Excited $[{\bf 70, \ell^+}]$ baryons in large $N_c$ QCD

The masses of the positive parity $[{\bf 70},0^+]$ and $[{\bf 70},2^+]$ non-strange baryons are calculated in large $N_c$ QCD by considering the most dominant operators in an $1/N_c$ expansion. The approach is based on the introduction of an excited core, obtained after the last particle (an excited quark) has been removed. Configuration mixing is neglected, for simplicity. Although being a sub-leading $1/N_c$ order, we find that the spin-spin interaction plays a dominant role in describing the data. The role of $N_c^ 0$ operators is also pointed out. We show how the contribution of the linear term in $N_c$, of the spin-spin and spin-orbit terms vary with the excitation energy.