We report an improved low-energy extrapolation of the cross section for the process ^ { 7 } \mathrm { Be } ( p, \gamma ) ^ { 8 } \mathrm { B } , 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 E < 0.5 MeV .
Most importantly , the S -factor at zero energy is constrained to be S ( 0 ) = 21.3 \pm 0.7 eV b , which is an uncertainty smaller by a factor of two than previously recommended .
That recommendation was based on the full range for S ( 0 ) 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 S ( E ) .