Halo effective field theory constrains the solar 7Be + p → 8B + γ rate

We report an improved low-energy extrapolation of the cross section for the process , which determines the 8 B neutrino flux from the Sun. Our extrapolant is derived from Halo Effective Field Theory (EFT) at next-to-leading order. We apply Bayesian methods to determine the EFT parameters and the low-energy S -factor, using measured cross sections and scattering lengths as inputs. Asymptotic normalization coefficients of 8 B are tightly constrained by existing radiative capture data, and contributions to the cross section beyond external direct capture are detected in the data at . Most importantly, the S -factor at zero energy is constrained to be , which is an uncertainty smaller by a factor of two than previously recommended. That recommendation was based on the full range for obtained among a discrete set of models judged to be reasonable. In contrast, Halo EFT subsumes all models into a controlled low-energy approximant, where they are characterized by nine parameters at next-to-leading order. These are fit to data, and marginalized over via Monte Carlo integration to produce the improved prediction for .