in a
companion paper. The "central sink approximation" is used to circumvent the
problem of describing the evolution inside the opaque central region for
densities greater than 10^11 cm^-3 and radii smaller than a few AUs. Only the
electrons are assumed to be attached to the magnetic field lines, and the
effects of both negatively and positively charged grains are accounted for.
After a mass of 0.1 solar mass accumulates in the central cell (forming star),
a series of magnetically driven outflows and associated outward propagating
shocks form in a quasi-periodic fashion. As a result, mass accretion onto the
protostar occurs in magnetically controlled bursts. We refer to this process as
spasmodic accretion. The shocks propagate outward with supermagnetosonic
speeds. The period of dissipation and revival of the outflow decreases in time,
as the mass accumulated in the central sink increases. We evaluate the
contribution of ambipolar diffusion to the resolution of the magnetic flux
problem of star formation during the accretion phase, and we find it to be very
significant although not sufficient to resolve the entire problem yet. Ohmic
dissipation is completely negligible in the disk during this phase of the
evolution. The protostellar disk is found to be stable against interchange-like
instabilities, despite the fact that the mass-to-flux ratio has temporary local
maxima.Comment: Astrophysical Journal, in press. 29 pages, 13 figure
Symmetric Versus Nonsymmetric Structure of the Phosphorus Vacancy on InP(110)
The atomic and electronic structure of positively charged P vacancies on
InP(110) surfaces is determined by combining scanning tunneling microscopy,
photoelectron spectroscopy, and density-functional theory calculations. The
vacancy exhibits a nonsymmetric rebonded atomic configuration with a charge
transfer level 0.75+-0.1 eV above the valence band maximum. The scanning
tunneling microscopy (STM) images show only a time average of two degenerate
geometries, due to a thermal flip motion between the mirror configurations.
This leads to an apparently symmetric STM image, although the ground state
atomic structure is nonsymmetric.Comment: 5 pages including 3 figures. related publications can be found at
http://www.fhi-berlin.mpg.de/th/paper.htm
Distribution of Fish Within Headwater Riffles of the Illinois River System, Washington County, Arkansas
Quantitative sampling of fish was performed in five headwater riffles of the Illinois River System, Washington County, Arkansas during low flow conditions. This study revealed differing fish species composition, biomass and feeding guild segregation between head and tail riffle reaches in 1st through 3rd order. Thirty species representing 10 families were identified. Of this number, darters (Percidae), sculpins (Cottidae), madtoms (Ictaluridae), and central stonerollers (Campostoma anomalum) (Cyprinidae) comprised 67 to 98 percent of riffle head populations. Fish biomass was greater for riffle head areas (0.58-6.6/0.28-2.0 g/m2 ) within sectivores and herbivores dominating. Total fish numbers decreased from riffleheads to tails, while number of species increased. Dominant fish groups in tail areas were minnows (Cyprinidae), darters (Percidae), and sunfishes (Centrarchidae). Feeding guild fish groups in tail areas were predominately insectivore and insectivore-piscivore. Stomach analysis of Cottus carolinae, the dominant headwater riffle predator, indicated selective feeding of macrobenthic invertebrates and fish based on size class. Abundance of herbivore and insectivore fishes in riffles, particularly head reaches, suggests a correlation with positive rheotaxic behavior, microhabitat preference or abundance of macrobenthic invertebrate populations
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