Color magnetism and the helicity-zero (γvN→Δ) transition amplitude

Abstract

We compute longitudinal and scalar multipoles in the γvN→Δ transition in nonrelativistic quark shell models as a function of the virtual-photon mass squared. We show that the gauge relation between these multipoles for arbitrary virtual-photon mass squared is violated in the truncated quark–shell-model basis. The computed scalar multipole, insensitive to this truncation effect, should provide an accurate test of quark models. Existing experiments support the role of color-magnetic tensor force in hadronic structure.