Spin-Orbit Correlations
  We summarize the intuitive connection between deformations of parton distributions in impact parameter space and single-spin asymmetries. Lattice results for the $x^2$-moment of the twist-3 polarized parton distribution $g_2(x)$ are used to estimate the average transverse force acting on the active quark in SIDIS in the instant after being struck by the virtual photon. 
Matthias Burkardt