Quark angular momentum and the Sivers asymmetry
  The determination of quark angular momentum requires the knowledge of the generalized parton distribution $E$ in the forward limit. We assume a connection between this function and the Sivers transverse-momentum distribution, based on model calculations and theoretical considerations. Using this assumption, we show that it is possible to fit at the same time nucleon anomalous magnetic moments and semi-inclusive single-spin asymmetries. This imposes additional constraints on the Sivers function and opens a plausible way to quantifying the quark angular momentum. 
Alessandro Bacchetta and Marco Radici
hep-ph hep-ex nucl-ex nucl-th