Measurement of the proton spin structure at long distances

CLAS Collaboration

Research output: Contribution to journalArticlepeer-review


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 GeV2. 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, for example, 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.

Original languageEnglish
Pages (from-to)736-741
Number of pages6
JournalNature Physics
Issue number6
Early online date12 Apr 2021
Publication statusPublished - Jun 2021

Bibliographical note

Funding Information:
All authors are members of The Jefferson Lab CLAS Collaboration. We thank the personnel of Jefferson Lab for their efforts that resulted in the successful completion of the experiment. We thank Kovacs for her contribution to the early analysis of the data. We are grateful to U.-G. Meißner and V. Pascalutsa for useful discussions on the theoretical χEFT calculations. This work was supported by the US Department of Energy (DOE), the US National Science Foundation, the US Jeffress Memorial Trust, the UK Science and Technology Facilities Council (STFC), the Italian Istituto Nazionale di Fisica Nucleare, the French Institut National de Physique Nucléaire et de Physique des Particules, the French Centre National de la Recherche Scientifique and the National Research Foundation of Korea. This material is based on work supported by the US Department of Energy, Office of Science, Office of Nuclear Physics under contract no. DE-AC05-06OR23177.

Publisher Copyright:
© 2021, The Author(s), under exclusive licence to Springer Nature Limited part of Springer Nature.


  • nucl-ex
  • hep-ex

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