Eur. Phys. J. A (2010) 44: 297-303
https://doi.org/10.1140/epja/i2010-10954-6

Regular Article - Theoretical Physics

The role of the orbital angular momentum in the proton spin

Department of Physics, Faculty of Science, Osaka University, 560-0043, Toyonaka, Osaka, Japan

^* e-mail: wakamatu@phys.sci.osaka-u.ac.jp

Received: 4 January 2010
Accepted: 6 March 2010
Published online: 27 March 2010

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 the 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.