Pierre M Stassart

Chiral theory of mesons in dense baryonic matter (I)

Abstract Chiral symmetry restoration at finite baryonic density is studied in a quark model involving both scalar and vector interactions. The presence of vector interactions makes the vacuum stiffer against chiral symmetry restoration. On-shell masses and coupling constants are calculated for the π, ω, ϱ and a 1 mesons. An attempt is made to relate the quark-meson interactions to the observed nucleon-meson coupling constants.

Chiral mesons in dense nuclear matter

Abstract Chiral symmetry restoration together with the σ- and π-meson propagators are calculated in dense baryonic matter. The vacuum is described by a Nambu-Jona-Lasinio model of quarks. Nuclear matter is described by a Fermi sea of either quarks or nucleons. The two descriptions are found to give different results for chiral symmetry restoration at high densities.

A model for the off-forward structure functions of the pion

Abstract 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 γ★π→γ★π scattering amplitude is evaluated in the chiral limit (mπ=0) and related to the twist-two and twist-three generalised parton distributions H, H3, H 3 . Non-perturbative effects, linked to the size of the pion and still preserving gauge invariance, are included. Remarkable new relations between H, H3 and H 3 are obtained and discussed.

The current quark mass and the regularisation of the Nambu-Jona-Lasinio action

We show that the Nambu-Jona-Lasinio model can distinguish observables, such as the meson masses, the pion decay constant and the constituent quark mass, which have logarithmic divergences, from quadratically divergent quantities such as the quark condensate 〈yψ〉 or the current quark mass. The latter can be eliminated from the lagrangian which can then be regularized with a single subtraction involving a cut-off mass. The range of possible cut-offs is considerably enlarged when it is realized that the quadratically divergent current quark mass requires two subtractions and therefore two cut-off parameters.

Clarifying the socioeconomic dimensions of agroecology: between principles and practices

ABSTRACT The concept of agroecology is being mobilized increasingly. However, its socioeconomic dimensions receive little attention from academia. This study helps to clarify the socioeconomic principles of agroecology by first identifying a list of principles in popular and scientific literature and, as a second step, by putting the principles to the test of a qualitative study of two Belgian organizations. Agribio is a grain cooperative, and Les Grosses Légumes is a network of consumers, farmers, and the members of an association set up to organize the production and distribution of vegetable boxes. Semi-directed interviews of the various actors linked to these organizations were conducted and then analyzed through an approach inspired by the convention theory in order to reveal the principles that the stakeholders have adopted. The main findings are then made explicit by analysis of four strong agreements (which concern the two organizations’ marketing schemes, a Participatory Guarantee System set up by Les Grosses Légumes and Agribio’s flour mill). The two case studies show the gap that exists between the principles that describe the horizon of agroecology and the principles that are actually put into practice by the parties in the field through various transition pathways.

Vector Mesons and Partial Restoration of Chiral Symmetry

A Nambu-Jona-Lasinio model with vector interactions is considered. Vector mesons are found to reduce significantly the partial restoration of chiral symmetry at normal nuclear density. The quark loop term is expanded in terms of the field amplitudes thus yielding on-shell meson masses. Results are compared with those obtained in gradient expansions. The model can describe on-shell π, ρ and ω mesons as bound q excitations but it is unable to form bound a1 and f1 mesons.

Pion Structure Function and violation of the Momentum Sum Rule

We present a method to evaluate the pion structure functions from a box diagram calculation. Pion and constituent quark fields are coupled through the simplest pseudoscalar coupling. The γ*π → qq cross‐section is evaluated and related to the structure functions. We then show that the introduction of non‐perturbative effects, related to the pion size and preserving gauge invariance, provides us with a straighforward relation with the quark distribution. It is predicted that higher‐twist terms become negligible for Q2 larger than about 2 GeV2 and that quarks in the pion have a momentum fraction smaller than in the proton. We enlarge the discussion concerning this violation of the momentum sum rule, emphasizing that the sum rule is recovered in the chiral limit and also when the finite size condition is not imposed.

On the scalar meson exchange in the baryon spectra

We explore the role of a scalar meson exchange interaction between quarks in a semirelativistic constituent quark model where the quarks are also subject to a linear confinement. We study how the spectrum evolves when the strength of the scalar meson exchange increases. Our results are relevant to the long-standing problem of the relative position of the first positive parity (Roper resonance) and the first orbitally excited states in the baryon spectrum.

Negative parity non-strange baryons

Abstract Our previos study is extended to negative parity baryon resonances up to J = 9 2 − . The framework is a semi-relativistic constituent quark model. The quark-quark interaction contains a Coulomb plus linear confinement terms and a short distance spin-spin and tensor terms. It is emphasized that a linear confinement potential gives too large a mass to the D35(1930) resonance.

The transition to conservation agriculture: an insularization process towards sustainability

Part of the Sustainability Transition Studies, this work addresses the question of the relationship between niches and regimes by examining the transition to conservation agriculture. It seeks to understand how farmers’ transition to conservation agriculture can contribute to a better understanding of the transition of agro-food systems towards sustainability. Based on an analysis of farmers’ trajectories in the Walloon region in Belgium, the paper develops the notion of insularization in order to characterize the emergence of conservation agriculture as a niche that is a dynamic process, growing from within and progressively detaching itself from the conventional agricultural regime. The analysis of farmers’ transition shows how, after an initial phase of destabilization of the conventional ploughing regime, learning and experiencing processes can lead to a transformation in soil and soil quality management perceptions. Our hypothesis is that this cognitive transformation constitutes a tipping point in the insularization process because of its effects on agricultural practices, which increase the detachment of conservation agriculture from the regime and thus embed the irreversibility and sustainability of the transition. Insularization describes an ecologizational pathway of agricultural practices endogenous to the regime that can not only lead to adaptive changes on the periphery of the system, but might also induce a deep and systemic transformation of conventional agricultural practices.