23 research outputs found
Covariant representations of the relativistic Brueckner T-matrix and the nuclear matter problem
We investigate nuclear matter properties in the relativistic Brueckner
approach. The in-medium on-shell T-matrix is represented covariantly by five
Lorentz invariant amplitudes from which we deduce directly the nucleon
self-energy. We discuss the ambiguities of this approach and the failure of
previously used covariant representations in reproducing the nucleon
self-energies on the Hartree-Fock level. To enforce correct Hartree-Fock
results we develop a subtraction scheme which treats the bare nucleon-nucleon
potential exactly in accordance to the different types of meson exchanges. For
the remaining ladder kernel, which contains the higher order correlations, we
employ then two different covariant representations in order to study the
uncertainty inherent in the approach. The nuclear matter bulk properties are
only slightly sensitive on the explicit representation used for the kernel.
However, we obtain new Coester lines for the various Bonn potentials which are
shifted towards the empirical region of saturation. In addition the nuclear
equation-of-state turns out to be significantly softer in the new approach.Comment: 39 pages Latex using Elsevier style, 16 PS figure
Scalar and vector decomposition of the nucleon self-energy in the relativistic Brueckner approach
We investigate the momentum dependence of the nucleon self-energy in nuclear
matter. We apply the relativistic Brueckner-Hartree-Fock approach and adopt the
Bonn A potential. A strong momentum dependence of the scalar and vector
self-energy components can be observed when a commonly used pseudo-vector
choice for the covariant representation of the T-matrix is applied. This
momentum dependence is dominated by the pion exchange. We discuss the problems
of this choice and its relations to on-shell ambiguities of the T-matrix
representation. Starting from a complete pseudo-vector representation of the
T-matrix, which reproduces correctly the pseudo-vector pion-exchange
contributions at the Hartree-Fock level, we observe a much weaker momentum
dependence of the self-energy. This fixes the range of the inherent uncertainty
in the determination of the scalar and vector self-energy components. Comparing
to other work, we find that extracting the self-energy components by a fit to
the single particle potential leads to even more ambiguous results.Comment: 35 pages RevTex, 7 PS figures, replaced by a revised and extended
versio
Efficiency and possibilities for Arabica coffee-banana management systems switching in the Mt. Elgon landscape of Uganda
Sustainably intensifying rural agricultural systems is now a
development goal that has gained momentum in the recent decades due to
a rapidly growing population and feeds directly into the Sustainable
Development Goals of ending poverty and hunger. By 2050, the world will
be inhabited by 10 billion people, 68% of whom will be city dwellers
which will pose serious food and livelihoods security threats to
millions of people, especially in Sub-Saharan Africa. The objective of
this study was to analyse technical efficiency of four production
systems in Arabica coffee-banana farming system of the Mt. Elgon in
Uganda and assesses possibilities for switching from one system to
another. The study was motivated by the notion that smallholder farmers
do not easily adopt new systems because of opportunity costs related to
input substitution, input and/or efficiency reduction and systems
redesigning. We estimated a production function to measure technical
efficiency and ordered the intensification pathways to create a
Technical Efficiency (TE) gradient. An ordered logit model was then
estimated to determine the factors influencing farmers to switch among
systems, by adopting one or more following a TE gradient. Results
showed that farmers produced 50% of the maximum possible Arabica coffee
output, indicating huge gaps between actual and potential yields. Use
of fertiliser for the lowest efficiency. Low-input-low-output pathway
and improved coffee genotypes, manure and labour intensification for
the higher technical efficiency clusters such as conventional and mild
agroecological would also significantly increase the chances of
switching from low to highly efficient and sustainable Arabica coffee
production systems in the Mt. Elgon watershed of Uganda.L\u2019intensification durable des syst\ue8mes agricoles ruraux est
un objectif de d\ue9veloppement durable qui a\ua0\ue9t\ue9
pris\ua0en compte pour mettre fin \ue0 la pauvret\ue9 et la faim
dans une population en croissance rapide. Le monde sera habit\ue9 par
10 milliards de personnes, dont 68% seront des citadins en 2050, ce qui
posera de graves menaces \ue0 la s\ue9curit\ue9 alimentaire et
des moyens de subsistance de millions de personnes, en particulier en
Afrique Sub-Saharienne. L\u2019objectif de cette \ue9tude \ue9tait
d\u2019analyser l\u2019efficacit\ue9 de technique de quatre
syst\ue8mes de production dans le syst\ue8me de culture
caf\ue9-banane Arabica du mont Elgon en Ouganda et \ue9value les
possibilit\ue9s de passer d\u2019un syst\ue8me \ue0 un autre.
