Proton spin crisis

Unsolved problem in physics:

How do the quarks and gluons carry the spin of protons?

The proton spin crisis (or proton spin puzzle) is a theoretical crisis precipitated by a 1987 experiment by the European Muon Collaboration (EMC),^[1] which tried to determine the distribution of spin within the proton.^[2]

Physicists expected that the quarks carry all a proton's spin. However, not only was the total proton spin carried by quarks far smaller than 100%, these results were consistent with almost zero (4–24%^[3]) proton spin being carried by quarks. This surprising and puzzling result was termed the "proton spin crisis".^[4] The problem is considered one of the important unsolved problems in physics.^[5]

^ Ashman, J.; Badelek, B.; Baum, G.; Beaufays, J.; Bee, C.P.; Benchouk, C.; et al. (European Muon Collaboration (EMC)) (1988). "A measurement of the spin asymmetry and determination of the structure function g₁ in deep inelastic muon-proton scattering". Physics Letters B. 206 (2): 364–370. Bibcode:1988PhLB..206..364A. doi:10.1016/0370-2693(88)91523-7. ISSN 0370-2693. CERN EP 87-230.
^ Ashman, J.; et al. (European Muon Collaboration (EMC)) (1988). "A measurement of the spin asymmetry and determination of the structure function g1 in deep inelastic muon-proton scattering" (PDF). Physics Letters B. 206 (2): 364. Bibcode:1988PhLB..206..364A. doi:10.1016/0370-2693(88)91523-7.
^ "Are scientists finally on the brink of understanding where proton spin comes from?". phys.org. June 2015.
^ Londergan, J. T. (2009). "Nucleon resonances and quark structure". International Journal of Modern Physics E. 18 (5–6): 1135–1165. arXiv:0907.3431. Bibcode:2009IJMPE..18.1135L. doi:10.1142/S0218301309013415. S2CID 118475917.
^ Hansson, Johan (July 2010) [8 March 2010]. "The "proton spin crisis" – a quantum query" (PDF). Progress in Physics. 3: 51–52. Archived from the original (PDF) on 2012-05-04.

[1] Ashman, J.; Badelek, B.; Baum, G.; Beaufays, J.; Bee, C.P.; Benchouk, C.; et al. (European Muon Collaboration (EMC)) (1988). "A measurement of the spin asymmetry and determination of the structure function g₁ in deep inelastic muon-proton scattering". Physics Letters B. 206 (2): 364–370. Bibcode:1988PhLB..206..364A. doi:10.1016/0370-2693(88)91523-7. ISSN 0370-2693. CERN EP 87-230.

[2] Ashman, J.; et al. (European Muon Collaboration (EMC)) (1988). "A measurement of the spin asymmetry and determination of the structure function g1 in deep inelastic muon-proton scattering" (PDF). Physics Letters B. 206 (2): 364. Bibcode:1988PhLB..206..364A. doi:10.1016/0370-2693(88)91523-7.

[3] "Are scientists finally on the brink of understanding where proton spin comes from?". phys.org. June 2015.

[4] Londergan, J. T. (2009). "Nucleon resonances and quark structure". International Journal of Modern Physics E. 18 (5–6): 1135–1165. arXiv:0907.3431. Bibcode:2009IJMPE..18.1135L. doi:10.1142/S0218301309013415. S2CID 118475917.

[5] Hansson, Johan (July 2010) [8 March 2010]. "The "proton spin crisis" – a quantum query" (PDF). Progress in Physics. 3: 51–52. Archived from the original (PDF) on 2012-05-04.

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