Abstract
The proton spin puzzle issue focused the attention on the parton spin and orbital angular momentum contributions to the proton spin. However, a complete characterization of the proton spin structure requires also the knowledge of the parton spin-orbit correlation. We showed that this quantity can be expressed in terms of moments of measurable parton distributions. Using the available phenomenological information about the valence quarks, we concluded that this correlation is negative, meaning that the valence quark spin and kinetic orbital angular momentum are, in average, opposite. The quark spin-orbit correlation can also be expressed more intuitively in terms of relativistic phase-space distributions, which can be seen as the mother distributions of the standard generalized and transverse-momentum dependent parton distributions. We present here for the first time some examples of the general multipole decomposition of these phase-space distributions.
Highlights
Unraveling the spin structure of the nucleon is one of the key questions in hadronic physics
The so-called quark orbital angular momentum (OAM) contribution to the proton spin corresponds to twice the correlation between the longitudinal components of the quark OAM lzq and the nucleon spin
We found a similar expression[4] for the quark spin-orbit correlation
Summary
Unraveling the spin structure of the nucleon is one of the key questions in hadronic physics. The so-called quark orbital angular momentum (OAM) contribution to the proton spin corresponds to twice the correlation between the longitudinal components of the quark OAM lzq and the nucleon spin. This is an Open Access article published by World Scientific Publishing Company. As discussed by Burkardt[5] in the case of the Ji relation, the extra x-factor representing the fraction of longitudinal momentum turns out to provide the “orbital” information It has been shown[6,7,8] that the (kinetic) quark OAM can alternatively be expressed in terms of twist-3 GPDs. Lqz = − dx xGq2(x, 0, 0). The difference between canonical and kinetic versions of the quark OAM has been investigated in some models.[9, 15] While the connection between these GTMDs and experimental observables is not yet clear, recent developments suggest that they could at least be computed on the lattice.[16]
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