He4+n+nContinuum within anAb initioFramework

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 nonlocal 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 Schr\"odinger equation is solved with three-body scattering boundary conditions by means of the hyperspherical-harmonics method on a Lagrange mesh. Using a soft similarity-renormalization-group evolved chiral nucleon-nucleon potential, we find the known ${J}^{\ensuremath{\pi}}={2}^{+}$ 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 $^{6}\mathrm{He}$ ground state. We also find resonances in the ${2}^{\ensuremath{-}}$, ${1}^{+}$, and ${0}^{\ensuremath{-}}$ channels, while no low-lying resonances are present in the ${0}^{+}$ and ${1}^{\ensuremath{-}}$ channels.