The $1/N_c$ expansion in hadron effective field theory

We study the $N_c$ scalings of pion-nucleon and nucleon-nucleon scatterings and find that a consistent large $N_c$ counting can be established if we assume Witten's counting rules are applied to matrix elements or scattering amplitudes which use the relativistic normalization for the nucleons. With the pionless effective field theory, we also find that the $S$-wave nucleon-nucleon interaction is so strong that it should be treated nonperturbatively at the leading order in the $1/N_c$ expansion. By summing all the leading order diagrams, we find that the deuteron binding energy is of order $1/N_c$. In contrast, meson-meson interaction is so weak that loosely-bound meson-meson molecular states may not exist in the large $N_c$ limit.