G. Chanfray

Chiral Lagrangians and quark condensate in nuclei

Abstract We study the evolution with density of the quark condensate in the nuclear medium with interacting nucleons and including the short-range correlations. We work with two chiral models, the linear sigma model and the non-linear one. For the last one we use two versions, one which does not satisfy PCAC, and another one which does. We show that the quark condensate, as well as the effective pion mass, is independent on the variant selected. The application to physical pions excludes the linear sigma model as a credible one. In the non-linear models our conclusions are the following: (i) There is no systematic reaction imposed by chiral symmetry against symmetry restoration. (ii) The quark condensate evolves essentially linearly with density, as if the nucleons were non-interacting. We examine here one correction due to the two-pion-exchange potential and found it to be negligible. The main correction arises from the one-pion-exchange interaction. (iii) In the s-wave optical potential chiral symmetry tolerates but does not impose two-body terms. On the other hand, the effect of correlations linked to the isospin-symmetric amplitude is negligible.

Two-pion production processes, chiral symmetry and NN interaction in the medium

Abstract.We study the two-pion propagator in the nuclear medium. This quantity appears in the ππ T-matrix and we show that it also enters the QCD scalar susceptibility. The medium effects on this propagator are due to the influence of the individual nucleon response to a scalar field through their pion clouds. This response is appreciably increased by the nuclear environment. It produces an important convergence effect between the scalar and pseudoscalar susceptibilities, reflecting the reshaping of the scalar strength observed in 2π production experiments. While a large modifications of the σ propagator follows, due to its coupling to two pion states, we show that the NN potential remains instead unaffected.

Proton and neutron contributions to the charge longitudinal response

Abstract The charge longitudinal response of nuclei, as measured by inclusive inelastic electron scattering, is investigated in the semiclassical RPA framework. The proton and neutron contributions to the total response are explicitly separated out. It is found that a sizable neutron ejection can be induced by RPA correlations, particularly at low momentum and energy transfer. A comparison of our theory with the experimental data is presented, also in the case of asymmetric nuclei ( N ≠ Z ). In particular, the differences in the neutron contributions between 30 Ca and 48 Ca are discussed.

Constraints on nuclear-matter properties from QCD susceptibilities

Abstract.In the framework of chiral theories, we establish the interrelation between the responses of the nuclear medium to probes which couple either to the quark scalar density fluctuation (i.e. the scalar susceptibility of quantum chromodynamics) or to the nucleonic ones. The second case concerns in fact the response to the scalar meson responsible for the nuclear binding in relativistic theories. In our approach both responses incorporate the nucleonic contributions. The aim of our work is to exploit this relation and the informations from lattice QCD to constrain the parameters which enter the problem of the nuclear binding. We show that in order to achieve a successful description of the binding properties, it is necessary to introduce the nucleon structure and confinement, in particular for the description of the three-body forces.

The semi-classical approach to the exclusive electron scattering

Abstract The semi-classical approach, successfully applied in the past to the inelastic, inclusive electron scattering off nuclei, is extended to the treatment of exclusive processes. The associated formalism goes beyond the plane wave impulse approximation, since it can account also for the final states interaction in the mean field approximation, respecting the Pauli principle. We then explore, in the frame of two admittedly crude approximations, the impact on the exclusive cross section of the distortion of the outgoing nucleon wave. In addition also the influence, on the same observable, of the shape of the potential binding the nucleons into the nucleus is investigated. In accord with the findings of fully quantum mechanical calculations, the exclusive scattering is found to be quite sensitive to the mean field final states interaction, unlike the inclusive one. Indeed we verify that the latter is not affected, as implied by unitarity, by the distortion of the outgoing nucleon wave except for the effect of relativity, which is modest in the range of momenta up to about 500 MeV/ c . Furthermore, depending upon the correlations between the directions of the outgoing and of the initial nucleon, the exclusive cross-section turns out to be remarkably sensitive to the shape of the potential binding the nucleons. These correlations also critically affect the domain in the missing energy - missing momentum plane where the exclusive process occurs. Finally the issue of the validity of the semi-classical framework, in leading order of the &ħ expansion, in providing a description of the exclusive processes is shortly addressed.

Scalar and Pseudoscalar Susceptibilities in Nuclei

We study the staticscalar and pseudoscalar susceptibilities of QCD in the nuclear medium. We show that they become much closer than in the vacuum at normal nuclear matter density, a strong signal of the partial restoration of chiral symmetry.

Scalar and pseudoscalar QCD susceptibilities in nuclei

Abstract.We study the staticscalar and pseudoscalar susceptibilities of QCD in the nuclear medium. We show that they become much closer than in the vacuum at normal nuclear matter density, a strong signal of the partial restoration of chiral symmetry.

Pion scalar density and chiral symmetry restoration at finite temperature and density

This paper is devoted to the evaluation of the pionic scalar density at finite temperature and baryonic density. We express the latter effect in terms of the nuclear response evaluated in the random phase approximation. We discuss the density and temperature evolution of the pionic density which governs the quark condensate evolution. Numerical evaluations are performed.