Pierre A.M. Guichon

How to reconcile the Rosenbluth and the polarization transfer methods in the measurement of the proton form factors.

The apparent discrepancy between the Rosenbluth and the polarization transfer methods for the ratio of the electric to magnetic proton form factors can be explained by a two-photon exchange correction which does not destroy the linearity of the Rosenbluth plot. Though intrinsically small, of the order of a few percent of the cross section, this correction is accidentally amplified in the case of the Rosenbluth method.

The role of nucleon structure in finite nuclei

Abstract The quark-meson coupling model, based on a mean field description of non-overlapping nucleon bags bound by the self-consistent exchange of σ, ω and ϱ mesons, is extended to investigate the properties of finite nuclei. Using the Born-Oppenheimer approximation to describe the interacting quark-meson system, we derive the effective equation of motion for the nucleon, as well as the self-consistent equations for the meson mean fields. The model is first applied to nuclear matter, after which we show some initial results for finite nuclei.

Virtual compton scattering and generalized polarizabilities of the proton

Abstract Threshold photon electroproduction off the proton allows one to measure new electromagnetic observables which generalize the usual polarizabilities. There are—a priori—ten “generalized polarizabilities”, functions of the virtual photon mass. The purpose of this paper is to lay down the appropriate formalism to extract these quantities from the photon electroproduction cross sections. We also give a first estimate of the generalized polarizabilities in the non-relativistic quark model.

Binding of hypernuclei in the latest quark-meson coupling model

Abstract The most recent development of the quark–meson coupling (QMC) model, in which the effect of the mean scalar field in-medium on the hyperfine interaction is also included self-consistently, is used to compute the properties of hypernuclei. The calculations for Λ and Ξ hypernuclei are of comparable quality to earlier QMC results without the additional parameter needed there. Even more significantly, the additional repulsion associated with the increased hyperfine interaction in-medium completely changes the predictions for Σ hypernuclei. Whereas in the earlier work they were bound by an amount similar to Λ hypernuclei, here they are unbound, in qualitative agreement with the experimental absence of such states. The equivalent non-relativistic potential felt by the Σ is repulsive inside the nuclear interior and weakly attractive in the nuclear surface, as suggested by the analysis of Σ -atoms.

Cold Uniform Matter and Neutron Stars in the Quark-Meson-Coupling Model

Abstract A new density dependent effective baryon–baryon interaction has been recently derived from the quark–meson-coupling (QMC) model, offering impressive results in application to finite nuclei and dense baryon matter. This self-consistent, relativistic, quark-level approach is used to construct the Equation of State (EoS) and to calculate key properties of high density matter and cold, slowly rotating neutron stars. The results include predictions for the maximum mass of neutron-star models, together with the corresponding radius and central density, as well the properties of neutron stars with mass of order 1.4  M ⊙ . Some conditions related to the direct URCA process are explored for the QMC EoS and the parameters relevant to slow rotation, namely the moment of inertia and the period of rotation, are investigated. The results of the calculation, which are found to be in good agreement with available observational data, are compared with the predictions of several more traditional EoS. The QMC EoS provides cold neutron-star models with maximum mass in the range 1.9–2.1  M ⊙ , with central density less than 6 times nuclear saturation density ( n 0 = 0.16 fm −3 ) and offers a consistent description of the stellar mass up to this density limit. In contrast with other models, QMC predicts no hyperon contribution at densities lower than 3 n 0 , for matter in β-equilibrium. At higher densities, Ξ − , 0 and Λ hyperons are present, with consequent lowering of the maximum mass. The absence of lighter Σ ± , 0 hyperons is understood as consequence of including the color hyperfine interaction in the response of the quark bag to the nuclear scalar field.

Quark structure and nuclear effective forces.

We formulate the quark meson coupling model as a many-body effective Hamiltonian. This leads naturally to the appearance of many-body forces. We investigate the zero range limit of the model and compare its Hartree-Fock Hamiltonian to that corresponding to the Skyrme effective force. By fixing the three parameters of the model to reproduce the binding and symmetry energy of nuclear matter, we find that it allows a very satisfactory interpretation of the Skyrme force.

Physical Origin of Density Dependent Force of the Skyrme Type within the Quark Meson Coupling Model

Abstract A density dependent, effective nucleon–nucleon force of the Skyrme type is derived from the quark–meson coupling model—a self-consistent, relativistic quark level description of nuclear matter. This new formulation requires no assumption that the mean scalar field is small and hence constitutes a significant advance over earlier work. The similarity of the effective interaction to the widely used SkM ∗ force encourages us to apply it to a wide range of nuclear problems, beginning with the binding energies and charge distributions of doubly magic nuclei. Finding acceptable results in this conventional arena, we apply the same effective interaction, within the Hartree–Fock–Bogoliubov approach, to the properties of nuclei far from stability. The resulting two neutron drip lines and shell quenching are quite satisfactory. Finally, we apply the relativistic formulation to the properties of dense nuclear matter in anticipation of future application to the properties of neutron stars.

The Axial Form-factor of the Nucleon and the Pion - Nucleon Vertex Function

Abstract The axial form factor of the nucleon in the Cloudy Bag Model is harder than the NNπ vertex function, and consistent with data for a bag radius about 1 fm. We examine how these results are altered by excluding the pion field from a smaller, interior volume.

Quarks and the deuteron asymptotic D state

Abstract Form factor effects associated with the nucleon size reduce significantly calculated values of the deuteron asymptotic D to S state ratio. We show that tensor forces due to pions and gluons exchanged between quarks compensate for such reductions.

Recoil corrections to the nucleon magnetic moment in the bag model

Abstract It is shown that the recoil correction to the value of the nucleon magnetic moment computed in the static bag model is small and negative. This is due to an over-cancellation of the spin precession effect by the retardation effect. The latter has been ignored in previous calculations, leading to a large positive correction.