Quark angular momentum in a spectator model

TL;DR

This paper explores the decomposition of quark angular momentum within a nucleon model using light-cone formalism, revealing discrepancies between canonical and kinetic angular momentum definitions and examining their relation to observable form factors.

Contribution

It introduces a detailed spectator model including scalar and axial-vector spectators to analyze quark angular momentum and tests the validity of model relations with gravitational form factors.

Findings

Canonical angular momentum does not equal half the sum of gravitational form factors.

The model relation between orbital angular momentum and pretzelosity is violated in the axial-vector case.

Even without gauge fields, canonical and kinetic angular momenta are not directly identifiable.

Abstract

We investigate the quark angular momentum in a model with the nucleon being a quark and a spectator. Both scalar and axial-vector spectators are included. We perform the calculations in the light-cone formalism where the parton concept is well defined. We calculate the quark helicity and canonical orbital angular momentum. Then we calculate the gravitational form factors which are often related to the kinetic angular momentums, and find that even in a no gauge field model we cannot identify the canonical angular momentums with half the sum of gravitational form factors. In addition, we examine the model relation between the orbital angular momentum and pretzelosity, and find it is violated in the axial-vector case.