Nonordinary light meson couplings and the 1/Nc expansion

We study the large Nc behavior of couplings among light meson states with different compositions in terms of quarks and gluons. We shortly review the most common compositions of mesons, which are of interest for the understanding of low-lying meson resonances, namely, the ordinary quark-antiquark states as well as the nonordinary glueball, tetraquark, and other states. We dedicate special attention to Jaffe’s generalization of the tetraquark with Nc − 1 q¯q pairs, which is the only type of state we have identified whose width does not necessarily vanish with Nc, though it does decouple exponentially with Nc from the ππ channel, so that it is weakly coupled to the meson-meson system