title: Proton spin structure from measurable parton distributions.
abstract: We present a systematic study of the proton spin structure in terms of measurable parton distributions. For a transversely polarized proton, we derive a polarization sum rule from the leading generalized parton distributions appearing in hard exclusive processes. For a longitudinally polarized proton, we obtain a helicity decomposition from well-known quark and gluon helicity distributions and orbital angular-momentum contributions. The latter are shown to be related to measurable subleading generalized parton distributions and quantum-phase space Wigner distributions.
authors.affiliation: Department of Physics, Shanghai Jiao Tong University, Shanghai 200240, People's Republic of China.
authors.forename: Xiangdong
authors.initials: X
authors.lastname: Ji
authors.forename: Xiaonu
authors.initials: X
authors.lastname: Xiong
authors.forename: Feng
authors.initials: F
authors.lastname: Yuan