Combining ab initio calculations and low-energy effective field theory for halo nuclear systems: The case of 7Li + n → 8Li + γ

We report a leading-order calculation of radiative Li-7 neutron captures to both the ground and first excited state of Li-8 in the framework of a low-energy effective field theory (halo-EFT). Each of the possible final states is treated as a shallow bound state composed of both n + Li-7 and n + Li-7* (core excitation) configurations. The ab initio variational Monte Carlo method is used to compute the asymptotic normalization coefficients of these bound states, which are then used to fix couplings in our EFT. We calculate the total and partial cross sections in the radiative capture process using this calibrated EFT. Fair agreement with measured total cross sections is achieved and excellent agreement with the measured branching ratio between the two final states is found. In contrast, a previous halo-EFT calculation [Rupak and Higa, Phys. Rev. Lett. 106, 222501 (2011)] assumes that the n-Li-7 couplings in different spin channels are equal, fits the P-wave effective-range parameter to the threshold cross section for Li-7 + n -> Li-8 + gamma, and assumes the core excitation is at high enough energy scale that it can be integrated out.