L\u2019\ue9tude \ue9tait motiv\ue9e par l\u2019id\ue9e que
les petits exploitants agricoles n\u2019adoptent pas facilement de
nouveaux syst\ue8mes en raison des co\ufbts
d\u2019opportunit\ue9 li\ue9s \ue0 la substitution des intrants,
la r\ue9duction des intrants et / ou de l\u2019efficacit\ue9 et la
refonte des syst\ue8mes. Nous avons estim\ue9 une fonction de
production pour mesurer l\u2019efficacit\ue9 de technique et nous
avons ordonn\ue9 les voies d\u2019intensification pour cr\ue9er un
gradient l\u2019efficacit\ue9 de technique (TE). Un mod\ue8le
logit ordonn\ue9 a ensuite \ue9t\ue9 estim\ue9 pour
d\ue9terminer les facteurs qui poussent les agriculteurs \ue0
basculer entre les syst\ue8mes, en adoptant un ou plusieurs suivant
un gradient TE. Les r\ue9sultats ont montr\ue9 que les agriculteurs
produisaient 50% de la production maximale possible de caf\ue9
Arabica, indiquant d\u2019\ue9normes \ue9carts entre les
rendements r\ue9els et potentiels. Utilisation d\u2019engrais pour
une efficacit\ue9 minimale. Une fili\ue8re \ue0 faibles intrants
et \ue0 faible rendement et des g\ue9notypes de caf\ue9
am\ue9lior\ue9s, du fumier et une intensification de la
main-d\u2019\u153uvre pour les grappes \ue0 plus haute
efficacit\ue9 de technique telles que l\u2019agro\ue9cologie
conventionnelle et douce augmenteraient \ue9galement
consid\ue9rablement les chances de passer de syst\ue8mes de
production de caf\ue9 Arabica faibles \ue0 tr\ue8s efficaces et
durables dans le bassin versant du Mont Elgon en Ouganda
Off shell behaviour of the in medium nucleon-nucleon cross section
The properties of nucleon-nucleon scattering inside dense nuclear matter are
investigated. We use the relativistic Brueckner-Hartree-Fock model to determine
on-shell and half off-shell in-medium transition amplitudes and cross sections.
At finite densities the on-shell cross sections are generally suppressed. This
reduction is, however, less pronounced than found in previous works. In the
case that the outgoing momenta are allowed to be off energy shell the
amplitudes show a strong variation with momentum. This description allows to
determine in-medium cross sections beyond the quasi-particle approximation
accounting thereby for the finite width which nucleons acquire in the dense
nuclear medium. For reasonable choices of the in-medium nuclear spectral width,
i.e. MeV, the resulting total cross sections are, however,
reduced by not more than about 25% compared to the on-shell values. Off-shell
effect are generally more pronounced at large nuclear matter densities.Comment: 31 pages Revtex, 12 figures, typos corrected, to appear in Phys. Rev.
The Relativistic Dirac-Brueckner Approach to Asymmetric Nuclear Matter
The properties of asymmetric nuclear matter have been investigated in a
relativistic Dirac-Brueckner-Hartree-Fock framework using the Bonn A potential.
The components of the self-energies are extracted by projecting on Lorentz
invariant amplitudes. Furthermore, the optimal representation scheme for the
matrix, the subtracted matrix representation, is applied and the
results are compared to those of other representation schemes. Of course, in
the limit of symmetric nuclear matter our results agree with those found in
literature. The binding energy fulfills the quadratic dependence on the
asymmetry parameter and the symmetry energy is 34 MeV at saturation density.
Furthermore, a neutron-proton effective mass splitting of is
found. In addition, results are given for the mean-field effective coupling
constants.Comment: 28 pages, 7 figures, to appear in Nucl. Phys. A, added additional
reference