The observed deep inelastic nucleon spin asymmetries $A_1^{p,n}$ and the spin dependent quark distributions break SU(6) spin-flavor symmetry. In quark models two mechanisms for breaking SU(6) symmetry are well known: the hyperfine interaction and the pion cloud of the nucleon. I re-evaluate these mechanisms and show how the breaking of SU(6) in each case affects spin dependent valence quark distributions of the proton. In particular I investigate the properties of these mechanisms in the kinematic region $0.3<x<0.7$ and attempt to test these against known data. Both mechanisms are able to explain the quantitative features of the spin asymmetries.

• Received 19 February 2017

DOI:https://doi.org/10.1103/PhysRevD.95.114010