Proton spin and chiral dynamics

Recent successful chiral models for the spin fractions of the proton rely on parametrizations that are inconsistent with deep inelastic lepton scattering unless the u, d, s quark masses are neglected and inconsistent with chiral quark models based on constituent quarks, because only chiral spinflip transitions are considered. Non-spinflip transitions are important for constituent quarks as they depend on the quark masses. Therefore, the models are valid at a scale of about 0.63 GeV to Λx where dynamical quark masses are expected to be close to current quark masses, rather than ΛQCD). When constituent quark masses are properly included, chiral quark models generate spin fractions that disagree with the proton spin data. (author)