Where Does the Proton Really Get Its Spin?

SEP 01, 1995

Polarized scattering experiments reveal that quarks contribute surprisingly little to the proton’s spin. This ‘spin crisis’ is doing much to clarify the subtle departures of the underlying field theory from the naive quark model.

DOI: 10.1063/1.881473

Robert L. Jaffe

In 1987 the European Muon Collaboration, which had been scattering muons off polarized protons at CERN, shocked the particle physics community with the announcement that little or none of the proton’s spin can be attributed to the spins of its three constituent quarks—two “up” quarks and one “down” quark. That report precipitated what became known as “the spin crisis.”

References

1. J. Ashman et al., Phys. Lett. B 206, 364 (1988); https://doi.org/PYLBAJ
J. Ashman, Nucl. Phys. B 328, 1 (1989).https://doi.org/NUPBBO
2. For a review of the experiments see C. Cavata, V. Hughes, eds., Internal Spin Structure of the Nucleon, World Scientific, Singapore (1995).
3. J. D. Bjorken, Phys. Rev. 148, 1467 (1966).https://doi.org/PHRVAO
4. L. Galfi, P. Gnadig, J. Kuti, F. Niedermeyer, A. Patkos, in Proc. 15th Intl. Conf. on High‐Energy Physics, Kiev 1970, V. Shelest et al., eds., Naukova Dumka, Kiev (1972).
J. Kuti, V. Weisskopf, Phys. Rev. D 4, 3418 (1971).https://doi.org/PRVDAQ
5. M. Gourdin, Nucl. Phys. B 38, 418 (1972).https://doi.org/NUPBBO
6. J. Ellis, R. Jaffe, Phys. Rev. D 9, 1444 (1974); https://doi.org/PRVDAQ
J. Ellis, R. Jaffe, 10, 1669 (1974).
7. L. Sehgal, Phys. Rev. D 10, 1663 (1974).https://doi.org/PRVDAQ
8. V. Hughes, in Proc. II Intl. Conf. on Polarized Targets, G. Shapiro, ed., U. Calif. P., Berkeley (1971).
9. M. Alguard et al., Phys. Rev. Lett. 37, 1261 (1976).https://doi.org/PRLTAO
10. G. Baum et al., Phys. Rev. Lett. 51, 1135 (1983).https://doi.org/PRLTAO
11. B. Adeva et al., Phys. Lett. B 302, 553 (1993).https://doi.org/PYLBAJ
12. P. Anthony et al., Phys. Rev. Lett. 71, 959 (1991).https://doi.org/PRLTAO
13. B. Adeva et al., Phys. Lett. B 320, 400 (1994); https://doi.org/PYLBAJ
B. Adeva, 329, 399 (1994).
14. K. Abe et al., Phys. Rev. Lett. 74, 346 (1995).https://doi.org/PRLTAO
15. G. Garvey, W. Louis, D. White, Phys. Rev. C 48, 761 (1993).https://doi.org/PRVCAN
16. X. Ji, P. Unrau, Phys. Lett. B 333, 228 (1994).https://doi.org/PYLBAJ
17. S. Brodsky, J. Ellis, M. Karliner, Phys. Lett. B 206, 309 (1988).https://doi.org/PYLBAJ
18. R. Jaffe, A. Manohar, Nucl. Phys. B 337, 509 (1990).https://doi.org/NUPBBO
19. For a review and references, see A. Manohar, in Symmetry and Spin in the Standard Model, Proc. 7th Lake Louise Winter Inst., B. Campbell et al., eds., World Scientific, Singapore (1992).
20. HERMES collaboration, “HERMES Technical Design Report,” DESY, Hamburg, July 1993.
21. J. Ralston, D. Soper, Nucl. Phys. B 152, 109 (1979).https://doi.org/NUPBBO
22. X. Artu, M. Mekhfi, Z. Phys. C 45, 669 (1990).https://doi.org/ZPCFD2
23. R. Jaffe, X. Ji, Phys. Rev. Lett. 67, 552 (1992); https://doi.org/PRLTAO
R. Jaffe, X. Ji, Nucl. Phys. B 375, 527 (1992). https://doi.org/NUPBBO
G. Bunce et al., Particle World 3, 1 (1992).

More about the Authors

Robert L. Jaffe. MITs Center for, Theoretical Physics, Cambridge, Massachusetts.