An excellent description of both spin-independent and spin-dependent quark distributions and structure functions has been obtained with a modified Nambu—Jona-Lasinio model, which is free of unphysical thresholds for nucleon decay into quarks—hence incorporating an important aspect of confinement. We utilize this model to investigate nuclear medium modifications to structure functions and find that we are readily able to reproduce both nuclear matter saturation and the experimental $F_{2N}^A/F_{2N}$ ratio, that is, the European Muon Collaboration (EMC) effect. Applying this framework to determine $g_{1p}^A$, we find that the ratio $g_{1p}^A/g_{1p}$ differs significantly from unity, with the quenching caused by the nuclear medium being about twice that of the spin-independent case. This represents an exciting result, which, if confirmed experimentally, will reveal much about the quark structure of nuclear matter.

• Received 27 April 2005

DOI:https://doi.org/10.1103/PhysRevLett.95.052302