Why the proton spin is not due to quarks

Abstract

Recent EMC data on the spin‐dependent proton structure function suggest that very little of the proton spin is due to the helicity of the quarks inside it. We argue that, at leading order in the 1/Nc expansion, none of the proton spin would be carried by quarks in the chiral limit where mq=0. This model‐independent result is based on a physical picture of the nucleon as a soliton solution of the effective chiral Lagrangian of large‐Nc QCD. The Skyrme model is then used to estimate quark contribution to the proton spin when chiral symmetry and flavor SU(3) are broken: this contribution turns out to be small, as suggested by the EMC. Next, we discuss the other possible contributions to the proton helicity in the infinite‐momentum frame–polarized gluons (ΔG), and orbital angular momentum (Lz). We argue on general grounds and by explicit example that ΔG=0 and that if the parameters of the chiral Lagrangian are adjusted so that gluons carry ∼50% of the proton momentum, most of the orbital angular momentum Lz is carried by quarks. We mention several experiments to test the EMC results and their interpretation.