Quark and gluon spin and orbital angular momentum in the proton

This is the authors' abstract. We don't add key points for this paper.

We address the proton spin puzzle with a fully relativistic and nonperturbative approach based on a light-front quantized Hamiltonian with quantum chromodynamics (QCD) input. From this, we calculate the effects from incorporating a dynamical gluon on the proton's gluon densities, helicity distribution, and orbital angular momentum that constitute the proton spin sum rule. We predict about 26% of the proton's spin is carried by the gluon's helicity and about 1.3% by its orbital angular momentum in low-momentum transfer experiments. Our approach also provides a good quality description of the proton's quark distribution functions following QCD scale evolution.