Orbital angular momentum, spin fractions, and scenarios for the proton's spin-weighted parton distributions

Abstract

New data from the European Muon Collaboration (EMC) on the spin-spin asymmetry in deep-inelastic lepton-proton scattering suggests that the total spin carried by valence quarks in a polarized proton may be approximately canceled by a strong negative polarization of the sea of $\overline{q}q$ pairs. The evolution in $Q^2$ of the fraction of proton spin carried by gluons depends on the initial spin fractions and it is possible to avoid the introduction of large orbital angular momentum by adopting a "hybrid" quark-Skyrme picture of the proton suggested by the EMC results.