Halo effective field theory constrains the solar Beryllium-7 + proton -> Boron-8 + photon rate

TL;DR

This paper uses Halo Effective Field Theory and Bayesian analysis to improve the extrapolation of the Beryllium-7 + proton to Boron-8 + photon cross section, reducing uncertainties in solar neutrino flux predictions.

Contribution

It introduces a Halo EFT-based method with Bayesian parameter estimation to more accurately determine the low-energy S-factor for the solar Beryllium-7 + proton reaction.

Findings

S(0) is constrained to 21.3 ± 0.7 eV b

Uncertainty in S(0) is halved compared to previous estimates

Model space is effectively encompassed by nine EFT parameters

Abstract

We report an improved low-energy extrapolation of the cross section for the process Beryllium-7+proton -> Boron-8+photon, which determines the Boron-8 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 Boron-8 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 + - 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).