Two types of hadrons

摘要

Resonances and enhancements in meson-meson scattering can be divided into two classes distinguished by their behavior as the number of colors (N-c) in QCD becomes large: The first are ordinary mesons that become stable as N-c -> infinity. This class includes textbook (q) over barq mesons as well as glueballs and hybrids. The second class, extraordinary mesons, are enhancements that disappear as Nc -> infinity; they subside into the hadronic continuum. This class includes indistinct and controversial objects that have been classified as (q) over bar(q) over bar qq mesons or meson-meson molecules. Pelaez's study of the N-c dependence of unitarized chiral dynamics illustrates both classes: the p-wave pi pi and K pi resonances, the rho (770) and K* (892), behave as ordinary mesons: the s-wave pi pi and K pi enhancements, the (7(600) and K(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. (c) 2008 Elsevier B.V. All rights reserved.