A percent-level determination of the nucleon axial coupling from quantum chromodynamics

The axial coupling of the nucleon, g_A, is the strength of its coupling to the weak axial current. Precision tests of the Standard Model in nuclear environments require a quantitative understanding of nuclear physics rooted in Quantum Chromodynamics. The prominence of g_A makes it a benchmark quantity to determine theoretically -- a difficult task because quantum chromodynamics is non-perturbative, precluding known analytical methods. Lattice Quantum Chromodynamics provides a rigorous, non-perturbative definition of quantum chromodynamics that can be implemented numerically. It has been estimated that a precision of two percent would be possible by 2020 if two challenges are overcome: contamination of g_A from excited states must be controlled in the calculations and statistical precision must be improved markedly. Here we report a calculation of g_A^QCD = 1.271±0.013, using an unconventional method inspired by the Feynman--Hellmann theorem that overcomes these challenges.