Quark spin-orbit correlations

TL;DR

This paper investigates the quark spin-orbit correlation within the proton, expressing it through measurable distributions, and finds that valence quark spin and orbital angular momentum tend to be oppositely aligned.

Contribution

It introduces a novel expression for the quark spin-orbit correlation in terms of moments of measurable parton distributions and phase-space distributions.

Findings

Valence quark spin and orbital angular momentum are generally opposite.

The quark spin-orbit correlation can be represented via relativistic phase-space distributions.

First examples of multipole decomposition of these distributions are presented.

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.