Halo excitation of He-6 in inelastic and charge-exchange reactions

Four-body distorted wave theory appropriate for nucleon-nucleus reactions leading to three-body continuum excitations of two-neutron Borromean halo nuclei is developed. The peculiarities of the halo bound state and three-body continuum are fully taken into account by using the method of hyperspherical harmonics. The procedure is applied for A=6 test-bench nuclei; thus we report detailed studies of inclusive cross sections for inelastic He-6(p,p')He-6* and charge-exchange Li-6(n,p)He-6* reactions at nucleon energy 50 MeV. The theoretical low-energy spectra exhibit two resonancelike structures. The first (narrow) is the excitation of the well-known 2(+) three-body resonance. The second (broad) bump is a composition of overlapping soft modes of multipolarities 1(-),2(+),1(+),0(+) whose relative weights depend on transferred momentum and reaction type. Inelastic scattering is the most selective tool for studying the soft dipole excitation mode.