65 research outputs found
Comparison of exact and approximate cross-sections in relativistic Coulomb excitation
We present a new method of obtaining time-dependent matrix elements of the
electromagnetic pulse produced by a highly-relativistic projectile. These
matrix elements are used in a coupled-channel calculation to predict the
cross-sections for population of 1- and 2-phonon states of the giant dipole
resonance. Comparisons are made with the predictions of the long-wavelength and
Born approximations.Comment: 26 pages, LaTex2
Light-Front Holography, Light-Front Wavefunctions, and Novel QCD Phenomena
Light-Front Holography, a remarkable feature of the AdS/CFT correspondence,
maps amplitudes in anti-de Sitter (AdS) space to frame-independent light-front
wavefunctions of hadrons in physical space-time. The model leads to an
effective confining light-front QCD Hamiltonian and a single-variable
light-front Schrodinger equation which determines the eigenspectrum and the
light-front wavefunctions of hadrons for general spin and orbital angular
momentum. The coordinate z in AdS space is identified with a Lorentz-invariant
coordinate zeta which measures the separation of the constituents within a
hadron at equal light-front time and determines the off-shell dynamics of the
bound-state wavefunctions and the fall-off in the invariant mass of the
constituents. The soft-wall holographic model, modified by a positive-sign
dilaton metric, leads to a remarkable one-parameter description of
nonperturbative hadron dynamics -- a semi-classical frame-independent first
approximation to the spectra and light-front wavefunctions of meson and
baryons. The model predicts a Regge spectrum of linear trajectories with the
same slope in the leading orbital angular momentum L of hadrons and the radial
quantum number n. The hadron eigensolutions projected on the free Fock basis
provides the complete set of valence and non-valence light-front Fock state
wavefunctions which describe the hadron's momentum and spin distributions
needed to compute measures of hadron structure at the quark and gluon level.
The effective confining potential also creates quark- antiquark pairs. The
AdS/QCD model can be systematically improved by using its complete orthonormal
solutions to diagonalize the full QCD light-front Hamiltonian or by applying
the Lippmann-Schwinger method to systematically include the QCD interaction
terms. A new perspective on quark and gluon condensates is also presented.Comment: Presented at LIGHTCONE 2011, 23 - 27 May, 2011, Dallas, T
NEOTROPICAL XENARTHRANS: a data set of occurrence of xenarthran species in the Neotropics
Xenarthrans â anteaters, sloths, and armadillos â have essential functions for ecosystem maintenance, such as insect control and nutrient cycling, playing key roles as ecosystem engineers. Because of habitat loss and fragmentation, hunting pressure, and conflicts with 24 domestic dogs, these species have been threatened locally, regionally, or even across their full distribution ranges. The Neotropics harbor 21 species of armadillos, ten anteaters, and six sloths. Our dataset includes the families Chlamyphoridae (13), Dasypodidae (7), Myrmecophagidae (3), Bradypodidae (4), and Megalonychidae (2). We have no occurrence data on Dasypus pilosus (Dasypodidae). Regarding Cyclopedidae, until recently, only one species was recognized, but new genetic studies have revealed that the group is represented by seven species. In this data-paper, we compiled a total of 42,528 records of 31 species, represented by occurrence and quantitative data, totaling 24,847 unique georeferenced records. The geographic range is from the south of the USA, Mexico, and Caribbean countries at the northern portion of the Neotropics, to its austral distribution in Argentina, Paraguay, Chile, and Uruguay. Regarding anteaters, Myrmecophaga tridactyla has the most records (n=5,941), and Cyclopes sp. has the fewest (n=240). The armadillo species with the most data is Dasypus novemcinctus (n=11,588), and the least recorded for Calyptophractus retusus (n=33). With regards to sloth species, Bradypus variegatus has the most records (n=962), and Bradypus pygmaeus has the fewest (n=12). Our main objective with Neotropical Xenarthrans is to make occurrence and quantitative data available to facilitate more ecological research, particularly if we integrate the xenarthran data with other datasets of Neotropical Series which will become available very soon (i.e. Neotropical Carnivores, Neotropical Invasive Mammals, and Neotropical Hunters and Dogs). Therefore, studies on trophic cascades, hunting pressure, habitat loss, fragmentation effects, species invasion, and climate change effects will be possible with the Neotropical Xenarthrans dataset
TRY plant trait database â enhanced coverage and open access
Plant traitsâthe morphological, anatomical, physiological, biochemical and phenological characteristics of plantsâdetermine how plants respond to environmental factors, affect other trophic levels, and influence ecosystem properties and their benefits and detriments to people. Plant trait data thus represent the basis for a vast area of research spanning from evolutionary biology, community and functional ecology, to biodiversity conservation, ecosystem and landscape management, restoration, biogeography and earth system modelling. Since its foundation in 2007, the TRY database of plant traits has grown continuously. It now provides unprecedented data coverage under an open access data policy and is the main plant trait database used by the research community worldwide. Increasingly, the TRY database also supports new frontiers of traitâbased plant research, including the identification of data gaps and the subsequent mobilization or measurement of new data. To support this development, in this article we evaluate the extent of the trait data compiled in TRY and analyse emerging patterns of data coverage and representativeness. Best species coverage is achieved for categorical traitsâalmost complete coverage for âplant growth formâ. However, most traits relevant for ecology and vegetation modelling are characterized by continuous intraspecific variation and traitâenvironmental relationships. These traits have to be measured on individual plants in their respective environment. Despite unprecedented data coverage, we observe a humbling lack of completeness and representativeness of these continuous traits in many aspects. We, therefore, conclude that reducing data gaps and biases in the TRY database remains a key challenge and requires a coordinated approach to data mobilization and trait measurements. This can only be achieved in collaboration with other initiatives
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