The proton spin crisis, first identified by the European Muon Collaboration (EMC) experiment in 1987, overturned the long-held assumption that the proton's spin arises predominantly from the intrinsic spins of its constituent quarks. Rather than accounting for the entirety of the proton’s spin, quarks were found to contribute only 4–24% a result that remains one of the unresolved puzzles in the Standard Model of particle physics. This work proposes a solution to this crisis through the Unified Fractal Quantum Field Theory (UFQFT) framework, which models particles as resonant structures of a fundamental energy field (Φ) and charge field (Ψ) in a spacetime with an effective fractal dimension of D≈2.7. Within UFQFT, spin is defined not merely as an intrinsic property of point-like quarks, but as a collective phenomenon emerging from the fractal resonance of the fields. The work proposes the decomposition of proton spin as follows: 𝑆𝑝 = 𝑆𝑞 + 𝑆𝛷−𝛹 + 𝐿𝑓 where 𝑆𝑞 represents the conventional quark spin contribution, 𝑆𝛷−𝛹 is the spin component arising from the fractal resonance between the Φ and Ψ fields, and 𝐿𝑓 represent the orbital angular momentum generated by the fractal geometry of the proton. This model attributes most of the proton spin to collective field dynamics after naturally accounting for the observed small quark spin contribution. These findings suggest that the proton spin crisis is not a fundamental paradox, but rather a component of the fractal and field-theoretic nature of the hadronic structure.
Hacı Soğukpınar (Thu,) studied this question.