Abstract
We discuss the orbital angular momentum of partons inside a longitudinally polarized proton in the recently proposed framework of spin decomposition. The quark orbital angular momentum defined by Ji can be decomposed into the ‘canonical’ and the ‘potential’ angular momentum parts, both of which are represented as the matrix element of a manifestly gauge invariant operator.
Highlights
Recent polarized beam experiments and global QCD analyses suggest that the contribution of the gluon helicity ∆G to the spin of the proton is rather small [1]
Efforts to improve upon this point have led Chen et al to propose a completely new decomposition scheme of the QCD angular momentum tensor [3,4] which has triggered a flurry of activity lately [5,6,7,8,9,10,11,12,13,14]
We focus on the orbital angular momentum of quarks inside a longitudinally polarized proton
Summary
Recent polarized beam experiments and global QCD analyses suggest that the contribution of the gluon helicity ∆G to the spin of the proton is rather small [1] This observation, together with the inexorable fact that the quark helicity contribution ∆Σ is small (less than 30%), lead one to suspect that the key to understand the proton spin puzzle is the orbital angular momentum (OAM) of quarks and gluons. Progress in this direction has been hindered by a number of difficulties in measuring, and even defining the OAM.
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