Selection rules and quark correlations in the $N^{\ast }$ resonance spectrum

A “$\mathrm{\Lambda }$ selection rule” for $N^{\ast }$ resonances in the presence of QCD mixing effects is identified. Due to the QCD mixing, excitations of {\bf 20}-plets are possible in SU(6). We show that this selection rule is useful for classifying PDG states at N=2, and for clarifying whether strongly correlated diquarks survive for $L>0$.