Quark orbital angular momentum as a chiral magnetic effect

Abstract

The flavor-nonsinglet ($u - d$) quark angular momentum (AM) in the proton is computed based on the effective spin-flavor dynamics emerging from chiral symmetry breaking by QCD instantons. The QCD AM operators are converted to effective spin-flavor operators expressing instanton-induced chiral interactions. A large negative orbital AM $L_{u - d}$ arises as a ``chiral magnetic effect'' of the interaction of the quarks with the chiral mean field in the proton in the large-$N_c$ limit. It cancels part of the large positive spin AM $S_{u-d}$ and reduces the total AM $J_{u-d} = L_{u-d} + S_{u-d}$, in agreement with lattice QCD calculations.