  Measuring the spin structure of nucleons (protons and neutrons) extensively
tests our understanding of how nucleons arise from quarks and gluons, the
fundamental building blocks of nuclear matter. The nucleon spin structure is
typically probed in scattering experiments using polarized beams and polarized
nucleon targets, and the results are compared with predictions from Quantum
Chromodynamics directly or with effective theories that describe the strong
nuclear force. Here we report on new proton spin structure measurements with
significantly better precision and improved coverage than previous data at low
momentum transfer squared between $0.012$ and $1.0$ GeV$^2$. This kinematic
range provides unique tests of effective field theory predictions. Our results
show that a complete description of the nucleon spin remains elusive. They call
for further theoretical works that include the more fundamental lattice gauge
method. Finally, our data agree with the Gerasimov-Drell-Hearn sum rule, a
fundamental prediction of quantum field theory.
