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

We report a leading-order (LO) calculation of Be-7(p,gamma)B-8 in a low-energy effective field theory. B-8 is treated as a shallow proton+Be-7 core and proton+Be-7(*) (core excitation) p-wave bound state. The couplings are fixed using measured binding energies and proton-Be-7 s-wave scattering lengths, together with B-8 asymptotic normalization coefficients from ab initio calculations. We obtain a zero-energy S-factor of 18.2 +/- 1.2(ANC only) eV b. Given that this is a LO result it is consistent with the recommended value S(0) = 20.8 +/- 1.6 eV b. Our computed S(E) compares favorably with experimental data on Be-7(p,gamma)B-8 for E < 0.4 MeV. We emphasize the important role of proton-Be-7 scattering parameters in determining the energy dependence of S(E), and demonstrate that their present uncertainties significantly limit attempts to extrapolate these data to stellar energies.