On the small-x behavior of the orbital angular momentum distributions in QCD

Yoshitaka Hatta and Dong-Jing Yang 2018 Physics Letters B  781 213-219

We present the numerical solution of the leading order QCD evolution equation for the
orbital angular momentum distributions of quarks and gluons and discuss its implications
for the nucleon spin sum rule. We observe that at small-x, the gluon helicity and orbital
angular momentum distributions are roughly of the same magnitude but with opposite signs, indicating a significant cancellation between them. A similar cancellation occurs also in the quark sector. We explain analytically the reason for this cancellation.