Baryon Magnetic Moments and Proton Spin: A Model with Collective Quark Rotation

Abstract

We analyse 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 3-quark correlation (such as a flux string) that rotates with angular momentum $⟨L_z⟩$ around the proton spin axis. A fit to the baryon magnetic moments, constrained by the measured values of the axial vector coupling constants $a^{(3)}=F+D$, $a^{(8)}=3F-D$, yields $⟨S_z⟩=0.08\pm 0.13$, $⟨L_z⟩=0.39\pm 0.09$, where the error is a theoretical estimate. A second fit, under slightly different assumptions, gives $⟨L_z⟩=0.37\pm 0.09$, with no constraint on $⟨S_z⟩$. The model provides a consistent description of axial vector couplings, magnetic moments and the quark polarization $⟨S_z⟩$ 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.

Comments: 16 pages, 3 ps-figures, uses RevTeX. Abstract, Sec. II, III and IV have been expanded