51 research outputs found
Demarcation of potentially mineral-deficient areas in Central and Northern Namibia by means of natural classification systems
Mineral deficiencies that lead to production losses often occur concurrently with climatic and management
changes. To diagnose these deficiencies in time to prevent production losses, long-term monitoring
of mineral status is advisable. Different classification systems were examined to determine whether
areas of possible mineral deficiencies could be identified, so that those which were promising could
then be selected for further monitoring purposes. The classification systems addressed differences in
soil, vegetation and geology, and were used to define the cattle-ranching areas in the central and northern
districts of Namibia.
Copper (Cu), iron (Fe), zinc (Zn), manganese (Mn) and cobalt (Co) concentrations were determined in
cattle livers collected at abattoirs. Pooled faecal grab samples and milk samples were collected by farmers,
and used to determine phosphorus (P) and calcium (Ca), and iodine (I) status, respectively.
Areas of low P concentrations could be identified by all classification systems. The lowest P concentrations
were recorded in samples from the Kalahari-sand area, whereas faecal samples collected from
cattle on farms in the more arid areas, where the harder soils are mostly found, rarely showed low P
concentrations.
In the north of the country, low iodine levels were found in milk samples collected from cows grazing on
farms in the northern Kalahari broad-leaved woodland. Areas supporting animals with marginal Cu status,
could be effectively identified by the detailed soil-classification system of irrigation potential. Copper
concentrations were lowest in areas of arid soils, but no indication of Co, Fe, Zn, or Mn deficiencies
were found . For most minerals, the geological classification was the best single indicator of areas of
lower concentrations. Significant monthly variation for all minerals could also be detected within the
classification system .
It is concluded that specific classification systems can be useful as indicators of areas with lower mineral
concentrations or possible deficiencies.The articles have been scanned in colour with a HP Scanjet 5590; 600dpi.
Adobe Acrobat X Pro was used to OCR the text and also for the merging and conversion to the final presentation PDF-format.mn201
Chiral nonperturvative approach to the isoscalar s-wave pion-pion interaction in a nuclear medium
The s-wave isoscalar amplitude for pion-pion scattering in a nuclear medium
is evaluated using a nonperturbative unitary coupled channels method and the
standard chiral Lagrangians. The method has proved successful to describe the
pion-pion properties in the scalar isoscalar channel up to 1.2 GeV giving rise
to poles in the t matrix for the f0(980) and the sigma. The extension of the
method to the nuclear medium implies not only the renormalization of the pions
in the medium, but also the introduction of interaction terms related to
contact terms in the pion-nucleon to pion-pion-nucleon interaction. Off shell
effects are also shown to be important leading to cancellations which reduce
the coupled channel integral equations to a set of algebraic equations. As the
density increases we find a reduction of strength below the region and
a certain accumulation of strength at energies around pion threshold. Our
results, based on chiral Lagrangians, provide similar results to those obtained
with phenomenological models which impose minimal chiral constraints.Comment: 18 pages, LaTeX, 8 PostScript figures.(Revised version Aug-98, with
corrected results and new references
The usefulness of faecal phosphorus and nitrogen in interpreting differences in live-mass gain and the response to P supplementation in grazing cattle in arid regions
The average daily gains of heifers and oxen on commercial and experimental farms in Namibia were
used to indicate production differences in several areas and at different rates of phosphorus and protein
supplementation. Faecal concentrations of phosphorus and nitrogen were used to indicate concentrations
of these nutrients in grazing.
Areas with high concentrations of nitrogen in faeces proved to support high levels of average daily gain.
Animals responded positively to phosphorus supplementation only when faecal nitrogen concentrations
were above 12 g/kg DM. Nitrogen concentrations in faeces were directly related to average daily gain
of heifers, but protein supplementation did not have a significantly positive effect on average daily gain.The articles have been scanned in colour with a HP Scanjet 5590; 600dpi.
