Measurement of the proton spin structure at long distances

Measuring the spin structure of protons and neutrons tests our understanding how they arise from quarks gluons, fundamental building blocks nuclear matter. At long distances coupling constant strong interaction becomes large, requiring non-perturbative methods to calculate quantum chromodynamics processes, such as lattice gauge theory or effective field theories. Here we report proton measurements scattering a polarized electron beam off protons. The spin-dependent cross-sections were measured at large distances, corresponding region low momentum transfer squared between 0.012 1.0 GeV$^2$. This kinematic range provides unique chiral predictions. Our results show that complete description nucleon remains elusive, call for further theoretical works, e.g. in chromodynamics. Finally, data extrapolated photon point agree with Gerasimov-Drell-Hearn sum rule, prediction relates anomalous magnetic moment its integrated cross-sections.