Coupled-channel treatment of $${\varvec{^7}}$$Li$${\varvec{(n,\gamma )^8}}$$Li in effective field theory

Abstract

The E1 contribution to the capture reaction $^7\mathrm{Li}(n,\gamma)^8\mathrm{Li}$ is calculated at low energies. We employ a coupled-channel formalism to account for the $^7\mathrm{Li}^\star$ excited core contribution. The halo effective field theory calculations show that the contribution of the $^7\mathrm{Li}^\star$ degree of freedom is negligible at momenta below 1 MeV and significant only beyond the $3^+$ resonance energy, though still compatible with a next-to-next-to-leading order correction. A power counting that accounts for the size of this correction is proposed. We compare our formalism with a previous halo effective field theory [Zhang, Nollett, and Phillips, Phys. Rev. C {\bf 89}, 024613 (2014)] that also treated the $^7\mathrm{Li}^\star$ core as an explicit degree of freedom. Our formal expressions and analysis disagree with this earlier work in several aspects.