Coupled-channel treatment of 7Li(n,γ)8Li in effective field theory
Abstract
The E1 contribution to the capture reaction 7Li(n,γ)8Li is calculated at low energies. We employ a coupled-channel formalism to account for the 7Li excited core contribution. We develop a halo effective field theory power counting where capture in the spin S=2 channel is enhanced over the S=1 channel. A next-to-leading order calculation is presented where the excited core contribution is shown to affect only the overall normalization of the cross section. The momentum dependence of the capture cross section, as a consequence, is the same in a theory with or without the excited 7Li degree of freedom at this order of the calculation. The kinematical signature of the 7Li core 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. We compare our formalism with a previous halo effective field theory calculation [Zhang, Nollett, and Phillips, Phys. Rev. C 89, 024613 (2014)] that also treated the 7Li core as an explicit degree of freedom. Our formal expressions and analysis disagree with this earlier work in several aspects.