Six-pion amplitude
Within the framework of the massive nonlinear sigma model extended to the next-to-leading order in the chiral counting [for corresponding to the two(-quark)-flavor Chiral Perturbation Theory], we calculate the relativistic six-pion scattering amplitude at low energy up to and including terms . Results for the pion mass, decay constant, and four-pion amplitude in the case of (meson) flavors at are