ELectron - Ion Scattering in a storage ring ( eA collider ) – ELISe

The ELISe experiment will be part of the installations envisaged at the NESR for FAIR. It offers worldwide unique opportunities to scatter electrons off exotic nuclei. Physics opportunities For all stable nuclei, it has been shown [1] that the central densities are about the same (≈ 0.17 nucleons/fm) and that the matter density radius is roughly proportional to the atomic number to the power of minus one-third (R ~ A). The surface thickness (or diffuseness, a ≈ 1 fm, see Fig. 1) is approximately independent of the mass number A. The spatial proton and neutron distributions have approximately the same form and only differ in magnitude. This is no longer the case for nuclei far from stability where skins (Rp ≠ Rn with ap ≈ an) and halos (Rp ≠ Rn with ap > an, or an > ap) were found. Heavy bubblelike nuclei (strong depression of central density) are also theoretically predicted. A pure electromagnetic probe like the electron allows to measure, with excellent precision, the proton distribution using elastic scattering. Together with elastic hadron scattering (e.g. p,p at medium energies) where only the matter density distribution can be measured, also the neutron density distributions can be extracted. The determination of charge radii and extraction of nuclear matter radii are crucial for studying the evolution of neutron and proton skins along isotopic chains. For theory, the precise knowledge of the skin thickness is decisive (e.g. [2]) to understand isovector interactions and the symmetry energy. The quantity Rn Rp is not only related to the symmetry energy but also to the slope of the equation of state (EOS) for neutrons which is proportional to the pressure. Several modern Skyrme Hartree-Fock models using different parameters [3] predict a clear linear and strong dependence between the neutron skin thickness S = Rn Rp for lead and the slope of the neutron EOS. In ref. [4] the relation of S to several quantities like the nuclear binding energy, the compressibility etc. is studied for a large amount of mean field theories. The diffuseness parameter reflects the behaviour of the nuclear potential (e.g. spin-orbit) and eigenfunctions at the nuclear surface. It is an important ingredient for computing the asymptotic behaviour of the wave function. This is relevant to understand e.g. hadron-nucleus reactions which are sensitive only to the outer region of the nucleus due to strong absorption. See for example the large differences between the spectroscopic factors measured via (e,e'p) and (d,He) [5]. Since ELISe can measure elastic form factors that are not accessible by other means, the diffuseness of charge density distributions will be uniquely determined. The isotopic dependence of higher moments of the charge distributions will also be determined. These are again related to the isovector interaction since the proton density follows the neutron density due to static polarization. Figure 1: Elastic charge form factor for different Ni isotopes (a), the corresponding point proton distributions (b) and the effect of the Coulomb distortion of the electrons(SDE curve includes it, BA doesn’t) on the form factor (c). Taken from ref. [6]. In recent years, experimental and theoretical investigations have been refocused on the nuclear electric dipole (E1) response. The investigations have been stimulated by results of nuclear resonance fluorescence and Coulomb dissociation experiments which yielded a significant concentration of E1 strength below the particle threshold in stable and non-stable nuclei (for a recent overview we refer to [7] and references therein). Besides the implica___________________________________________ *Work supported by EC under INTAS contract number 03-54-6545 [email protected] FAIR-NUSTAR-ELISE-01