Adobe Acrobat X Pro was used to OCR the text and also for the merging and conversion to the final presentation PDF-format.mn201
Unitary chiral dynamics in J/Psi to VPP decays and the role of scalar mesons
We make a theoretical study of the J/Psi decays into \omega\pi\pi,
\phi\pi\pi, \omega K \bar{K} and \phi K\bar{K} using the techniques of the
chiral unitary approach stressing the important role of the scalar resonances
dynamically generated through the final state interaction of the two
pseudoscalar mesons. We also discuss the importance of new mechanisms with
intermediate exchange of vector and axial-vector mesons and the role played by
the OZI rule in the J/\Psi\phi\pi\pi vertex, quantifying its effects. The
results nicely reproduce the experimental data for the invariant mass
distributions in all the channels considered.Comment: 29 pages, 10 figure
Observational hints on the Big Bounce
In this paper we study possible observational consequences of the bouncing
cosmology. We consider a model where a phase of inflation is preceded by a
cosmic bounce. While we consider in this paper only that the bounce is due to
loop quantum gravity, most of the results presented here can be applied for
different bouncing cosmologies. We concentrate on the scenario where the scalar
field, as the result of contraction of the universe, is driven from the bottom
of the potential well. The field is amplified, and finally the phase of the
standard slow-roll inflation is realized. Such an evolution modifies the
standard inflationary spectrum of perturbations by the additional oscillations
and damping on the large scales. We extract the parameters of the model from
the observations of the cosmic microwave background radiation. In particular,
the value of inflaton mass is equal to GeV. In
our considerations we base on the seven years of observations made by the WMAP
satellite. We propose the new observational consistency check for the phase of
slow-roll inflation. We investigate the conditions which have to be fulfilled
to make the observations of the Big Bounce effects possible. We translate them
to the requirements on the parameters of the model and then put the
observational constraints on the model. Based on assumption usually made in
loop quantum cosmology, the Barbero-Immirzi parameter was shown to be
constrained by from the cosmological observations. We have
compared the Big Bounce model with the standard Big Bang scenario and showed
that the present observational data is not informative enough to distinguish
these models.Comment: 25 pages, 8 figures, JHEP3.cl
Chiral unitary approach to S-wave meson baryon scattering in the strangeness S=0 sector
We study the S-wave interaction of mesons with baryons in the strangeness S=0
sector in a coupled channel unitary approach. The basic dynamics is drawn from
the lowest order meson baryon chiral Lagrangians. Small modifications inspired
by models with explicit vector meson exchange in the t-channel are also
considered. In addition the pi pi N channel is included and shown to have an
important repercussion in the results, particularly in the isospin 3/2 sector.Comment: 23 pages, LaTeX, 21 figure
Separable potential model for interactions at low energies
The effective separable meson-baryon potentials are constructed to match the
equivalent chiral amplitudes up to the second order in external meson momenta.
We fit the model parameters (low energy constants) to the threshold and low
energy data. In the process, the -proton bound state problem is
solved exactly in the momentum space and the 1s level characteristics of the
kaonic hydrogen are computed simultaneously with the available low energy
cross sections. The model is also used to describe the
mass spectrum and the energy dependence of the amplitude.Comment: 31 pages, v2 - added corrections to make it compatible with the
published versio
Chiral Symmetry and light resonances in hot and dense matter
We present a study of the scattering amplitude in the and
channels at finite temperature and nuclear density within a chiral
unitary framework. Meson resonances are dynamically generated in our approach,
which allows us to analyze the behavior of their associated scattering poles
when the system is driven towards chiral symmetry restoration. Medium effects
are incorporated in three ways: (a) by thermal corrections of the unitarized
scattering amplitudes, (b) by finite nuclear density effects associated to a
renormalization of the pion decay constant, and complementarily (c) by
extending our calculation of the scalar-isoscalar channel to account for finite
nuclear density and temperature effects in a microscopic many-body
implementation of pion dynamics. Our results are discussed in connection with
several phenomenological aspects relevant for nuclear matter and Heavy-Ion
Collision experiments, such as mass scaling vs broadening from dilepton
spectra and chiral restoration signals in the channel. We also
elaborate on the molecular nature of resonances.Comment: 14 pages, 14 figures. Contribution to Hard Probes 2008, Illa de A
Toxa, Spain, June 8th-14th 200
Effective Lagrangian Approach to the Theory of Eta Photoproduction in the Region
We investigate eta photoproduction in the resonance region
within the effective Lagrangian approach (ELA), wherein leading contributions
to the amplitude at the tree level are taken into account. These include the
nucleon Born terms and the leading -channel vector meson exchanges as the
non-resonant pieces. In addition, we consider five resonance contributions in
the - and - channel; besides the dominant , these are:
and . The amplitudes for the
and the photoproduction near threshold have significant
differences, even as they share common contributions, such as those of the
nucleon Born terms. Among these differences, the contribution to the
photoproduction of the -channel excitation of the is the most
significant. We find the off-shell properties of the spin-3/2 resonances to be
important in determining the background contributions. Fitting our effective
amplitude to the available data base allows us to extract the quantity
, characteristic of the
photoexcitation of the resonance and its decay into the
-nucleon channel, of interest to precise tests of hadron models. At the
photon point, we determine it to be from
the old data base, and from a
combination of old data base and new Bates data. We obtain the helicity
amplitude for to be from the old data base, and from the combination of the old data base and new Bates
data, compared with the results of the analysis of pion photoproduction
yielding , in the same units.Comment: 43 pages, RevTeX, 9 figures available upon request, to appear in
Phys. Rev.
Photoproduction of the f2(1270) resonance
We have performed a calculation of the γp→π+π−p reaction, where the two pions have been separated in D-wave producing the f2(1270) resonance. We use elements of the local hidden gauge approach that provides the interaction of vector mesons in which the f2(1270) resonance appears as a ρ-ρ molecular state in L=0 and spin 2. The vector meson dominance, incorporated in the local hidden gauge approach converts a photon into a ρ0 meson and the other meson connects the photon with the proton. The picture is simple and has no free parameters, since the parameters of the theory have been constrained in the previous study of the vector-vector states. In a second step we introduce new elements, not present in the local hidden gauge approach, adapting the ρ propagator to Regge phenomenology and introducing the ρNN tensor coupling. We find that both the differential cross section as well as the t dependence of the cross section are in good agreement with the experimental results and provide support for the molecular picture of the f2(1270) in the first baryonic reaction where it has been tested
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