Two types of hadrons

Resonances and enhancements in meson–meson scattering can be divided into two classes distinguished by their behavior as the number of colors (Nc) in QCD becomes large: The first are ordinary mesons that become stable as Nc→∞. This class includes textbook mesons as well as glueballs and hybrids. The second class, extraordinary mesons, are enhancements that disappear as Nc→∞; they subside into the hadronic continuum. This class includes indistinct and controversial objects that have been classified as mesons or meson–meson molecules. Peláez's study of the Nc dependence of unitarized chiral dynamics illustrates both classes: the p-wave ππ and Kπ resonances, the ρ(770) and K∗(892), behave as ordinary mesons; the s-wave ππ and Kπ enhancements, the σ(600) and κ(800), behave like extraordinary mesons. Ordinary mesons resemble Feshbach resonances while extraordinary mesons look more like effects due to potentials in meson–meson scattering channels. I build and explore toy models along these lines. Finally I discuss some related dynamical issues affecting the interpretation of extraordinary mesons.