Baryon magnetic moments and proton spin: A model with collective quark rotation

Abstract

We analyze the baryon magnetic moments in a model that relates them to the parton spins {Delta}u, {Delta}d, {Delta}s, and includes a contribution from orbital angular momentum. The specific assumption is the existence of a three-quark correlation (such as a flux string) that rotates with angular momentum {l_angle}L{sub z}{r_angle} around the proton spin axis. A fit to the baryon magnetic moments, constrained by the measured values of the axial vector coupling constants a{sup (3)}=F+D, a{sup (8)}=3F{minus}D, yields {l_angle}S{sub z}{r_angle}=0.08{plus_minus}0.13, {l_angle}L{sub z}{r_angle}=0.39{plus_minus}0.09, where the error is a theoretical estimate. A second fit, under slightly different assumptions, gives {l_angle}L{sub z}{r_angle}=0.37{plus_minus}0.09, with no constraint on {l_angle}S{sub z}{r_angle}. The model provides a consistent description of axial vector couplings, magnetic moments, and the quark polarization {l_angle}S{sub z}{r_angle} measured in deep inelastic scattering. The fits suggest that a significant part of the angular momentum of the proton may reside in a collective rotation of the constituent quarks. {copyright} {ital 1997} {ital The American Physical Society}