4,142 research outputs found
Influence of latitude and climate on spread, radiation and rise to dominance of early angiosperms during the Cretaceous in the Northern Hemisphere
Our aim is to evaluate the influence of climate and latitude on the rise to dominance of angiosperms in space and time during the Cretaceous. The main objectives of the study are: 1) to determine whether a relationship existed between plant biogeographical distribution and Cretaceous climate changes; 2) to explore latitude-dependent forcing on early angiosperm ecology; 3) to propose a mechanism explaining the observed stages of radiation in early angiosperms. The study focuses on 18 Cretaceous megafossil localities and reviews on microfossils in the Northern Hemisphere. A database has been compiled using literature and personal unpublished data. The data document occurrences of micro- and megafossil plant remains including spores, pollen grains, leaves and whole plants. They are placed in context through the use of Cretaceous geographical maps and temperature curves or values. There is a clear correlation between latitude and the composition of Cretaceous floras. Latitudinal vegetation belts fluctuated in concert with climate changes during the Cretaceous. Differences in original plant associations may have driven the gradual plant turnover that resulted in the rise to dominance of early angiosperms during the Cretaceous. Cretaceous climate changes created dispersal bottlenecks. Bottlenecks induced the extinction of some plant groups and the radiation of others. Those that successfully radiated continued to spread
Coherent two pion photoproduction on 12C
We develop the formalism for coherent two pion photoproduction in nuclei and
perform actual calculations of cross sections for and
photoproduction on . We find that due to the isospin symmetry the cross
section for production is very small and has a maximum when the
pions propagate together. However, the kinematical region where the energies
and polar angles of the two mesons are equal and their relative
azimuthal angle is forbidden. Conversely in the
production the pions prefer to have a relative azimuthal angle 180 and the
production of the pions propagating together is suppressed. The dominant
one-body mechanism in both channels is related to the excitation of the
isobar. Hence the reaction can serve as a source of information about
's properties in nucleus. We have found that the reaction is sensitive
to effects of the pion and renormalization in the nuclear medium,
similar to those found in the coherent reaction, but magnified
because of the presence of the two pions.Comment: 17 pages LATEX and 11 postscript figure
Helicity asymmetries in double pion photoproduction on the proton
Based on a prior model on double pion photoproduction on the proton,
successfully tested in total cross sections and invariant mass distributions,
we make a theoretical study of the angular dependence of helicity asymmetries
from the interaction of circularly polarized photons with unpolarized protons.
We show that this observable is sensitive to details of the internal mechanisms
and, thus, represents a complementary test of the theoretical model.Comment: 16 pages, 6 figures, version accepted for publication in Nuclear
Physics
Les plantes fossiles du gisement hettangien de Talmont-Saint-Hilaire (Vendée, France) : intérêts systématique et paléoécologique
The Liassic Talmont-Saint-Hilaire (Vendée, France) outcrop has been studied here according to the abundance of fossil plants of the Cheirolepidiaceous family (Gymnosperms, Coniferales). A new species of Brachyphyllum has been studied in light microscopy, even in scanning electron microscopy and transmission electron microscopy. It is, at that time, one of the most complete study realized on a fossil taxon. Epidermal cells, subsidiary cells and guard cells have been described on both adaxial and abaxial surfaces, in transversal and longitudinal sections. Cuticle ultrastructural variations have been observed between epidermal and stomatal cells. To conclude, we propose a discussion about the cuticle structure variations related to environmental conditions and the functional morphology of the different cells
The reaction at threshold in chiral perturbation theory
In the framework of heavy baryon chiral perturbation theory, we give thIn the
framework of heavy baryon chiral perturbation theory, we give the chiral
expansion for the threshold amplitudes and to
quadratic order in the pion mass. The theoretical results agree within one
standard deviation with the empirical values. We also derive a relation between
