${}^4\mathrm{He}$+$n$+$n$ continuum within an ab initio framework

Abstract

The low-lying continuum spectrum of the ${}^6\mathrm{He}$ nucleus is investigated for the first time within an ab initio framework that encompasses the ${}^4\mathrm{He}$+$n$+$n$ three-cluster dynamics characterizing its lowest decay channel. This is achieved through an extension of the no-core-shell model combined with the resonating-group method, in which energy-independent non-local interactions among three nuclear fragments can be calculated microscopically starting from realistic nucleon-nucleon interactions and consistent ab initio many-body wave functions of the clusters. The three-cluster Schrödinger equation is solved with three-body scattering boundary conditions by means of the hyperspherical-harmonic method on a Lagrange mesh. Using a soft similarity-renormalization-group evolved chiral nucleon-nucleon potential, we find the known $J^{\pi }=2^{+}$ resonance as well as a result consistent with a new low-lying second $2^{+}$ resonance recently observed at GANIL at $2.6$ MeV above the ${}^6$He ground state. We also find resonances in the $2^{-}$, $1^{+}$ and $0^{-}$ channels, while no low-lying resonances are present in the $0^{+}$ and $1^{-}$ channels.

Comments: 5 pages 4 figures