Dwaves in the nucleon: A test of color magnetism

The Becchi-Morpurgo selection rule that the decay $\Delta \to N\gamma$ is pure $M1\mathrm{}$ was an early success of the quark model. We show that quark hyperfine interactions produce $D$ waves in the nucleon and $\Delta$ which lead to a small violation of this rule. The observation of this effect, especially when combined with earlier tests for mixed-symmetry $S$ waves in the nucleon, would provide good evidence for the presence of the color magnetism expected from one-gluon exchange. Existing measurements give some indications for the predicted effect.