the two threshold amplitudes of the reaction and the S--wave scattering lengths, and , respectively, to order
. We show that there are uncertainties mostly related to
resonance excitation which make an accurate determination of the
scattering length from the threshold amplitudes at present
very difficult. The situation is different in the isospin two final
state. Here, the chiral series converges and one finds consistent with the one--loop chiral perturbation theory prediction.Comment: 30 pp, LaTeX file, uses epsf, 6 figures (appended), corrections in
sections 5 and 6, conclusions unchange
Nonlocal SU(3) chiral quark models at finite temperature: the role of the Polyakov loop
We analyze the role played by the Polyakov loop in the description of the
chiral phase transition within the framework of nonlocal SU(3) chiral models
with flavor mixing. We show that its presence provides a substantial
enhancement of the predicted critical temperature, bringing it to a better
agreement with the most recent results of lattice calculations.Comment: 9 pages, 1 figur
Double Photoproduction off the Proton at Threshold
The reaction has been measured using the TAPS
BaF calorimeter at the tagged photon facility of the Mainz Microtron
accelerator. Chiral perturbation theory (ChPT) predicts that close to threshold
this channel is significantly enhanced compared to double pion final states
with charged pions. In contrast to other reaction channels, the lower order
tree terms are strongly suppressed in 2 photoproduction. The consequence
is the dominance of pion loops in the 2 channel close to threshold - a
result that opens new prospects for the test of ChPT and in particular its
inherent loop terms. The present measurement is the first which is sensitive
enough for a conclusive comparison with the ChPT calculation and is in
agreement with its prediction. The data also show good agreement with a
calculation in the unitary chiral approach.Comment: Submitted to PL
On the integrability of N=2 supersymmetric massive theories
In this paper we propose a criteria to establish the integrability of N=2
supersymmetric massive theories.The basic data required are the vacua and the
spectrum of Bogomolnyi solitons, which can be neatly encoded in a graph
(nodes=vacua and links= Bogomolnyi solitons). Integrability is then equivalent
to the existence of solutions of a generalized Yang-Baxter equation which is
built up from the graph (graph-Yang-Baxter equation). We solve this equation
for two general types of graphs: circular and daisy, proving, in particular,
the inte- grability of the following Landau-Ginzburg superpotentials: A_n(t_1),
A_n(t_2), D_n(\tau),E_6(t_7), E_8(t_16). For circular graphs the solutions are
intertwiners of the affine Hopf algebra , while for
daisy graphs the solution corresponds to a susy generalization of the Boltzmann
weights of the chiral Potts model in the trigonometric regime. A chiral Potts
like solution is conjectured for the more tricky case . The
scattering theory of circular models, for instance or ,
is Toda like. The physical spectrum of daisy models, as or
, is given by confined states of radial solitons. The scattering
theory of the confined states is again Toda like. Bootstrap factors for the
confined solitons are given by fusing the susy chiral Potts S-matrices of the
elementary constituents, i.e. the radial solitons of the daisy graph.Comment: 26 pages, Latex (this version replaces a previously corrupted one;
epic.sty macro needed, available from hep-th in compressed form
epic.sty.tar.Z
Bethe-Salpeter Approach for the Elastic Pion-Nucleon Scattering in Heavy Baryon Chiral Perturbation Theory
Heavy Baryon Chiral Perturbation Theory (HBChPT) to leading order provides a
kernel to solve the Bethe-Salpeter equation for the
(-channel) system, in the infinite nucleon mass limit.
Crossed Born terms include, when iterated within the Bethe-Salpeter equation,
both {\it all} one- and {\it some} two-pion intermediate states, hence
preserving elastic unitarity below the two-pion production threshold. This
suggests searching for a solution with the help of dispersion relations and
suitable subtraction constants, when all in-elasticities are explicitly
neglected. The solution allows for a successful description of the experimental
phase shift from threshold up to MeV in terms of four
subtraction constants. Next-to-leading order HBChPT calculations are also used
to estimate the unknown subtraction constants which appear in the solution.
Large discrepancies are encountered which can be traced to the slow convergence
rate of HBChPT.Comment: 11 pages, 3 figure
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