Large Nc and chiral dynamics -: art. no. 034005

We study the dependence on the number of colors of the leading pipi scattering amplitude in chiral dynamics. We demonstrate the existence of a critical number of colors for and above which the low energy pipi scattering amplitude computed from the simple sum of the current algebra and vector meson terms is crossing symmetric and unitary at leading order in a truncated and regularized 1/N-c expansion. The critical number of colors turns out to be N-c=6 and is insensitive to the explicit breaking of chiral symmetry. Below this critical value, an additional state is needed to enforce the unitarity bound; it is a broad one, most likely of a "four quark" nature.