Measurement of the proton spin structure at long distances

TL;DR

This study measures proton spin structure at long distances using polarized electron scattering, testing theoretical predictions and highlighting the need for further non-perturbative QCD calculations.

Contribution

First measurements of proton spin structure at low momentum transfer, providing data to test chiral effective field theories and sum rules.

Findings

Results align with the Gerasimov-Drell-Hearn sum rule

Data challenge current theoretical models of nucleon spin

Highlights need for advanced lattice QCD calculations

Abstract

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