  The $\textit{axial coupling of the nucleon}$, $g_A$, is the strength of its
coupling to the $\textit{weak}$ axial current of the Standard Model of particle
physics, in much the same way as the electric charge is the strength of the
coupling to the electromagnetic current. This axial coupling dictates the rate
at which neutrons decay to protons, the strength of the attractive long-range
force between nucleons and other features of nuclear physics. Precision tests
of the Standard Model in nuclear environments require a quantitative
understanding of nuclear physics rooted in Quantum Chromodynamics, a pillar of
the Standard Model. 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 $g_A^{QCD} = 1.271\pm0.013$, using an unconventional
method inspired by the Feynman-Hellmann theorem that overcomes these
challenges.
