The role of orbital angular momentum in the proton spin

Abstract

The orbital angular momenta $L^u$ and $L^d$ of up and down quarks in the proton are estimated as functions of the energy scale as model-independently as possible, on the basis of Ji's angular momentum sum rule. This analysis indicates that $L^u-L^d$ is large and negative even at low energy scale of nonperturbative QCD, in contrast to Thomas' similar analysis based on the refined cloudy bag model. We pursuit the origin of this apparent discrepancy and suggest that it may have a connection with the fundamental question of how to define quark orbital angular momenta in QCD.

14 pages, 3 figures, 1 table A slightly extended version to appear in Eur. Phys. J. A

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• Proper identification of the gluon spin
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• Calculating Dihadron Fragmentation Functions in the NJL-jet model
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• Dihadron Fragmentation Functions from the NJL-jet model and their QCD Evolution
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• Wigner distributions and quark orbital angular momentum
• The role of the GPD E in the determination of the parton angular momenta
• Proton spin: a topological invariant
• Contribution of Orbital Angular Momentum to the Nucleon Spin
• Transverse Spin Effects in Future Drell-Yan Experiments
• Spectrum of the excited $N^{\ast }$ and ${\Delta }^{\ast }$ baryons in a relativistic chiral quark model
• Gauge-invariant decomposition of nucleon spin