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Agriculture biologique et verrouillage des systèmes de connaissances: Conventionalisation des filières agroalimentaire bio

La question de la co-existence entre deux systèmes bio, l’un porté par les acteurs historiques, l’autre par les nouveaux acteurs agroindustriels, selon un modèle de bifurcation, est traitée à travers l’hypothèse de conventionalisation. Nous utilisons le cas du système Blanc Bleu Belge et de l’élevage bovin bio belge pour montrer que le problème de la co-existence de ces deux systèmes est à la fois une question empirique et une question théorique. Notre argument est que le concept de « référentiel » (au sens de Jobert et Muller, 1987) est non seulement supérieur d’un point de vue théorique mais qu’il ouvre d’intéressantes perspectives pratiques et ceci pour deux raisons. D’abord parce qu’il permet de comprendre la question et l’enjeu de la co-existence de deux systèmes de connaissances différents, l’un alternatif et l’autre conventionnel. Ensuite parce qu’il permet de comprendre les effets d’irréversibilité qui rendent les systèmes conventionnel et biologique incompatibles l’un avec l’autre

A Model for the Off-forward Structure Functions of the Pion

We extend our model for the pion, which we used previously to calculate its diagonal structure function, to the off-forward case. The imaginary part of the off-forward gamma* pi -> gamma* pi scattering amplitude is evaluated in the chiral limit (m_pi=0) and related to the twist-two and twist-three generalised parton distributions, H, H3, H3tilde. Non-perturbative effects, linked to the size of the pion and still preserving gauge invariance, are included. Remarkable new relations between H, H3 and H3tilde are obtained and discussed.Comment: 9 pages, 20 figures, LaTeX, uses ulg_ptf.cls (included); v2: Form factors dropped in favour of GPD's, comparison between Wandzura-Wilczek approximation and our relations adde

The Valence Quark Distribution of the Pion

The pion structure function is investigated in a simple model, where the pion and its constituent quark fields are coupled through the simplest pseudoscalar coupling. The imaginary part of the forward gamma* pi -> gamma* pi scattering amplitude is evaluated and related to the structure functions. It is shown that the introduction of non-perturbative effects, linked to the size of the pion, allows a connection with the quark distribution. It is predicted that higher-twist terms become negligible for Q^2 larger than ~2 GeV^2, that quarks in the pion have a momentum fraction smaller than in the proton case, and that the momentum sum rule is violated for the pion.Comment: Presented by J. P. Lansberg at the XXII Physics in Collision Conference (PIC02), Stanford, California, June 20-22, 2002, 3 pages, 4 figures, LaTeX, uses pic02.sty (included

A Model for the Generalized Parton Distribution of the Pion

We calculate the off-forward structure function of the pion within a simple model where the size of the pion is accounted for using a momentum cut-off. Twist-two and twist-three generalized parton distributions are extracted. Relations between twist-three and twist-two contributions are obtained, the origin of which is not kinematical as they differ from those arising from the Wandzura-Wilczek approximation.Comment: Presented at 6th Conference on Quark Confinement and the Hadron Spectrum, Villasimius, Sardinia, Italy, 21-25 Sep 2004, 3 pages, 4 figures, LaTeX, uses aip-6s.clo, aipproc.cls and aipxfm.sty (included

AGROECOLOGIE : LE CHAINON MANQUANT ROLE DE CONSOMMATEURS ET D'ONG DANS LES PROCESSUS EMERGEANT D'APPRENTISSAGES

N° ISBN - 978-2-7380-1284-5International audienceAcademic texts on agro-ecology are often too limited to production systems. When notions of food systems and food value chains are described they introduce the consumer as a choice taker, reflecting neoclassical economic theory. The study of the emergence of two agroecological chains in Benin and Belgium explains how consumer demands are key learning resources for chain co-creation. In the case of the fair trade rice chain an NGO stimulates the cocreation process while in the beef chain in Gaume consumers who are directly involved in a deliberative process bring about this questioning. Agroecology, Food Systems, Participatory Research, Consumers, Learning process, Food Chain, Knowledge's, Biodiversity, Stock Farming, Rice Farmin