42,616 research outputs found
Quark Propagation in the Quark-Gluon Plasma
It has recently been suggested that the quark-gluon plasma formed in
heavy-ion collisions behaves as a nearly ideal fluid. That behavior may be
understood if the quark and antiquark mean-free- paths are very small in the
system, leading to a "sticky molasses" description of the plasma, as advocated
by the Stony Brook group. This behavior may be traced to the fact that there
are relatively low-energy resonance states in the plasma leading to
very large scattering lengths for the quarks. These resonances have been found
in lattice simulation of QCD using the maximum entropy method (MEM). We have
used a chiral quark model, which provides a simple representation of effects
due to instanton dynamics, to study the resonances obtained using the MEM
scheme. In the present work we use our model to study the optical potential of
a quark in the quark-gluon plasma and calculate the quark mean-free-path. Our
results represent a specific example of the dynamics of the plasma as described
by the Stony Brook group.Comment: 17 pages, 4 figures, revtex
Software that goes with the flow in systems biology
A recent article in BMC Bioinformatics describes new advances in workflow systems for computational modeling in systems biology. Such systems can accelerate, and improve the consistency of, modeling through automation not only at the simulation and results-production stages, but also at the model-generation stage. Their work is a harbinger of the next generation of more powerful software for systems biologists
Mapping Crop Cycles in China Using MODIS-EVI Time Series
As the Earth’s population continues to grow and demand for food increases, the need for improved and timely information related to the properties and dynamics of global agricultural systems is becoming increasingly important. Global land cover maps derived from satellite data provide indispensable information regarding the geographic distribution and areal extent of global croplands. However, land use information, such as cropping intensity (defined here as the number of cropping cycles per year), is not routinely available over large areas because mapping this information from remote sensing is challenging. In this study, we present a simple but efficient algorithm for automated mapping of cropping intensity based on data from NASA’s (NASA: The National Aeronautics and Space Administration) MODerate Resolution Imaging Spectroradiometer (MODIS). The proposed algorithm first applies an adaptive Savitzky-Golay filter to smooth Enhanced Vegetation Index (EVI) time series derived from MODIS surface reflectance data. It then uses an iterative moving-window methodology to identify cropping cycles from the smoothed EVI time series. Comparison of results from our algorithm with national survey data at both the provincial and prefectural level in China show that the algorithm provides estimates of gross sown area that agree well with inventory data. Accuracy assessment comparing visually interpreted time series with algorithm results for a random sample of agricultural areas in China indicates an overall accuracy of 91.0% for three classes defined based on the number of cycles observed in EVI time series. The algorithm therefore appears to provide a straightforward and efficient method for mapping cropping intensity from MODIS time series data
Polarization and decoherence in a two-component Bose-Einstein Condensate
We theoretically investigate polarization properties of a two-component
Bose-Einstein condensate (BEC) and influence of decoherence induced by
environment on BEC polarization through introducing four BEC Stokes operators
which are quantum analog of the classical Stokes parameters for a light field.
BEC polarization states can be geometrically described by a Poincar\'{e} sphere
defined by expectation values of BEC Stokes operators. Without decoherence, it
is shown that nonlinear inter-atomic interactions in the BEC induce periodic
polarization oscillations whose periods depend on the difference between
self-interaction in each component and inter-component interaction strengths.
In particular, when inter-atomic nonlinear self-interaction in each BEC
component equals inter-component nonlinear interaction, Stokes vector
associated with Stokes operators precesses around a fixed axis in the dynamic
evolution of the BEC. The value of the processing frequency is determined by
the strength of the linear coupling between two components of the BEC. When
decoherence is involved, we find each component of the Stokes vector decays
which implies that decoherence depolarizes the BEC.Comment: 10 pages, 2 figure
Hidden Caldeira-Leggett dissipation in a Bose-Fermi Kondo model
We show that the Bose-Fermi Kondo model (BFKM), which may find applicability
both to certain dissipative mesoscopic qubit devices and to heavy fermion
systems described by the Kondo lattice model, can be mapped exactly onto the
Caldeira-Leggett model. This mapping requires an ohmic bosonic bath and an
Ising-type coupling between the latter and the impurity spin. This allows us to
conclude unambiguously that there is an emergent Kosterlitz-Thouless quantum
phase transition in the BFKM with an ohmic bosonic bath. By applying a bosonic
numerical renormalization group approach, we thoroughly probe physical
quantities close to the quantum phase transition.Comment: Final version appearing in Physical Review Letter
The Marangoni flow of soluble amphiphiles
Surfactant distribution heterogeneities at a fluid/fluid interface trigger
the Marangoni effect, i.e. a bulk flow due to a surface tension gradient. The
influence of surfactant solubility in the bulk on these flows remains
incompletely characterized. Here we study Marangoni flows sustained by
injection of hydrosoluble surfactants at the air/water interface. We show that
the flow extent increases with a decrease of the critical micelle
concentration, i.e. the concentration at which these surfactants self-assemble
in water. We document the universality of the surface velocity field and
predict scaling laws based on hydrodynamics and surfactant physicochemistry
that capture the flow features.Comment: 5 pages, 4 figures, submitte
Flow probe of symmetry energy in relativistic heavy-ion reactions
Flow observables in heavy-ion reactions at incident energies up to about 1
GeV per nucleon have been shown to be very useful for investigating the
reaction dynamics and for determining the parameters of reaction models based
on transport theory. In particular, the elliptic flow in collisions of
neutron-rich heavy-ion systems emerges as an observable sensitive to the
strength of the symmetry energy at supra-saturation densities. The comparison
of ratios or differences of neutron and proton flows or neutron and hydrogen
flows with predictions of transport models favors an approximately linear
density dependence, consistent with ab-initio nuclear-matter theories.
Extensive parameter searches have shown that the model dependence is comparable
to the uncertainties of existing experimental data. Comprehensive new flow data
of high accuracy, partly also through providing stronger constraints on model
parameters, can thus be expected to improve our knowledge of the equation of
state of asymmetric nuclear matter.Comment: 20 pages, 24 figures, review to appear in EPJA special volume on
nuclear symmetry energ
Classification of Reductive Monoid Spaces Over an Arbitrary Field
In this semi-expository paper we review the notion of a spherical space. In
particular we present some recent results of Wedhorn on the classification of
spherical spaces over arbitrary fields. As an application, we introduce and
classify reductive monoid spaces over an arbitrary field.Comment: This is the final versio
Estimation of absorption line indices of early-type galaxies using colours
Context. Absorption line indices are widely used to determine the stellar
population parameters such as age and metallicity of galaxies, but it is not
easy to obtain the line indices of some distant galaxies that have colours
available. Aims. This paper investigates the correlations between absorption
line indices and colours. Methods. A few statistical fitting methods are mainly
used, via both the observational data of Sloan Digital Sky Survey and a widely
used theoretical stellar population model. Results. Some correlations between
widely used absorption line indices and ugriz colours are found from both
observational data of early-type galaxies and a theoretical simple stellar
population model. In particular, good correlations between colours and widely
used absorption line indices such as Dn(4000), HgammaA, HgammaF, HdeltaA, Mg1,
Mg2, and Mgb, are shown in this paper. Conclusions. Some important absorption
line indices of early-type galaxies can be estimated from their colours using
correlations between absorption line indices and colours. For example,
age-sensitive absorption line indices can be estimated from (u-r) or (g-r)
colours and metallicity-sensitive ones from (u - z) or (g - z). This is useful
for studying the stellar populations of distant galaxies, especially for
statistical investigations.Comment: 9 pages, 21 figures, will be shown in A&
- …