Proton Spin Structure from Measurable Parton Distributions

TL;DR

This paper investigates the proton's spin structure using measurable parton distributions, deriving sum rules and decompositions for both transverse and longitudinal polarization states, linking them to generalized distributions and quantum phase space.

Contribution

It introduces new sum rules and decompositions for proton spin based on measurable generalized parton and Wigner distributions, advancing understanding of spin contributions.

Findings

Derived a polarization sum rule for transversely-polarized protons.

Obtained a helicity decomposition for longitudinally-polarized protons.

Linked orbital angular momentum to measurable distributions.

Abstract

We present a systematic study of the proton spin structure in terms of measurable parton distributions. For a transversely-polarizedproton, 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 is shown to be related to measurable subleading generalized parton distributions and quantum-phase space Wigner distributions.