Halo Excitation of ${}^6$He in Inelastic and Charge-Exchange Reactions

Abstract

Four-body distorted wave theory appropriate for nucleon-nucleus reactions leading to 3-body continuum excitations of two-neutron Borromean halo nuclei is developed. The peculiarities of the halo bound state and 3-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 ${}^6$He(p,p')${}^6$He${}^{\ast }$ and charge-exchange ${}^6$Li(n,p)${}^6$He${}^{\ast }$ reactions at nucleon energy 50 MeV. The theoretical low-energy spectra exhibit two resonance-like 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.

Comments: Submitted to Phys. Rev. C., 11 figures using epsf