Excitation modes of 6He from proton collisions

The Borromean two-neutron halo nuclei 11Li and 6He have been attracting many theoretical and experimental studies since the ground state and the excited modes are not both well understood yet, and knowledge of the continuum properties will contribute to our understanding of the properties of these nuclei. To illustrate the puzzling features half-unveiled in this field, various theories [1–3] predict a variety of excitation/resonance modes at low energies in the continuum of the above Borromean nuclei, but up to now there is only a clear evidence of a J = 2+ resonance for 6He [4]. Thus, for more than a decade the existence and the understanding of the excitation modes is far from being achieved, now a detailed study of the resonances in the continuum sea is one of the priority tasks of radioactive beam research. From a theoretical point of view, structure calculations predict a rich continuum. Within few body cluster models that include the three-body dynamics of the Borromean system [5], two kinds of phenomena have been predicted: (i) true three-body resonances, with three-body phase shifts crossing π/2 in all partial waves, and (ii) not-fully-fledged resonant contributions with fast growing three-body phase shifts up to π/2 in some partial waves. These few-body cluster models rely on fitted effective interactions between the cluster constituents. In addition, the simultaneous treatment of the Pauli principle and continuum pairing in these approaches remains today an open problem. Shell model approaches have also been developed but the coupling to the continuum presents a difficult problem. Most often, excited states may appear to be resonances in some structure models, in particular for the soft dipole mode in Borromean nuclei. This calls for an improvement of the structure approaches, and for new work to test the structure theories by means of reaction observables.