Angular momentum limit for fusion of halo and weakly bound systems

Recent experimental results for the fusion of the proton-halo system ⁸B + ⁵⁸Ni are discussed. A strong enhancement is observed both below and above the barrier. This is qualitatively different from the usual sub-barrier enhancement observed for many heavy-ion systems. Arguments are given that relate the enhancement to a static halo effect directly associated to the extended size of the proton-halo state. A simple single-barrier model is proposed which describes the fusion cross sections in terms of a maximum angular momentum for fusion, L_f. It is shown that a linear energy dependence for L_f gives a surprisingly good description of the experimental fusion excitation function not only for ⁸B + ⁵⁸Ni but also for the neutron-halo systems ⁶He + (²⁰⁹Bi,⁶⁴Zn). The model is also applied to several weakly bound systems.