Three-Quark Currents and Baryon Spin

Abstract

We show that three-quark axial currents as required by broken SU(6) spin-flavor symmetry reduce the quark spin contribution to proton spin from ${{\Sigma}_p = 1}$ (one-quark axial current value) to ${{\Sigma}_p = 0.41(12)}$ consistent with the empirical value ${{\Sigma}_{p, exp} = 0.33(08)}$ . In the case of the ${{\Delta}^+(1232)}$ baryon, we find that three-quark axial currents increase the one-quark axial current value ${{\Sigma}_{{\Delta}^+} = 3}$ to ${{\Sigma}_{{\Delta}^+} = 3.87(22)}$ . We also calculate the quark orbital angular momenta L _u and L _d in the proton and ${{\Delta}^+}$ and interpret our results in terms of the prolate and oblate geometric shapes of these baryons consistent with their intrinsic quadrupole moments.