Radiative capture study by combining EFT with ab initio calculations: Lithium-7(neutron,photon)Lithium-8 and Beryllium-7(proton,photon)Boron-8

The (isospin-mirror) radiative nucleon captures, Li7(n,photon)Li8 and Be7(p,photon)B8, are subjects of long-standing interest for astrophysics. In this talk, I combine ab initio quantum-Monte-Carlo (QMC) calculations with the Halo-Effective-Field-Theory (Halo-EFT) framework to study them in the low-energy region. The Li8(B8) nucleus is considered as a shallow neutron+Li7 core and neutron+Li7* (proton+Be7 core and proton+Be7*) p-wave bound state. Li7* and Be7* are the core excitations. The scattering and bound states can be studied in Halo-EFT, in which both core and nucleon are treated as fundamental degrees of freedom. The couplings in the EFT Lagrangian are not known a priori and are difficult to extract from experiment. They can, however, be obtained from QMC calculations of the Li8 and B8 nuclei. In our leading order calculation, we use asymptotic normalization coefficients from QMC calculations to fix the parameters in our EFT Lagrangian, which we then apply to study radiative capture reactions. This reduces the need to employ numerically intensive QMC methods to directly compute radiative capture separately at each reaction energy, while still incorporating the ab initio information on nuclear dynamics that these methods provide in the calculation. Our results for both captures compare favorably with available data on total cross sections and branching ratios, within the estimated theoretical uncertainty. I will emphasize the important role of proton-Be7 scattering parameters in determining the energy dependence of the cross section (S factor), and demonstrate that their present uncertainties significantly limit attempts to extrapolate data to stellar energies. If time permits, I would mention briefly my works on neutrino-nucleus reactions and perhaps jet quenching physics in heavy ion collisions.