Xiangdong Ji, Xiaonu Xiong, Feng Yuan
We analyze the proton's longitudinal and transverse spin in terms of parton's
spin and orbital angular momentum contributions in the infinite momentum frame,
emphasizing experiment measurability. The simple partonic sum rule is shown to
exist only for the transverse polarization and is related to the twist-two
generalized parton distributions (GPD's). The longitudinal spin, however,
necessarily involves parton transverse momentum and parton correlations. In a
gauge-invariant approach, parton contributions to the proton helicity are shown
to be related to twist-two and -three GPD's and are amenable to lattice QCD
calculations. A simpler but subtler parton picture emerges in a light-cone
gauge approach through a quantum phase-space Wigner distribution.
View original:
http://arxiv.org/abs/1202.2843
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