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. $\Gamma\leq 40$ 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 $T$ matrix, the subtracted $T$ 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 $E_b$ 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 $m_n^* < m_p^*$ 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