Non-ordinary light meson couplings and the $1/N_c$ expansion
  We study the large $N_c$ 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 non-ordinary, glueball, tetraquark, etc. We dedicate special attention to Jaffe's generalization of the tetraquark with $N_c-1$ $q\bar{q}$ pairs, that is the only type of state we have identified, whose width does not necessarily vanish with $N_c$, while it does decouple exponentially with $N_c$ from the $\pi\pi$ channel, so that is weakly coupled to the meson-meson system. 
Thomas Cohen, Felipe J. Llanes-Estrada, J. R. Pelaez and J. Ruiz de   Elvira
hep-ph