Quark and gluon spin and orbital angular momentum in the proton

Abstract

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.

Topics & Concepts