124 research outputs found
Production, Persistence and Diversity of Species in Temperate Grasslands
Temperate grassland areas are considered to be semi-arid, with rainfall typically between 250 and 500 mm, much of it occurring in the late spring and early summer. Grazing plays an important role in all these grasslands with impacts on diversity and persistence. Against this assumption that seasonal productivity would be more uniform within a pasture that was diverse, the research suggested that species rich pastures were less stable, because this species richness was composed of non-perennial, volunteer and weed species. This paper explores the concept that systems diversity of temperate grassland does not always equate to production. In these systems the functional role of perennial grasses is important both for the productive capacity and protection of soil through the maintenance of cover, hence represents the key element which needs to be preserved through management strategies
Longitudinal-Transverse Separations of Structure Functions at Low for Hydrogen and Deuterium
We report on a study of the longitudinal to transverse cross section ratio,
, at low values of and , as determined from
inclusive inelastic electron-hydrogen and electron-deuterium scattering data
from Jefferson Lab Hall C spanning the four-momentum transfer range 0.06 GeV. Even at the lowest values of , remains
nearly constant and does not disappear with decreasing , as expected. We
find a nearly identical behaviour for hydrogen and deuterium.Comment: 4 pages, 2 gigure
Measurements of electron-proton elastic cross sections for
We report on precision measurements of the elastic cross section for
electron-proton scattering performed in Hall C at Jefferson Lab. The
measurements were made at 28 unique kinematic settings covering a range in
momentum transfer of 0.4 5.5 . These measurements
represent a significant contribution to the world's cross section data set in
the range where a large discrepancy currently exists between the ratio of
electric to magnetic proton form factors extracted from previous cross section
measurements and that recently measured via polarization transfer in Hall A at
Jefferson Lab.Comment: 17 pages, 18 figures; text added, some figures replace
Charged pion form factor between =0.60 and 2.45 GeV. I. Measurements of the cross section for the H() reaction
Cross sections for the reaction H() were measured in Hall
C at Thomas Jefferson National Accelerator Facility (JLab) using the CEBAF
high-intensity, continous electron beam in order to determine the charged pion
form factor. Data were taken for central four-momentum transfers ranging from
=0.60 to 2.45 GeV at an invariant mass of the virtual photon-nucleon
system of =1.95 and 2.22 GeV. The measured cross sections were separated
into the four structure functions , , , and
. The various parts of the experimental setup and the analysis
steps are described in detail, including the calibrations and systematic
studies, which were needed to obtain high precision results. The different
types of systematic uncertainties are also discussed. The results for the
separated cross sections as a function of the Mandelstam variable at the
different values of are presented. Some global features of the data are
discussed, and the data are compared with the results of some model
calculations for the reaction H().Comment: 26 pages, 23 figure
Separated Response Function Ratios in Exclusive, Forward pi^{+/-} Electroproduction
The study of exclusive electroproduction on the nucleon,
including separation of the various structure functions, is of interest for a
number of reasons. The ratio is
sensitive to isoscalar contamination to the dominant isovector pion exchange
amplitude, which is the basis for the determination of the charged pion form
factor from electroproduction data. A change in the value of
from unity at small , to 1/4 at
large , would suggest a transition from coupling to a (virtual) pion to
coupling to individual quarks. Furthermore, the mentioned ratios may show an
earlier approach to pQCD than the individual cross sections. We have performed
the first complete separation of the four unpolarized electromagnetic structure
functions above the dominant resonances in forward, exclusive
electroproduction on the deuteron at central values of 0.6, 1.0, 1.6
GeV at =1.95 GeV, and GeV at =2.22 GeV. Here, we
present the and cross sections, with emphasis on and , and
compare them with theoretical calculations. Results for the separated ratio
indicate dominance of the pion-pole diagram at low , while results
for are consistent with a transition between pion knockout and quark
knockout mechanisms.Comment: 6 pages, 3 figure
Charged pion form factor between Q^2=0.60 and 2.45 GeV^2. II. Determination of, and results for, the pion form factor
The charged pion form factor, Fpi(Q^2), is an important quantity which can be
used to advance our knowledge of hadronic structure. However, the extraction of
Fpi from data requires a model of the 1H(e,e'pi+)n reaction, and thus is
inherently model dependent. Therefore, a detailed description of the extraction
of the charged pion form factor from electroproduction data obtained recently
at Jefferson Lab is presented, with particular focus given to the dominant
uncertainties in this procedure. Results for Fpi are presented for
Q^2=0.60-2.45 GeV^2. Above Q^2=1.5 GeV^2, the Fpi values are systematically
below the monopole parameterization that describes the low Q^2 data used to
determine the pion charge radius. The pion form factor can be calculated in a
wide variety of theoretical approaches, and the experimental results are
compared to a number of calculations. This comparison is helpful in
understanding the role of soft versus hard contributions to hadronic structure
in the intermediate Q^2 regime.Comment: 18 pages, 11 figure
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